limitations of interstellar space travel

Is Interstellar Travel Really Possible?

Interstellar flight is a real pain in the neck.

Paul M. Sutter is an astrophysicist at The Ohio State University , host of Ask a Spaceman and Space Radio , and author of " Your Place in the Universe. " Sutter contributed this article to Space.com's Expert Voices: Op-Ed & Insights . 

Interstellar space travel . Fantasy of every five-year-old kid within us. Staple of science fiction serials. Boldly going where nobody has gone before in a really fantastic way. As we grow ever more advanced with our rockets and space probes, the question arises: could we ever hope to colonize the stars? Or, barring that far-flung dream, can we at least send space probes to alien planets, letting them tell us what they see?

The truth is that interstellar travel and exploration is technically possible . There's no law of physics that outright forbids it. But that doesn't necessarily make it easy, and it certainly doesn't mean we'll achieve it in our lifetimes, let alone this century. Interstellar space travel is a real pain in the neck. 

Related: Gallery: Visions of Interstellar Starship Travel

Voyage outward

If you're sufficiently patient, then we've already achieved interstellar exploration status. We have several spacecraft on escape trajectories, meaning they're leaving the solar system and they are never coming back. NASA's Pioneer missions, the Voyager missions , and most recently New Horizons have all started their long outward journeys. The Voyagers especially are now considered outside the solar system, as defined as the region where the solar wind emanating from the sun gives way to general galactic background particles and dust.

So, great; we have interstellar space probes currently in operation. Except the problem is that they're going nowhere really fast. Each one of these intrepid interstellar explorers is traveling at tens of thousands of miles per hour, which sounds pretty fast. They're not headed in the direction of any particular star, because their missions were designed to explore planets inside the solar system. But if any of these spacecraft were headed to our nearest neighbor, Proxima Centauri , just barely 4 light-years away, they would reach it in about 80,000 years.

I don't know about you, but I don't think NASA budgets for those kinds of timelines. Also, by the time these probes reach anywhere halfway interesting, their nuclear batteries will be long dead, and just be useless hunks of metal hurtling through the void. Which is a sort of success, if you think about it: It's not like our ancestors were able to accomplish such feats as tossing random junk between the stars, but it's probably also not exactly what you imagined interstellar space travel to be like.

Get the Space.com Newsletter

Breaking space news, the latest updates on rocket launches, skywatching events and more!

Related: Superfast Spacecraft Propulsion Concepts (Images)

Speed racer

To make interstellar spaceflight more reasonable, a probe has to go really fast. On the order of at least one-tenth the speed of light. At that speed, spacecraft could reach Proxima Centauri in a handful of decades, and send back pictures a few years later, well within a human lifetime. Is it really so unreasonable to ask that the same person who starts the mission gets to finish it?

Going these speeds requires a tremendous amount of energy. One option is to contain that energy onboard the spacecraft as fuel. But if that's the case, the extra fuel adds mass, which makes it even harder to propel it up to those speeds. There are designs and sketches for nuclear-powered spacecraft that try to accomplish just this, but unless we want to start building thousands upon thousands of nuclear bombs just to put inside a rocket, we need to come up with other ideas.

Perhaps one of the most promising ideas is to keep the energy source of the spacecraft fixed and somehow transport that energy to the spacecraft as it travels. One way to do this is with lasers. Radiation is good at transporting energy from one place to another, especially over the vast distances of space. The spacecraft can then capture this energy and propel itself forward.

This is the basic idea behind the Breakthrough Starshot project , which aims to design a spacecraft capable of reaching the nearest stars in a matter of decades. In the simplest outline of this project, a giant laser on the order of 100 gigawatts shoots at an Earth-orbiting spacecraft. That spacecraft has a large solar sail that is incredibly reflective. The laser bounces off of that sail, giving momentum to the spacecraft. The thing is, a 100-gigawatt laser only has the force of a heavy backpack. You didn't read that incorrectly. If we were to shoot this laser at the spacecraft for about 10 minutes, in order to reach one-tenth the speed of light, the spacecraft can weigh no more than a gram.

That's the mass of a paper clip.

Related: Breakthrough Starshot in Pictures: Laser-Sailing Nanocraft to Study Alien Planets

A spaceship for ants

This is where the rubber meets the interstellar road when it comes to making spacecraft travel the required speeds. The laser itself, at 100 gigawatts, is more powerful than any laser we've ever designed by many orders of magnitude. To give you a sense of scale, 100 gigawatts is the entire capacity of every single nuclear power plant operating in the United States combined.

And the spacecraft, which has to have a mass no more than a paper clip, must include a camera, computer, power source, circuitry, a shell, an antenna for communicating back home and the entire lightsail itself.  

That lightsail must be almost perfectly reflective. If it absorbs even a tiny fraction of that incoming laser radiation it will convert that energy to heat instead of momentum. At 100 gigawatts, that means straight-up melting, which is generally considered not good for spacecraft. 

Once accelerated to one-tenth the speed of light, the real journey begins. For 40 years, this little spacecraft will have to withstand the trials and travails of interstellar space. It will be impacted by dust grains at that enormous velocity. And while the dust is very tiny, at those speeds motes can do incredible damage. Cosmic rays, which are high-energy particles emitted by everything from the sun to distant supernova, can mess with the delicate circuitry inside. The spacecraft will be bombarded by these cosmic rays non-stop as soon as the journey begins.

Is Breakthrough Starshot possible? In principle, yes. Like I said above, there's no law of physics that prevents any of this from becoming reality. But that doesn't make it easy or even probable or plausible or even feasible using our current levels of technology (or reasonable projections into the near future of our technology). Can we really make a spacecraft that small and light? Can we really make a laser that powerful? Can a mission like this actually survive the challenges of deep space?

The answer isn't yes or no. The real question is this: are we willing to spend enough money to find out if it's possible?

• Building Sails for Tiny Interstellar Probes Will Be Tough — But Not Impossible
• 10 Exoplanets That Could Host Alien Life
• Interstellar Space Travel: 7 Futuristic Spacecraft to Explore the Cosmos

Learn more by listening to the episode "Is interstellar travel possible?" on the Ask A Spaceman podcast, available on iTunes and on the Web at http://www.askaspaceman.com . Thanks to @infirmus, Amber D., neo, and Alex V. for the questions that led to this piece! Ask your own question on Twitter using #AskASpaceman or by following Paul @PaulMattSutter and facebook.com/PaulMattSutter .  

Follow us on Twitter @Spacedotcom or Facebook . 

Join our Space Forums to keep talking space on the latest missions, night sky and more! And if you have a news tip, correction or comment, let us know at: [email protected].

Paul M. Sutter is an astrophysicist at SUNY Stony Brook and the Flatiron Institute in New York City. Paul received his PhD in Physics from the University of Illinois at Urbana-Champaign in 2011, and spent three years at the Paris Institute of Astrophysics, followed by a research fellowship in Trieste, Italy, His research focuses on many diverse topics, from the emptiest regions of the universe to the earliest moments of the Big Bang to the hunt for the first stars. As an "Agent to the Stars," Paul has passionately engaged the public in science outreach for several years. He is the host of the popular "Ask a Spaceman!" podcast, author of "Your Place in the Universe" and "How to Die in Space" and he frequently appears on TV — including on The Weather Channel, for which he serves as Official Space Specialist.

Space-based solar power may be one step closer to reality, thanks to this key test (video)

DJI Avata 2 drone review

Iconic British meteorite 'Winchcombe' found to have a smashing past

Most Popular

• 2 Venus is leaking carbon and oxygen, a fleeting visit by BepiColombo reveals
• 3 I flew Boeing's Starliner spacecraft in 4 different simulators. Here's what I learned (video, photos)
• 4 NASA's Mars sample return plan is getting a major overhaul: 'The bottom line is $11 billion is too expensive'
• 5 NASA astronaut Loral O'Hara missed the total solar eclipse, but saw Earth 'moving' below her during spacewalk (photos)

An official website of the United States government

The .gov means it’s official. Federal government websites often end in .gov or .mil. Before sharing sensitive information, make sure you’re on a federal government site.

The site is secure. The https:// ensures that you are connecting to the official website and that any information you provide is encrypted and transmitted securely.

• Publications
• Account settings

Preview improvements coming to the PMC website in October 2024. Learn More or Try it out now .

• Advanced Search
• Journal List

Human Health during Space Travel: State-of-the-Art Review

Chayakrit krittanawong.

1 Department of Medicine and Center for Space Medicine, Section of Cardiology, Baylor College of Medicine, Houston, TX 77030, USA

2 Translational Research Institute for Space Health, Houston, TX 77030, USA

3 Department of Cardiovascular Diseases, New York University School of Medicine, New York, NY 10016, USA

Nitin Kumar Singh

4 Biotechnology and Planetary Protection Group, Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA

Richard A. Scheuring

5 Flight Medicine, NASA Johnson Space Center, Houston, TX 77058, USA

Emmanuel Urquieta

6 Department of Emergency Medicine and Center for Space Medicine, Baylor College of Medicine, Houston, TX 77030, USA

Eric M. Bershad

7 Department of Neurology, Center for Space Medicine, Baylor College of Medicine, Houston, TX 77030, USA

Timothy R. Macaulay

8 KBR, Houston, TX 77002, USA

Scott Kaplin

9 Department of Dermatology, Baylor College of Medicine, Houston, TX 77030, USA

Stephen F. Kry

10 Department of Radiation Physics, University of Texas MD Anderson Cancer Center, Houston, TX 77030, USA

Thais Russomano

11 InnovaSpace, London SE28 0LZ, UK

Marc Shepanek

12 Office of the Chief Health and Medical Officer, NASA, Washington, DC 20546, USA

Raymond P. Stowe

13 Microgen Laboratories, La Marque, TX 77568, USA

Andrew W. Kirkpatrick

14 Department of Surgery and Critical Care Medicine, University of Calgary, Calgary, AB T2N 1N4, Canada

Timothy J. Broderick

15 Florida Institute for Human and Machine Cognition, Pensacola, FL 32502, USA

Jean D. Sibonga

16 Division of Biomedical Research and Environmental Sciences, NASA Lyndon B. Johnson Space Center, Houston, TX 77058, USA

Andrew G. Lee

17 Department of Ophthalmology, University of Texas Medical Branch School of Medicine, Galveston, TX 77555, USA

18 Department of Ophthalmology, Blanton Eye Institute, Houston Methodist Hospital, Houston, TX 77030, USA

19 Department of Ophthalmology, University of Texas MD Anderson Cancer Center, Houston, TX 77030, USA

20 Department of Ophthalmology, Texas A and M College of Medicine, College Station, TX 77807, USA

21 Department of Ophthalmology, University of Iowa Hospitals and Clinics, Iowa City, IA 52242, USA

22 Departments of Ophthalmology, Neurology, and Neurosurgery, Weill Cornell Medicine, New York, NY 10021, USA

Brian E. Crucian

23 National Aeronautics and Space Administration (NASA) Johnson Space Center, Human Health and Performance Directorate, Houston, TX 77058, USA

Associated Data

The field of human space travel is in the midst of a dramatic revolution. Upcoming missions are looking to push the boundaries of space travel, with plans to travel for longer distances and durations than ever before. Both the National Aeronautics and Space Administration (NASA) and several commercial space companies (e.g., Blue Origin, SpaceX, Virgin Galactic) have already started the process of preparing for long-distance, long-duration space exploration and currently plan to explore inner solar planets (e.g., Mars) by the 2030s. With the emergence of space tourism, space travel has materialized as a potential new, exciting frontier of business, hospitality, medicine, and technology in the coming years. However, current evidence regarding human health in space is very limited, particularly pertaining to short-term and long-term space travel. This review synthesizes developments across the continuum of space health including prior studies and unpublished data from NASA related to each individual organ system, and medical screening prior to space travel. We categorized the extraterrestrial environment into exogenous (e.g., space radiation and microgravity) and endogenous processes (e.g., alteration of humans’ natural circadian rhythm and mental health due to confinement, isolation, immobilization, and lack of social interaction) and their various effects on human health. The aim of this review is to explore the potential health challenges associated with space travel and how they may be overcome in order to enable new paradigms for space health, as well as the use of emerging Artificial Intelligence based (AI) technology to propel future space health research.

1. Introduction

Until now space missions have generally been either of short distance (Low Earth Orbit—LEO) or short duration (Apollo Lunar Missions). However, upcoming missions are looking to push the boundaries of space travel, with plans to travel for longer distances and durations than ever before. Both the National Aeronautics and Space Administration (NASA) and several commercial space companies (e.g., Blue Origin, SpaceX) have already started the process of preparing for long distance, long-duration space exploration and currently plan to explore inner solar planets (e.g., Mars) by the 2030s [ 1 ].

Within the extraterrestrial environment, a multitude of exogenous and endogenous processes could potentially impact human health in several ways. Examples of exogenous processes include exposure to space radiation and microgravity while in orbit. Space radiation poses a risk to human health via a number of potential mechanisms (e.g., alterations of gut microbiome biosynthesis, accelerated atherosclerosis, bone remodeling, and hematopoietic effects) and prolonged microgravity exposure presents additional potential health risks (e.g., viral reactivation, space motion sickness, muscle/bone atrophy, and orthostatic intolerance) [ 2 , 3 , 4 , 5 , 6 , 7 ]. Examples of endogenous processes potentially impacted by space travel include alteration of humans’ natural circadian rhythm (e.g., sleep disturbances) and mental health disturbances (e.g., depression, anxiety) due to confinement, isolation, immobilization, and lack of social interaction [ 8 , 9 , 10 ]. Finally, the risk of unknown exposures, such as yet undiscovered pathogens, remain persistent threats to consider. Thus, prior to the emergence of long distance, long duration space travel it is critical to anticipate the impact of these varied environmental factors and identify potential mitigating strategies. Here, we review the available medical literature on human experiments conducted during space travel and summarize our current knowledge on the effects of living in space for both short and long durations of time. We also discuss the potential countermeasures currently employed during interstellar travel, as well as future directions for medical research in space.

1.1. Medical Screening and Certification Prior to Space Travel

When considering preflight medical screening and certification, the requirements and recommendations vary based on the duration of space travel. Suborbital spaceflight, part of the new era of space travel, has participants launching to the edge of space (defined as the Karman line, 100 Km above mean sea level) for brief 3–5 min microgravity exposures. Orbital spaceflight, defined as microgravity exposure for up to 30 days, involves healthy individuals with preflight medical screening. In addition to a physical examination and metabolic screening, preflight medical screening assessing aerobic capacity (VO 2max ), and muscle strength and function may be sufficient to ensure proper conditioning prior to mission launch [ 11 , 12 , 13 , 14 ]. Age-appropriate health screening tests (e.g., colonoscopy, serum prostate specific antigen in men, and mammography in women) are generally recommended for astronauts in the same fashion as their counterparts on Earth. In individuals with cardiovascular risk factors or with specific medical conditions, additional screening may be required [ 15 ]. The goal of these preflight screening measures is to ensure that medical conditions that may result in sudden incapacitation are identified and either disqualified or treated before the mission begins. In addition to the medical screening described above, short-duration space travelers are also required to undergo acceleration training, hypobaric and hypoxia exposure training, and hypercapnia awareness procedures as part of the preflight training phase.

In preparation for long-duration space travel, astronauts generally undergo a general physical examination, as well as imaging and laboratory studies at the time of initial selection. These screening tests would then be repeated annually, as well as upon assignment to an International Space Station (ISS) mission. ISS crew members are medically certified for long-duration spaceflight missions through individual agency medical boards (e.g., NASA Aerospace Medical Board) and international medical review boards (e.g., Multilateral Space Medicine Board) [ 16 , 17 ]. In order for an individual to become certified for long-duration space travel, an individual must be at the lowest possible risk for the occurrence of medical events during the preflight, infight, and postflight periods. Following spaceflight, it is recommended that returning astronauts undergo occupational surveillance for the remainder of their lifetime for the detection of health issues related to space travel (e.g., NASA’s Lifetime Surveillance of Astronaut Health program) [ 18 ]. Table 1 summarizes the preflight, inflight, and postflight screening recommendations for each organ system. Further research utilizing data from either long-term space missions or simulated environments is required in order to develop an adequate preflight scoring system capable of predicting inflight and postflight health outcomes in space travelers based on various risk factors.

Summarizes the pre-flight, in-flight and post-flight screening in each system.

Below we discuss potential Space Hazards for each organ system along with possible countermeasures ( Table 2 ). Table 3 lists prospective opportunities for artificial intelligence (AI) implementation.

Summary of Space Hazards to each organ system and potential countermeasures.

Potential AI applications in space health.

1.2. Effects on the Cardiovascular System

During short-duration spaceflight, microgravity alters cardiovascular physiology by reducing circulatory blood volume, diastolic blood pressure, left ventricular mass, and cardiac contractility [ 42 , 123 ]. Several studies have demonstrated that peak exercise performance is reduced both inflight and immediately after short-duration spaceflight due primarily to a reduction in maximal cardiac output and O 2 delivery [ 124 , 125 ]. Prolonged exposure to microgravity does cause unloading of the cardiovascular system (e.g., removal of expected loading effects from Earth’s gravity when upright during the day), resulting in cardiac atrophy. These changes may be an example of adaptive physiologic changes (“physiologic atrophy”) that returns to baseline after returning from spaceflight. This process may be similar to the adaptive physiologic changes to the cardiovascular system seen during athletic training (“physiologic hypertrophy”). Thus far, there is no evidence that the observed short-term cardiac atrophy could permanently impair systolic function. However, this physiologic adaptation to microgravity in space could lead to orthostatic hypotension/intolerance upon returning to Earth’s gravity due to changes in the comparative position of peripheral resistance and sympathetic nerve activity [ 41 , 126 , 127 ]. Figure 1 demonstrates potential effects of the space environment on each organ system.

Potential effects of the space environment on each organ system.

Another potential effect of microgravity exposure is that an alteration of hydrostatic forces in the vertical gravitational (Gz) axis could lead to the formation of internal jugular vein thromboses [ 28 , 29 ]. Anticoagulation would not be an ideal choice for prevention as astronauts have an increased risk of suffering traumatic injury during spaceflight, thus potentially inflating the risk of developing an intracerebral hemorrhage or subdural hematoma. In addition, if a traumatic accident were to occur during spaceflight, the previously discussed cardiovascular adaptations could impair the body’s ability to tolerate blood loss and shock [ 45 , 46 , 47 ].

During long-duration spaceflight, one recent study demonstrated that astronauts did not experience orthostatic hypotension/intolerance during routine activities or after landing following 6 months in space [ 128 ]. It is worth noting that all of these astronauts performed aggressive exercise countermeasures while in flight [ 128 ]. Another study of healthy astronauts after 6 months of space travel showed that the space environment caused transient changes in left atrial structure/electrophysiology, increasing the risk of developing atrial fibrillation (AF) [ 129 ]. However, there was no definitive evidence of increased incidence of supraventricular arrhythmias and no identified episodes of AF [ 129 ]. Evaluation with echocardiography or cardiac MRI may be considered following long-duration spaceflight in certain cases.

Prior human studies with supplemental data obtained from animal studies, have shown that healthy individuals with prolonged exposure to ionizing radiation may be at increased risk for the development of accelerated atherosclerosis secondary to radiation-induced endothelial damage and a subsequent pro-inflammatory response [ 3 , 4 , 57 , 58 , 59 , 60 , 123 ]. One study utilizing human 3D micro-vessel models showed that ionizing radiation inhibits angiogenesis via mechanisms dependent on the linear energy transfer (LET) of charged particles [ 130 ], which could eventually lead to cardiac dysfunction [ 131 , 132 ]. In fact, specific characteristics of the radiation encountered in space may be an important factor to understanding its effects. For example, studies of pediatric patients undergoing radiotherapy have shown an increase in cardiac-related morbidity/mortality due to radiation exposure, but not until radiation doses exceeded 10 Gy [ 133 ]. At lower dose levels the risk is less clear: while a study of atomic bomb survivors with more than 50 years of followup demonstrated elevated cardiovascular risks at doses < 2 Gy [ 134 ]. A recent randomized clinical trial with a 20-year follow-up showed no increase in cardiac mortality in irradiated breast cancer patients with a median dose of 3.0 Gy (1.1–8.1 Gy) [ 135 ]. The uncertainty in cardiovascular effects of ionizing radiation, are accentuated in a space environment as the type and quality of radiation likely play an important role as well.

Further research is required to understand the radiation dosage, duration, and quality necessary for cardiovascular effects to manifest, as well as develop preventive strategies for AF and internal jugular vein thrombosis during space travel.

1.3. Effects on the Gastrointestinal System

During short-duration spaceflight, the presence of gastrointestinal symptoms (e.g., diarrhea, vomiting, and inflammation of the gastrointestinal tract) are common due to microgravity exposure [ 35 , 136 , 137 ]. Still unknown however is whether acute, surgical conditions such as cholecystitis and appendicitis occur more frequently due to microgravity-induced stone formation or alterations in human physiology/anatomy, and immunosuppression [ 40 ]. Controlling for traditional risk factors associated with the development of these conditions (e.g., adequate hydration, maintenance of a normal BMI, dietary fat avoidance, etc.) may help mitigate the risk.

During long-duration spaceflight, it is possible that prolonged radiation exposure could lead to radiation-induced gastrointestinal cancer. Gamma radiation exposure is a known risk factor for colorectal cancer via an absence of DNA methylation [ 138 ]. NASA has recently developed a space radiation simulator, named the “GCR Simulator”, which allows for the more accurate radiobiologic research into the development and mitigation of radiation-induced malignancies [ 139 ]. Preflight colorectal cancer screening via colonoscopy or inflight screening via gut microbiome monitoring may be beneficial, but further research is required to demonstrate their clinical utility. Several studies, including the NASA Twins study have shown that microgravity could lead to alterations in an individual’s gut microbial community (i.e., gut dysbiosis) [ 2 , 140 , 141 , 142 ]. While changes to an individual’s gut microbiome can cause inflammation of the gastrointestinal tract [ 143 , 144 ], it remains unclear whether the specific alterations observed during spaceflight pose a risk to astronaut health. In fact, increased gut colonization by certain bacterial species is even associated with a beneficial effect on the gastrointestinal tract [ 2 , 140 ]. ( Table S1 ) Certain limitations of these studies, such as variations in genomic profile, diet, and a lack of adjusted confounders (e.g., the microbial content of samples) should be considered. Another potential consequence of prolonged microgravity exposure is the possibility of increased fatty-acid processing [ 145 ], leading to the development of non-alcoholic fatty liver disease (NAFLD) and hepatic fibrosis [ 146 , 147 ].

Further research is required to better understand gut microbial dynamics during space travel, as well as spaceflight-associated risk factors for the development of NAFLD, cholecystitis, and appendicitis.

1.4. Effects on the Immune System

During spaceflight, exposure to microgravity could potentially induce modifications in the cellular function of the human immune system. For example, it has been hypothesized that microgravity exposure could lead to an increase in the production of inflammatory cytokines [ 148 ] and stress hormones [ 149 , 150 ], alterations in the function of certain cell lines (NK cells [ 151 , 152 ], B cells [ 153 ], monocytes [ 154 ], neutrophils [ 154 ], T cells [ 5 , 155 ]), and impairments of leukocyte distribution [ 156 ] and proliferation [ 155 , 157 , 158 ]. The resultant immune system dysfunction could lead to the reactivation of latent viruses such as Epstein-Bar Virus (EBV), Varicella-Zoster Virus (VZV), and Cytomegalovirus (CMV) [ 31 , 32 ]. Persistent low-grade pro-inflammatory responses microgravity could lead to space fever. [ 159 ] Studies are currently underway to evaluate countermeasures to improve immune function and reduce reactivation of latent herpesviruses [ 33 , 160 , 161 , 162 ]. Microgravity exposure could also lead to the development of autoantibodies, predisposing astronauts to various autoimmune conditions [ 136 , 163 ]. ( Table S2 ) Most importantly, studies have shown that bacteria encountered within the space environment appear to be more resistant to antibiotics and more harmful in general compared to bacteria encountered on Earth [ 164 , 165 ]. This is in addition to the threat of novel bacteria species (e.g., Methylobacterium ajmalii sp. Nov. [ 76 ]) that we have not yet discovered.

Upon returning from the space environment astronauts remain in an immunocompromised state, which has been particularly problematic in the era of the COVID-19 pandemic. Recently, NASA has recommended postflight quarantine and immune status monitoring (i.e., immune-boosting protocol) to mitigate the risk of infection [ 77 ]. This is similar to the Apollo and NASA Health Stabilization Programs that helped establish the preflight protocol (pre-mission quarantine) currently used for this purpose.

Further research is required to understand the mechanisms of antibiotic resistance and the modifications in inflammatory cytokine dynamics, in order to develop immune boosters and surrogate immune biomarkers.

1.5. Effects on the Hematologic System

During short-duration spaceflight, the plasma volume and total blood volume de-crease within the first hours and remain reduced throughout the inflight period, a finding previously identified as space anemia [ 166 ]. Space anemia during spaceflight is perhaps due to a normal physiologic adaptation of newly released blood cells and iron metabolism to microgravity [ 167 ].

During long-duration spaceflight, microgravity exposure could potentially induce hemoglobin degradation, leading to hemolytic anemia. In a recent study of 14 astronauts who were on 6-month missions onboard the ISS, a 54% increase in hemolysis was ob-served after landing one year later [ 50 ]. In another small study, nearly half of astronauts (48%) landing after long duration missions were anemic and hemoglobin levels were characterized as having a dose–response relationship with microgravity exposure [ 51 ]. An additional study collected whole blood sample from astronauts during and after up to 6 months of orbital spaceflight [ 168 ]. Upon analysis, once the astronauts returned to Earth RBC and hemoglobin levels were significantly elevated. It is worth noting that these studies analyzed blood samples from astronauts collected after spaceflight, which may be influenced by various factors (e.g., the stress of landing and re-adaptation to conditions on Earth). In addition, these studies may be confounded by other extraterrestrial environmental factors such as fluid shifts, dehydration, and alteration of the circadian cycle.

Further research is urgently needed to understand plasma volume physiology dur-ing spaceflight and delineate the etiology and degree of hemolysis with longer space exposure, such as 1-year ISS or Mars exploration missions.

1.6. Oncologic Effects

Even during short-duration spaceflight, the stochastic nature of cancer development makes it possible that space radiation exposure could cause cancer via epigenomic modifications [ 63 ]. Currently, our epidemiological understanding of radiation-induced cancer risk is based primarily on atomic bomb survivors and accidental radiation exposures, which both show a clear association between radiation exposure and cancer risk [ 169 , 170 ]. However, these studies are hard to generalize to spaceflight as the patient populations vary significantly (generally healthy astronauts vs. atomic bomb survivors [NCRP 126]) [ 171 ]. Moreover, the radiation encountered in space is notably different than that associated with atomic bomb exposure. Most terrestrial exposures are based on low LET radiation (e.g., atomic bomb survivors received <1% dose from high LET neutrons) [ 172 ], whereas space radiation is comprised of higher LET ions (solar energetic particles and galactic cosmic rays) [ 173 , 174 ].

During long-duration spaceflight, our current understanding of cancer risk is also largely unknown. Our current epidemiologic understanding of long-duration radiation exposure and cancer risk is primarily based on the study of chronic occupational exposures and medically exposed individuals, supplemented with data obtained from animal studies, which are again based overwhelmingly on low LET radiation [ 169 , 170 , 175 , 176 ]. In animal studies, exposure to ionizing radiation (up to 13.5 months) has been associated with an increased risk of developing a variety of cancers [ 162 , 177 , 178 , 179 , 180 ]. Ionizing radiation exposure may cause DNA methylation patterns similar to the specific patterns observed in human adenocarcinomas and squamous cell carcinomas [ 63 ]; however, this response is not yet certain [ 181 , 182 ].

For the purposes of risk prediction, the elevated biological potency of heavy ions is modeled through concepts such as the radiation weighting factor, with NASA recently releasing unique quality factors ( Q NASA ) focused on high density tracks [ 183 ]. Although these predictive models can only estimate the impact of radiation exposure, extrapolation of current terrestrial-based data suggest that this risk could be at least substantial for astronauts. NASA, for example, has updated crew permissible career exposure limits to 0.6Sv, independent of age and sex. This degree of exposure results in a 2–3% mean increased risk of death from radiation carcinogenesis (NCRP 2021) [ 184 ]. This limit would be reached between 200 and 400 days of space travel (depending on degree of radiation shielding) [ 48 ].

Further research is urgently needed to understand the true risk of space radiation exposure. This is especially important for individuals with certain genotype-phenotype profiles (e.g., BRCA1 or DNA methylation signatures) who may be more sensitive to the effects of radiation exposure. Most importantly, the utilization of genotype-phenotype profiles of astronauts or space travelers is valuable not just for pre-flight screening, but also during in-flight travel, especially for long-duration flights to deeper space. An individual’s genetic makeup will in-variably change during spaceflight due to the shifting epigenetic microenvironment. Future crewed-missions to deep space will have to adapt to these anticipated changes, be-come aware of impending red-flag situations, and determine whether any meaningful shift or change to ones’ genetic makeup is possible. For example, personalized radiation shields could potentially be tailored to an individuals’ genotype-phenotype profile, individualized pulmonary capillary wedge pressure under microgravity may be different due to transient changes in left atrial structure, or preflight analysis of the globin gene for the prediction of space anemia [ 50 , 129 , 185 ]. This research should be designed to identify the radiation type, dose, quality, frequency, and duration of exposure required for cancer development.

1.7. Effects on the Neurologic System

During the initial days of spaceflight, space motion sickness (SMS) is the most commonly encountered neurologic condition. Microgravity exposure during spaceflight commonly leads to alterations in spatial orientation and gaze stabilization (e.g., shape recognition [ 186 ], depth perception and distance [ 187 , 188 ]). Postflight, impairments in object localization during pitch and roll head movements [ 189 , 190 ] and fine motor control (e.g., force modulation [ 191 ], keyed pegboard completion time [ 192 ], and bimanual coordination [ 193 ]) are common. Anecdotally, astronauts also reported alterations in smell and taste sensations during their missions [ 27 , 194 , 195 ]. The observed impairment in olfactory function is perhaps due to elevated intracranial pressure (ICP) with increased cerebrospinal fluid outflow along the cribriform plate pathways [ 196 ]. However, to date, there have been no studies directly measuring ICP during spaceflight.

Upon returning from spaceflight, studies have observed that astronauts experience decrements in postural and locomotor control that can increase fall risk [ 197 ]. These decrements have been observed in both standard sensorimotor testing and functional tasks. While recovery of sensorimotor function occurs rapidly following short-duration spaceflight (within the first several days after return) [ 192 , 198 ], recovery after long-duration spaceflight often takes several weeks. Similar to SMS, post-flight motion sickness (PFMS) is very common and occurs soon after g-transition [ 30 ]. Deficits in dexterity, dual-tasking, and vehicle operation [ 199 ] are also commonly observed immediately after spaceflight. Therefore, short-duration astronauts are recommended to not drive automobiles for several days, and only after a sensorimotor evaluation (similar to a field sobriety test).

Similarly to the effects seen following short-duration spaceflight, those returning from long-duration spaceflight can also experience deficits in dexterity, dual-tasking, and vehicle operation. Long-duration astronauts are recommended to not drive automobiles for several weeks, and also require a sensorimotor evaluation. While central nervous system (CNS) changes [ 53 ] associated with long-duration spaceflight are commonly observed, the resulting effects of these changes both during and immediately after spaceflight remain unclear [ 199 ]. Observed CNS changes include structural and functional alterations (e.g., upward shift of the brain within the skull [ 54 ], disrupted white matter structural connectivity [ 55 ], increased fluid volumes [ 56 ], and increased cerebral vasoconstriction [ 200 ]), as well as modifications to adaptive plasticity [ 53 ]. Adaptive reorganization is primarily observed in the sensory systems. For example, changes in functional connectivity during plantar stimulation have been observed within sensorimotor, visual, somatosensory, and vestibular networks after spaceflight [ 201 ]. In addition, functional responses to vestibular stimulation were altered after spaceflight―reducing the typical deactivation of somatosensory and visual cortices [ 202 ]. These studies provide evidence for sensory reweighting among visual, vestibular, and somatosensory inputs.

Further research is required to fully understand the observed CNS changes. In addition, integrated countermeasures are needed for the acute effects of g-transitions on sensorimotor and vestibular function.

1.7.1. Effects on the Neuro-Ocular System

Prolonged exposure to ionizing radiation is well known to produce secondary cataracts [ 61 , 62 ]. Most importantly, Spaceflight Associated Neuro-Ocular Syndrome (SANS) is a unique constellation of clinical and imaging findings which occur to astronauts both during and after spaceflight, and is characterized by: hyperopic refractive changes (axial hyperopia), optic disc edema, posterior globe flattening, choroidal folds, and cotton wool spots [ 43 ]. Ophthalmologic screening for SANS, including both clinical and imaging assessments is recommended. ( Figure 2 ) Although the precise etiology and mechanism for SANS remain ill-defined, some proposed risk factors for the development of SANS include microgravity related cephalad fluid shifts [ 203 ], rigorous resistive exercise [ 204 ], increased body weight [ 205 ], and disturbances to one carbon metabolic pathways [ 206 ]. Many scientists believe that the cephalad fluid shift secondary to microgravity exposure is the major pathophysiological driver of SANS [ 203 ]. Although inflight lumbar puncture has not been attempted, several mildly elevated ICPs have been recorded in astronauts with SANS manifestations upon returning to Earth [ 43 ]. Moreover, changes to the pressure gradient between the intraocular pressure (IOP) and ICP (the translaminar gradient) have been proposed as a pathogenic mechanism for SANS [ 207 ]. The translaminar gradient may explain the structural changes seen in the posterior globe such as globe flattening and choroidal folds [ 207 ]. Alternatively, the microgravity induced cephalad fluid shift may impair venous or cerebrospinal drainage from the cranial cavity and/or the eye/orbit (e.g., choroid or optic nerve sheath). Impairment of the glymphatic system has also been proposed as a contributing mechanism to SANS, but this remains unproven [ 208 , 209 ]. Although permanent visual loss has not been observed in astronauts with SANS, some structural changes (e.g., posterior globe flattening) may persist and have been documented to remain for up to 7 years of long-term follow-up [ 210 ]. Further research is required to better understand the mechanism of SANS, and to develop effective countermeasures prior to longer duration space missions.

Ophthalmologic screening for SANS.

1.7.2. Effects on the Neuro-Behavioral System

The combination of mission-associated stressors with the underlying confinement and social isolation of space travel has the potential to lead to cognitive deficits and the development of psychiatric disorders [ 211 ]. Examples of previously identified cognitive deficits associated with spaceflight include impaired concentration, short-term memory loss, and an inability to multi-task. These findings are most evident during G-transitions, and are likely due to interactions between vestibular and cognitive function [ 212 , 213 ]. Sopite syndrome, a neurologic component of motion sickness, may account for some cognitive slowing. The term “space fog”, has been used to describe the generalized lack of focus, altered perception of time, and cognitive impairments associated with spaceflight, which can occur throughout the mission. This may be related to chronic sleep deprivation as deficiencies (including decreased sleep duration and quality of sleep) are prevalent despite the frequent use of sleep medications [ 71 ]. These results highlight the broad impact of space travel on cognitive and behavioral health, and support the need for integrated countermeasures for long-duration explorative missions.

1.8. Effects on the Musculoskeletal System

During short-duration spaceflight, low back pain and disk herniation are common due to the presence of microgravity. While the pathogenesis of space-related low back pain and disk herniation is complex, the etiology is likely multifactorial in nature (e.g., microgravity induced hydration and swelling of the vertebral disk, muscle atrophy of the neck and lower back) [ 19 , 214 , 215 ]. Additionally, various joint injuries (e.g., space-suited shoulder injuries) can also occur in space due to the presence of microgravity [ 16 , 216 , 217 , 218 ]. Interestingly, one study showed that performing specific exercises could potentially promote automatic and tonic activation of lumbar multifidus and transversus abdominis as well as prevent normal lumbopelvic positioning against gravity following bed rest as a simulation of space flight [ 219 ], and the European Space Agency suggested that exercise program could relieve low back pain during spaceflight [ 220 ]. Further longitudinal studies are required to develop specialized exercise protocols during space travel.

During long-duration spaceflight, the presence of microgravity could cause an alteration in collagen fiber orientation within tendons, reduce articular cartilage and meniscal glycosaminoglycan content, and impair the wound healing process [ 22 , 23 , 24 , 221 ]. These findings seen in animal studies suggest that mechanical loading is required in order for these processes to occur in a physiologic manner. It is theorized that there is a mandatory threshold of skeletal loading necessary to direct balanced bone formation and resorption during healthy bone remodeling [ 222 , 223 ]. Despite the current countermeasure programs, the issue of skeletal integrity is still not solved [ 224 , 225 , 226 ].

Space radiation could also impact bone remodeling, though the net effect differs based on the amount of radiation involved [ 6 ]. In summary, high doses of space radiation lead to bone destruction with increased bone resorption and reduced bone formation, while low doses of space radiation actually have a positive impact with increased mineralization and reduced bone resorption. Most importantly, space radiation, particularly solar particle events in the case of a flare, may induce acute radiation effects, leading to hematopoietic syndrome [ 7 ]. This risk is highest for longer duration missions, but can be substantially minimized with current spacecraft shielding options.

Longitudinal studies are required to develop special exercise protocols and further assess the aforementioned risk of space radiation on the development of musculoskeletal malignancies.

1.9. Effects on the Pulmonary System

During short-duration spaceflight, a host of changes to normal, physiologic pulmonary function have been observed [ 73 , 227 ]. Studies during parabolic flight have shown that the diaphragm and abdomen are displaced cranially due to microgravity, which is accompanied by an increase in the diameter of the lower rib cage with outward movement. Due to the observed changes to the shape of the chest wall, diaphragm, and abdomen, alterations to the pressure-volume curve resulted in a net reduction in lung volumes [ 228 ]. In five subjects who underwent 25 s of microgravity exposure during parabolic flight, functional residual capacity (FRC) and vital capacity (VC) were found to be reduced [ 229 ]. During the Spacelab Life Sciences-1 mission, microgravity exposure resulted in 10%, 15%, 10–20%, and 18% reductions in VC, FRC, expiratory reserve volume (ERV), and residual volume (RV), respectively, compared to values seen in Earth’s gravity [ 227 ]. The observed physiologic change in FRC is primarily due to the cranial shift of the diaphragm and abdominal contents described previously, and secondarily to an increase in intra-thoracic blood volume and more uniform alveolar expansion [ 227 ].

One surrogate measure for the inhomogeneity of pulmonary perfusion can be assessed through changes in cardiogenic oscillations of CO 2 (oscillations in exhaled gas composition due to differential flows from different lung regions with differing gas composition). Following exposure to microgravity, the size of cardiogenic oscillations were significantly reduced to 60% in comparison to the preflight standing values [ 230 , 231 ]. Possible causes of the observed inhomogeneity of ventilation include regional differences in lung compliance, airway resistance, and variations in motion of the chest wall and diaphragm. Access to arterial blood gas analysis would allow for enhanced physiologic evaluations, as well as improved management of clinical emergencies (e.g., pulmonary embolism) occurring during space travel. However, there is currently no suitable method for assessing arterial blood in space. The earlobe arterialized blood technique for collecting blood gas has been proposed, but evidence is limited [ 232 ]. Further research is required in this area to establish an effective means for sampling arterial blood during spaceflight.

In comparison to the changes seen during short-duration spaceflight, studies conducted during long-duration spaceflight showed that the heterogeneity of ventilation/perfusion (V/Q) was largely unchanged, with preserved gas exchange, VC, and respiratory muscle strength [ 73 , 233 , 234 ]. This resulted in overall normal lung function. This is supported by long-duration studies (up to 6 months) in microgravity which demonstrated that the function of the normal human lungs is largely unchanged following the removal of gravity [ 233 , 234 ]. It is worth noting that there were some small changes which were observed (e.g., an increase in ERV in the standing posture) following long-duration spaceflight, which can perhaps be attributed to a reduction in circulating blood volume [ 233 , 234 ]. However, while microgravity can causes temporary changes in lung function, these changes were reversible upon return to Earth’s gravity (even after 6 months of exposure to microgravity). Based on the currently available data, the overall effect of acute and sustained exposure to microgravity does not appear to cause any deleterious effects to gas exchange in the lungs. However, the biggest challenge for long-duration spaceflight is perhaps extraterrestrial dust exposure. Further research is required to identify the long term consequences of extraterrestrial dust exposure and develop potential countermeasures (e.g., specialized face masks) [ 73 ].

1.10. Effects on the Dermatologic System

During short-duration space travel, skin conditions such as contact dermatitis, skin sensitivity, biosensor electrolyte paste reactions, and thinning skin are common [ 44 , 235 ]. However, these conditions are generally mild and unlikely to significantly impact astronaut safety or prevent completion of space missions [ 44 ].

The greatest dermatologic concern for long-duration space travelers is the theoretical increased risk of developing skin cancer due to space radiation exposure. This hypothesis is supported by one study which found the rate of basal cell carcinoma, melanoma, and squamous cell carcinoma of the skin to be higher among astronauts compared to a matched cohort [ 236 ]. While the three-fold increase in prevalence was significant, there were a number of confounders (e.g., the duration of prolonged UV exposure on Earth for training or recreation, prior use of sunscreen protection, genetic predisposition, and variations in immune system function) that must also be taken into account. A potential management strategy for dealing with various skin cancers during space travel involves telediagnostic and telesurgical procedures. Further research is needed to improve the telediagnosis and management of dermatological conditions (e.g., adjustment for a lag in communication time) during spaceflight.

1.11. Diagnostic Imaging Modalities in Space

In addition to routine physical examination, various medical imaging modalities may be required to monitor and diagnose medical conditions during long-duration space travel. To date, ultrasound imaging acquired on space stations has proven to be helpful in diagnosing a wide array of medical conditions, including venous thrombosis, renal and biliary stones, and decompression sickness [ 29 , 237 , 238 , 239 , 240 , 241 , 242 ]. Moreover, the Focused Assessment with Sonography for Trauma (FAST), utilized by physicians to rapidly evaluate trauma patients, may be employed during space missions to rule out life-threatening intra-abdominal, intra-thoracic, or intra-ocular pathology [ 243 ]. Remote telementored ultrasound (aka tele-ultrasound) has been previously investigated during the NASA Extreme Environment Missions Operations (NEEMO) expeditions [ 244 ]. Today, the Butterfly iQ portable ultrasound probe can be linked directly to a smartphone through cloud computing, allowing physicians/specialists to promptly analyze remote ultrasound images [ 245 ].

Currently, alternative imaging modalities such as X-ray, CT, PET and MRI scan are unable to be used in space due to substantial limitations (e.g., limited space for large imaging structures, difficulties in interpretation due to microgravity). However, it is possible that the future development of a photocathode-based X-ray source may one day make this a possibility [ 101 , 246 ]. If X-ray imaging was possible, certain caveats would need to be taken into account for accurate interpretation. For example, pleural effusions, air-fluid levels, and pulmonary cephalization commonly seen on terrestrial imaging, would need to be interpreted in an entirely different way due to the effect of microgravity [ 247 ]. While this adjustment might be challenging, the altered principles of weightless physiology may provide some advantages as well. For example, one study found that intra-abdominal fluid was better able to be detected in space than in the terrestrial environment due to gravitational alterations in fluid dynamics [ 248 ]. Further research is required to identify and optimize inflight imaging modalities for the detection and treatment of various medical conditions.

1.12. Medical and Surgical Procedures in Space

Despite the presence of microgravity, both basic life support and advanced cardiac life support are feasible during space travel with some modifications [ 249 , 250 ]. For example, the recent guidelines for CPR in microgravity recommend specialized techniques for delivering chest compressions [ 251 ]. The use of mechanical ventilators, and moderate sedation or general anesthesia in microgravity are also possible but the evidence is extremely limited [ 252 , 253 ]. In addition, there are several procedures such as endotracheal intubation, percutaneous tracheostomy, diagnostic peritoneal lavage, chest tube insertion, and advanced vascular access which have only been studied through artificial stimulation [ 254 , 255 ].

Once traditionally “surgical” conditions are appropriately diagnosed, the next step is to determine whether these conditions should be managed medically, percutaneously, or surgically (laparoscopic vs. open procedures) [ 47 , 256 ]. For example, acute appendicitis or cholecystitis that would historically be managed surgically in terrestrial hospitals, could instead be managed with antibiotics rather than surgery. While the use of antibiotics for these conditions is usually effective on Earth, there remain concerns due to space-induced immune alterations, increased pathogenicity and virulence of microorganisms, and limited resources to “rescue” cases of antibiotic failure [ 39 ]. In cases of antibiotic failure, one potential minimally invasive option could be ultrasound-guided percutaneous drainage, which has previously been demonstrated to be possible and effective in microgravity [ 257 ]. Another potential approach is to focus on the early diagnosis and minimally invasive treatment of appropriate conditions, rather than treating late stage disease. In addition to expediting the patient’s post-operative recovery, minimally invasive surgery in space has the added benefit of protecting the cabin environment and the remainder of the crew [ 258 , 259 ].

As in all aspects of healthcare delivery in space, the presence of microgravity can complicate even the most basic of procedures. However, based on collective experience to date, if the patient, operators, and all required equipment are restrained, the flow of surgical procedures remains relatively unchanged compared to the traditional, terrestrial experience [ 260 ]. A recent animal study confirmed that it was possible to perform minor surgical procedures (e.g., vessel and wound closures) in microgravity [ 261 ]. Similar study during parabolic flight has further confirmed that emergent surgery for the purpose of “damage control” in catastrophic scenarios can be conducted in microgravity [ 262 ]. As discussed previously, telesurgery may be feasible if the surgery can be performed with an acceptably brief time lag (<200 ms) and if the patient is within a low Earth orbit [ 263 , 264 ]. However, further research and technological advancements are required for this to come to fruition.

1.13. Lifestyle Management in Space

Based on microgravity simulation studies, NASA has proposed several potential biomedical countermeasures in space [ 33 , 160 , 161 ]. Mandatory exercise protocols in space are crucial and can be used to maintain physical fitness and counteract the effects of microgravity. While these protocols may be beneficial, exercise alone may not be enough to prevent certain effects of microgravity (e.g., an increase in arterial thickness/stiffness) [ 20 , 265 , 266 , 267 ]. For example, a recent study found that resistive exercise alone could not suppress the increase in bone resorption that occurs in space [ 20 ]. Hence, a combination of resistance training and an antiresorptive medication (e.g., bisphosphonate) appears to be optimal for promotion of bone health [ 20 , 21 ]. Further research is needed to identify the optimal exercise regimen including recommended exercises, duration, and frequency.

In addition to exercise, dietary modification may be another potential area for optimization. The use of a diet based on caloric restriction (CR) in space remains up for debate. Based on data from terrestrial studies, caloric restriction may be useful for improving vascular health; however, this benefit may be offset by the associated muscle atrophy and osteoporosis [ 268 , 269 ]. Given that NASA encourages astronauts to consume adequate energy to maintain body mass, there has been an attempt to mimic the positive effects of CR on vascular health while providing appropriate nutrition. Further research is needed this area to identify the ideal space diet.

Based on current guidelines, only vitamin D supplementation during space travel is recommended. Supplementation of A, B6, B12, C, E, K, Biotin, folic acid are not generally recommended at this time due to insufficient evidence [ 64 ] ( Table S3 ). The use of traditional prescription medications may not function as intended on Earth. Therefore, alternative methods such as synthetic biologic agents or probiotics may be considered [ 35 , 38 ]. However, evidence in this area is extremely limited, and it is possible that the synthetic agents or probiotics may themselves be altered due to microgravity and radiation exposure. Further research is needed to investigate the relationship between these supplements and potential health benefits in space.

Currently, most countermeasures are directed towards cardiovascular system and musculoskeletal pathologies but there is little data against issues like immune and sleep deprivation, SANS, skin, etc. Artificial Gravity (AG) has been postulated as adequate multi-system countermeasure especially the chronic exposure in a large radius systems. Previously, the main barrier is the huge increase in costs [ 270 , 271 , 272 ]. However, there are various studies that show the opposite and also the recent decrease in launch cost makes the budget issue nearly irrelevant especially when a huge effort is paid to counteract the lack of gravity. The use of AG especially long-radius chronic AG is feasible. Further studies are needed to determine the utilization of AG in long-duration space travel.

1.14. Future Directions for Precision Space Health with AI

In this new era of space travel and exploration, ‘future’ tools and novel applications are needed in order to prepare deep space missions, particularly pertaining to strategies for mitigating extraterrestrial environmental factors, including both exogenous and en-dogenous processes. Such ‘future’ tools could help assist and ensure a safe travel to deep space, and more importantly, help bring space travelers and astronauts back to Earth. These tools and methods may initially be ‘remotely’ controlled, or have its data sent back to Earth for analysis. Primarily efforts should be focused on analyzing data in situ, and on site during the mission itself, both for the purpose of efficiency, and for the progressive purpose of slowly weaning off a dependency on Earth.

AI is an emerging tool in the big data era and AI is considered a critical aspect of ‘fu-ture’ tools within the healthcare and life science fields. A combination of AI and big data can be used for the purposes of decision making, data analysis and outcome prediction. Just recently, there have been encourage in advancements in AI and space technologies. To date, AI has been employed by astronauts for the purpose of space exploration; however, we may just be scratching the surface of AI’s potential. In the area of medical research, AI technology can be leveraged for the enhancement of telehealth delivery, improvement of predictive accuracy and mitigation of health risks, and performance of diagnostic and interventional tasks [ 273 ]. The AI model can then be trained and have its inference leveraged through cloud computing or Edge TPU or NVIDIA Jetson Nano located on space stations. ( Table 3 and Table S4 ) Figure 3 demonstrates potential AI applications in space.

Potential AI applications in space.

As described previously, the capability to provide telemedicine beyond LEO is primarily limited by the inability to effectively communicate between space and Earth in real-time [ 274 ]. However, AI integration may be able to bridge the gap and advance communication capabilities within the space environment [ 275 , 276 ]. One study demonstrated a potential mechanism for AI incorporation in which an AI-generated predictive algorithm displayed the projected motion of surgical tools to adjust for excess communication lag-time [ 277 ]. This discovery could potentially enable AI-enhanced robotics to complete repetitive, procedural tasks in space without human inputs (e.g., vascular access) [ 278 ]. Today, procedures performed with robotic assistance are not yet fully autonomous (they still require at least one human expert). It is possible with future iterations that an AI integration could be created with the ability to fully replicate the necessary human steps to make terrestrial procedures (e.g., percutaneous coronary intervention, incision and drainage [ 103 ], telecholecystectomy [ 105 , 106 ], etc.) feasible in space [ 275 , 279 ]. The seventh NEEMO mission previously demonstrated that robotic surgery controlled by a remote physician is feasible within the environment of a submarine, but it remains to be seen whether this can be expanded to the space environment [ 280 ].

On space stations, Edge TPU-accelerated AI inference could be used to generate accurate risk prediction models based on data obtained from simulated environments (e.g., NASA AI Risk Prediction Challenge) [ 281 ]. For example, AI could potentially utilize data (e.g., -omics) obtained from research conducted both on Earth and in simulated environments (e.g., NASA GCR Simulator) to predict an astronaut’s risk of developing cancer due to high-LET radiation exposure (cytogenetic damage, mitochondrial dysregulation, epigenetic alterations, etc.) [ 63 , 78 , 79 , 282 , 283 , 284 ].

Another potential area for AI application is through integration with wearable technology to assist in the monitoring and treatment of a variety of medical conditions. For example, within the field of cardiovascular medicine, wearable sensor technology has the capability to detect numerous biosignals including an individual’s cardiac output, blood pressure, and heart rate [ 285 ]. AI-based interpretation of this data can facilitate prompt diagnosis and treatment of congestive heart failure and arrhythmias [ 285 ]. In addition, several wearable devices in various stages of development are being created for the detection and treatment of a wide array of medical conditions (obstructive sleep apnea, deep vein thrombosis, SMS, etc.) [ 285 , 286 , 287 , 288 ].

As discussed previously, the confinement and social isolation associated with prolonged space travel can have a profound impact on an astronaut’s mental health [ 8 , 10 , 67 ]. AI-enhanced facial and voice recognition technology can be implemented to detect the early signs of depression or anxiety better than standardized screening questionnaires (e.g., PHQ-9, GAD-7) [ 68 , 69 ]. Therefore, telepsychology or telepsychiatry can be used pre-emptively for the diagnosis of mental illness [ 68 , 69 , 289 ].

2. Conclusions

Over the next decade, NASA, Russia, Europe, Canada, Japan, China, and a host of commercial space companies will continue to push the boundaries of space travel. Space exploration carries with it a great deal of risk from both known (e.g., ionizing radiation, microgravity) and unknown risk factors. Thus, there is an urgent need for expanded research to determine the true extent of the current limitations of long-term space travel and to develop potential applications and countermeasures for deep space exploration and colonization. Researchers must leverage emerging technology, such as AI, to advance our diagnostic capability and provide high-quality medical care within the space environment.

Acknowledgments

The authors would like to thank Tyson Brunstetter, (NASA Johnson Space Center, Houston, TX) for his suggestions and comments on this article as well as providing the update NASA’s SANS Evidence Report, Ajitkumar P Mulavara, (Neurosciences Laboratory, KBRwyle, Houston, TX), Jonathan Clark, (Neurology & Space Medicine, Center for Space Medicine, Houston, TX), Scott M. Smith, (Nutritional Biochemistry, Biomedical Research and Environmental Sciences Division, Human Health and Performance Directorate, NASA Johnson Space Center, Houston, TX) for his suggestions and providing the update NASA’s Nutrition Report, G. Kim Prisk, (Department of Medicine, Division of Physiology, University of California, San Diego, La Jolla, CA), Lisa C. Simonsen (NASA Langley Research Center, Hampton, VA), Siddharth Rajput, (Royal Australasian College of Surgeons, Australia and Aerospace Medical Association and Space Surgery Association, USA), David S. Martin, MS, (KBR, Houston, TX), ‪David W. Kaczka, (Department of Anesthesia, University of Iowa Carver College of Medicine, Iowa City, Iowa), Benjamin D. Levine (Institute for Exercise and Environmental Medicine, Texas Health Presbyterian Hospital, Dallas, University of Texas Southwestern Medical Center), Afshin Beheshti (NASA Ames Research Center), Christopher Wilson (NASA Goddard Space Flight Center), Michael Lowry (NASA Ames Research Center), Graham Mackintosh (NASA Advanced Supercomputing Division), and staff from NASA Goddard Space Flight Center for their suggestions. In addition, the authors would like to thank the anonymous reviewers for their careful reading of our manuscript, constructive criticism, and insightful comments and suggestions.

Abbreviations

Supplementary materials.

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/cells12010040/s1 , Table S1: title Summary of gut microbial alteration during spaceflight; Table S2: title Summary of immune/cytokine changes during spaceflight; Table S3: title Summary of diet recommendation during spaceflight; Table S4: title Summary of AI technology and potential applications in space.

Funding Statement

This research received no external funding.

Conflicts of Interest

Krittanawong discloses the following relationships-Member of the American College of Cardiology Solution Set Oversight Committee, the American Heart Association Committee of the Council on Genomic and Precision Medicine, the American College of Cardiology/American Heart Association (ACC/AHA) Joint Committee on Clinical Data Standards (Joint Committee), and the American College of Cardiology/American Heart Association (ACC/AHA) Task Force on Performance Measures, The Lancet Digital Health (Advisory Board), European Heart Journal Digital Health (Editorial board), Journal of the American Heart Association (Editorial board), JACC: Asia (Section Editor), and The Journal of Scientific Innovation in Medicine (Associate Editor). Other authors have no disclosure.

Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). MDPI and/or the editor(s) disclaim responsibility for any injury to people or property resulting from any ideas, methods, instructions or products referred to in the content.

Blue Marble Space Institute of Science

Space exploration begins at home

The limits of human exploration: Problems and solutions to cosmic space travel

by Oliver Kimmance

Introduction

The concept of traveling between stars throughout the universe has been envisioned by humanity for thousands of years. Until recently, this has purely been through our imagination, but with recent leaps in technological development in the last 50 years, this notion has turned from fiction into a real and exciting possibility. Not only this, interstellar space travel is becoming increasingly necessary as our pursuit to find and understand extraterrestrial life grows exponentially, and as we uncover more information about the finite resources and environmental problems we face here on Earth. 

In the unlikely absence of any major natural disasters, it has been theorised that Earth can continue to support life for another 1.75 billion years (Parry, 2013). This figure arises from the fact that around that time our Sun will have exhausted a considerable amount of its hydrogen fuel in a process called nuclear fusion, reducing its mass but releasing more and more energy in the form of solar radiation. This will dramatically alter the Goldilocks Zone of our solar system (the region in which liquid water can stand at the surface of a planet based solely on orbital characteristics), with Earth being withdrawn from this zone and becoming increasingly too hot to support life as we know it after approximately 1.75 billion years. You may think this is a huge range of time for humanity to leave Earth and settle in other solar systems, and you would be correct, however it also assumes that Earth is not made uninhabitable by other factors such as a large planetary impact or runaway climate change within that time frame. 

Either way our time on planet Earth is finite, making it inevitable that in the near or distant future we must settle on other planets and even other star systems in the Milky Way. This of course requires transporting large numbers of people across vast interstellar distances (at least assuming we maintain our current physical form), a task unsurprisingly met by numerous problems.

Problems with interstellar travel: Current technologies

Interstellar distances are of course immense, but it can be difficult to fathom quite how immense. Travelling to our closest other star, Proxima Centauri, would be equivalent to travelling to and back from Pluto at its furthest distance from us in its orbit 2667 times. Even light, the fastest travelling entity in the universe, takes 4.24 years to reach us here on Earth from Proxima Centauri.

Unfortunately, current spacecraft technologies simply cannot travel such astronomical distances in feasible time frames, especially not when carrying the weight of hundreds of people to settle in other solar systems. Even using the fastest ever crewed spacecraft, Apollo 10, which travelled at approximately 40,000 km/h , it would take over 115,000 years to complete the journey to Proxima Centauri. To put this into perspective, the Homo sapiens species only developed speech as a cognitive function around 50,000-150,000 years ago, so the time it would take the generations of humans on the spacecraft to reach their destination would be equivalent to the time taken for the entirety of human verbal communicative history to play out. 

In the confinements of a spacecraft with little to no selective pressures (except perhaps the ability to cope in such an environment), and with the ever increasing time taken to send and receive transmissions with humans back on Earth, the rate of adaptation and evolution of people on such a mission would dramatically decrease. Provided humanity back on Earth avoided any major disasters and continued to learn and develop new technologies, hypothetically, by the time such a crew reached Proxima Centauri they may be as underdeveloped compared to humans back on Earth as cavemen are to us right now! This is a disconcerting thought, but it highlights the necessity for new ideas and faster space travel if humanity is to leave Earth and venture into other solar systems.

All rockets used to date have been powered by chemical-based fuel which is heavy to carry and extremely inefficient. Chemical fuels exploit the energy released from the rearrangement of bonds during chemical reactions to generate thrust and accelerate the spacecraft. However these types of reactions only rearrange the electrons in atoms which for hydrogen, the predominantly used fuel source, makes up a mere 0.05% of the atom’s total mass. As explained by the equation for mass-energy equivalence from Einstein’s General Relativity: E = mc 2 , the smaller the mass involved with an interaction, the less energy is released. This can be used to calculate the efficiency of chemical fuel, coming in at a measly 0.0001% (Siegel, 2020). 

Evidently we require the development of more efficient sources of power that can accelerate spacecraft to much faster speeds than is currently possible, in order to reduce the travel time of interstellar missions to a practical length. Fortunately, there are multiple exciting new technologies under development, and many more theoretical solutions that with increased scientific information may become feasible in the near future.

Potential new technologies

One exciting new concept is nuclear fusion fuel. Nuclear fusion has been recognised for some time, it is the process that occurs in the core of the Sun and other stars as the incredibly high temperatures achieved as a result of immense gravitational pressure causes small atoms such as hydrogen to fuse together and form helium and larger atoms, releasing energy in the process. Using nuclear fusion fuel to power spacecraft attempts to replicate this process, but the temperatures required to initiate these reactions are extremely hard to reach without the immense pressures achieved in the centres of stars due to their gravity. Consequently, new methods are required to stably heat hydrogen to the temperatures required for nuclear fusion reactions to take place. 

One such method is the ‘ITER magnetic confinement reactor’ currently under development in France. A circle of enormous electromagnetic superconductors generate enough pressure to squeeze a central ring of hydrogen gas into a plasma, allowing their nuclei to fuse into helium and generate energy. However, to prevent the superconductors vaporising themselves, a network of tubes containing liquid helium cooled to -270°C  must surround the superconductors to stabilise the system. This requires huge amounts of energy to maintain and currently the energy input to carry out the reaction is more than is obtained from nuclear fusion. 

Nevertheless, continued improvements to this technology could approve nuclear fusion as a more efficient and powerful means of powering the next generation of spacecraft. Compared to chemical-based fuel which utilises only 0.05% of the reagents total mass, nuclear fusion reactions rearrange the proton and neutron nuclei of atoms, entities with significantly more mass than elections and subsequently producing around 10,000 times more energy per unit mass of fuel. Spacecraft equipped with nuclear-based fuel could therefore be accelerated for much longer periods of time and achieve much faster speeds than current spacecraft. Furthermore, the only reagents necessary for hydrogen nuclear fusion are hydrogen isotopes, the most abundant elements in the universe. Theoretically, future spacecraft using this technology could gather cosmic hydrogen as they travel, feed the hydrogen into the reactor, and continue generating thrust. This would also reduce the amount of initial hydrogen required to power the spacecraft, greatly reducing its mass and therefore allowing it to be accelerated to greater speeds.

Another proposed method for dramatically increasing the speed of spacecraft is laser-powered propulsion, well known for being used in the ‘Breakthrough Starshot’ program. This revolutionary technology removes the need for on board fuel since the energy is provided from a ground based source. This dramatically reduces the weight of the spacecraft allowing it to be accelerated to much faster speeds than previously possible. A ground-based array of high power lasers all fire a beam into space that converges on a huge reflective light-sail attached to the spacecraft in orbit to accelerate it. To be effective, the mass of the entire spacecraft must be in the order of grams, whilst the combined energy output of the laser array must be in the order of hundreds of Megawatts. 

Nevertheless, the Breakthrough Starshot program predicts that their ~1 gram probes can be accelerated to around 20% the speed of light, an incredible achievement that would reduce the travel time to Proxima Centauri down to only 22 years. This is more than a 5000-fold decrease in travel time compared to the fastest crewed spacecraft to date, and would allow the mission to be completed and data to be returned to Earth well within a human’s lifetime. The success of a mission like Breakthrough Starshot would be invaluable for gaining information on other solar systems and potentially habitable exoplanets, and would be an immense achievement, however a 1 gram spacecraft is clearly insufficient for transporting people across interstellar space in the eventuality of settling on worlds around other stars. 

The obvious problem of scaling up the technology to power a spacecraft capable of accommodating passengers on its journey is a massive engineering dilemma, and perhaps is not possible. This is coupled with the fact that there is also no current solution as to how human passengers would survive the force experienced when traveling at such speeds. One problem will be the construction of a light-sail large enough to reflect enough energy to power a spacecraft many orders of magnitude heavier than those currently under development, but the most difficult problem will be providing the amount of energy required in the first place. Let’s say we wish to accelerate a spacecraft to 20% the speed of light with the mass of Apollo 10 (28,834 kg), bearing in mind this had only 3 crew members. The energy required to accelerate the Breakthrough Starshot probe to this speed is around 100 MW so the energy required to accelerate this manned spacecraft to the same speed would be 2.9 million GW, almost double the entire energy consumption of India in a one hour period . Evidently this amount of energy usage is far from possible, however there are theories for future energy harnessing methods that may bring this into the realms of possibility. Such proposals include construction of a Dyson sphere or harvesting cosmic hydrogen for fuel cells, but are beyond the scope of this article. 

Further problems with laser-beam propulsion include collision of the spacecraft with interstellar dust particles, which would completely destroy the lightweight light-sail at such high speeds. Breakthrough Starshot will attempt to evade this by propelling thousands of probes in the hope that at least a few reach Proxima Centauri undamaged. 

Limitations of intergalactic space travel

The prolonged future survival of humanity noticeably depends on the settlement of other planets and eventually other star systems in our galaxy. Accomplishing this would be a tremendous achievement and would allow humanity to expand and develop exponentially as the raw materials from more planets and the energy from more stars could be harnessed. However, unfortunately there seems to be a physical limit to the extent of human expansion that is built into the physics of space and time.

At the instant of the Big Bang 13.8 billion years ago the universe was a tiny pocket of energy, with high density and low density regions due to quantum fluctuations. Moments after the big bang, cosmic inflation stretched these sub-atomic density differences into enormous differences, millions of light years across in distance. After this event, gravity began to combine the more dense regions back together, forming clusters of galaxies, whilst the less dense regions had insufficient mass for gravity to prevail and so continued expanding. The result was the formation of millions of clusters of galaxies, all separated by expanding space. This also means that we are only gravitationally bound to our ‘local group’, a cluster of galaxies including our Milky Way, the Andromeda Galaxy, and around 50 dwarf galaxies, whilst all other clusters of galaxies are moving away from us as a consequence of the expansion of the Universe. Well, technically these superclusters are not moving away from us themselves, rather the space in between us and them is expanding and so the relative distance is increasing. Regrettably, this means that even if we could travel near the speed of light, the local group is likely to be the only area of the Universe humanity will be able to explore. 

Moreover, as a result of dark energy, a phenomenon not officially discovered but whose effects can be observed, the expansion of the universe is accelerating. The further and further away we look into space, the faster and faster those galaxies and superclusters are moving away from us. This provides us with a ‘reachable zone’, much like a black hole event horizon where anything that passes beyond this horizon is traveling faster relative to us than the speed of light and is therefore unreachable and impossible to interact with. Earth’s ‘reachable zone’ is approximately 18 million light years in radius and makes up only 6% of the observable universe, comprising our local group and many other clusters that are accelerating away from us but at a slower rate relative to us than the speed of light. This can be a discouraging statistic, but the reachable zone still contains billions and billions of stars and probably even more planets to be explored. 

Theoretical solutions to intergalactic space travel

There are even hypothetical means permitted by our current laws of physics that may make it possible to travel faster than light, making the unreachable universe theoretically reachable. Two dominating and exciting theories are wormholes and warp drives. 

Wormholes are hypothetical phenomena that form when two extremely dense entities such as two black holes distort and bend the fabric of spacetime so much that a tunnel between the two is formed. This may be between two points in space or even between two points in parallel Universes. There is currently no evidence of wormholes existing, but Einstein’s theory of general relativity allows for their existence. Unfortunately even if wormholes were discovered, it is also theorised that any matter passing through the wormhole, even a single particle, would trigger catastrophe and cause the wormhole to break down. One theoretical solution to this is to feed antimatter into the wormhole along with whatever matter is attempting to pass through, which would potentially stabilise the effects of regular matter passing through.

The concept of warp drives similarly plays on our understanding of space-time given by general relativity. This is a theoretical technology that would be attached to future spacecraft and distort space-time in front of the craft, massively reducing the distance between the spacecraft and its destination. By cleverly and carefully exploiting large quantities of matter and antimatter, space-time could be shortened in front of the spacecraft and lengthened behind it, creating a stable bubble of space surrounding the spacecraft and allowing it to travel to a particular destination far quicker than the speed of light.

Evidently, these two potential means of traveling faster than light are far from plausible at the present date, but the fact that they are both possible under our current laws of physics is extremely exciting. Perhaps with future discoveries and information gained on dark energy, dark matter, antimatter, and black holes, these models will become real-world technologies and allow humanity to freely explore the universe!

Oliver Kimmance is a Research Associate with the Blue Marble Space Institute of Science and is an undergraduate student in Biochemistry at the University of Bristol, UK.

STEM Fellowship Journal

ReTHINKING Space Propulsion for Interstellar Travel

By Kisothan Suthakaran

In 2016, astronomers discovered an Earth-like planet orbiting Proxima Centauri, our nearest-known star, situated such that its temperature allows liquid water to possibly exist on its surface. Our current knowledge about this exoplanet is however limited by the data provided by telescopes and spectrographs. The only effective way of gaining more insights about this exoplanet is to get there, either physically or remotely. The underlying issue is that our closest star system is 4.25 light-years away from the sun. To reach this destination promptly without running out of fuel, space propulsion must be rethought and revitalized because current technology is inadequate for long-distance spaceflight. This article will discuss the three most promising propulsion schemes for efficient short-term interstellar flight currently in development: ion thrusters, fusion-driven rockets, and laser-pushed light sail.

Tyranny of the Rocket Equation

        Conventional rocket engines, which use the exhaust gas from the combustion reaction to propel the spacecraft as depicted in Figure 1, generate high thrust but consume a large amount of fuel in a short period.

Figure 1: Chemical Rocket Engine Schematic (Source: EFDA)

This type of rocket is best suited to lift off the ground and reach low Earth orbit. For long-distance spaceflight, the Tsiolkovskjy rocket equation (1), which governs the motion of all rockets, becomes a major concern (1).

v = ve In m0mf                                                     (1)

This equation relates the velocity gain of vehicle, v, to the exhaust velocity, ve,  of the reaction mass and the ratio of the initial mass of the rocket, m0, to its final mass, mf . The plot of this equation in Figure 2 shows that as the desired momentum change v increases, the required amount of fuel increases exponentially.

Figure 2: Plot of the Rocket Equation

On the other hand, trading velocity gain for lower fuel consumption results in longer flight durations. This dilemma between fuel consumption and flight time puts interstellar travel in a problematic situation. The propulsion solutions presented below seek to overcome this obstacle by either pushing the practical limits of the rocket equation or simply circumventing it. 

Potential Propulsion Technologies

Ion Thruster 

        An ion thruster is a form of electric propulsion that relies on injecting charged particles into an electric field to accelerate them. The resulting force propels the spacecraft using Newton’s third law. This is still a rocket concept, but instead of ejecting high-temperature combustion products, ions are discharged. This has a significant impact on the fuel consumption of this propulsion system, making it more efficient than combustion engines (2). The anatomy of the ion engine is illustrated in Figure 3.

Figure 3: Electrostatic Ion Thruster Diagram (Source: NASA)

Fusion-Driven Rocket

        The fusion-driven rocket scheme attempts to exploit the tremendous amount of nuclear energy released by atomic synthesis to either directly expel hot plasma or heat and accelerate a propellant. The physics of fusion is governed by Einstein’s mass-energy equivalence equation (2). 

E = mc²                                                            (2)

When two atoms collide and fuse, the resulting reaction produces a new atom and the mass difference m between the reactants and the products are converted to energy, E , as shown in Figure 4.  This equation states that the conversion factor between mass and energy is the square of the speed of light c, which is about 300 000 km/s. The large value for c is the reason behind the enormous amount of energy that is released from these collisions (3).

Figure 4: Schematic of the Fusion Reaction (Source: EFDA)

Laser-Pushed Lightsail 

        Unlike electric and nuclear propulsion, the laser-pushed light sail is a propellantless scheme that relies on the principle of direct momentum transfer. The energy source is a stationary laser that sends a large light beam from Earth to a thin sheet of material moving in space called a light sail, which carries the probe (4).  Although the momentum equation (3) from classical physics suggests that massless objects like photons can’t carry momentum, the laws of quantum mechanics and special relativity allow any particle that carries energy to have momentum, regardless of whether they have mass or not, as demonstrated in  the general form of the relativistic equation (4). In the quantum world, all wave-like particles have energy since they have a non-zero frequency as shown by Plank’s equation (5). 

p = mv                                                                      (3)

p = E2 – (mc2)2(c2)                                                         (4)

E = hf                                                                      (5)

Hence, as illustrated in Figure 5, the momentum carried by the photons can be transferred to the sail throughout the interstellar journey. This simple solution allows for high-velocity missions without the limitation of the rocket equation.

Figure 5: Schematic of the Laser Propulsion Concept [5]

Efficiency and Technical Feasibility

        An ion thruster is a low-thrust engine that builds up momentum with time. This means that this rocket starts with a low propulsive force and reaches the optimal thrust for interstellar travel only later on in the journey.  On the other hand, ion thrusters expel ions at a faster rate than at which chemical rockets expel reaction mass. As shown in equation (6), this high exhaust velocity, Ve, provides ion rockets with greater specific impulse ISP than combustion engines, which implies that they burn their fuel much more efficiently than their traditional counterparts.

Isp= Ve (g)                                              (6)                

This combination of low thrust and high specific impulse allows these rockets to sustain long-distance spaceflight, but the associated long-duration flight times are inconvenient for short-term interstellar missions. That being said, the ion propulsion scheme is the most feasible solution among the three candidates as it has already been implemented and heavily tested. Figure 6 shows a prototype ion thruster built by NASA in 2005.

Figure 6: Ion Engine Testing at NASA (Source: NASA)

The testing results confirm the low propulsive force of the engine. The current research on ionic propulsion is more geared towards using ion thrusters for low-thrust orbital maneuvers than for high-thrust deep space missions because there is a theoretical limitation on the amount of propulsive force that can be achieved by these engines. Child-Langmuir’s law, illustrated by equation (7), stipulates that there is a fundamental limit to the amount of charge that can be contained in space.

J = K  Vd (d2)                                              (7)        

This equation provides the maximum current density J between two infinite parallel electrodes separated by a distance d with a potential difference of Vd. K, is a physical constant. Since the current density is closely related to the intensity of the electric field, there is a limit to how rapidly ions can be ejected out of the rocket. Thus, although ion engines are feasible, they are limited in thrust.  

        The fusion-driven rocket is the most powerful jet propulsion concept. The relativistic energy emerging from the fusion reaction provides these rockets with a great amount of thrust. Research has shown that a fusion core that uses hot plasma exhaust to produce thrust could enable rockets to reach ‘‘several hundred times higher exhaust velocities than today’s high power chemical rocket engines.’’ [6] The most attractive feature of fusion rocket engines is that their high thrust capacity is coupled with high specific impulse as opposed to ion thrusters and chemical rockets, which exhibit a tradeoff between thrust and fuel efficiency. For this reason, fusion rockets are often considered the ideal technology for interstellar propulsion. However, implementing this solution is limited by some intricate engineering challenges that are far from being solved. The biggest one of them is the problem of controlling fusion inside a rocket. Scientists and engineers have made some significant progress in controlling fusion on Earth through various schemes. Figure 7 shows one of them, where a magnetic field generated by the flow of current in the coils forces hot plasma to swirl within the torus shape vessel (7).

Figure 7: Small Scale (Top) and Large Scale (Bottom) View of the Magnetic Confinement of Plasma in the Tokamak Fusion Reactor (Source: Fusion for Energy and JET/UKAEA)

However, current technology could only barely confine this highly conductive ionized substance and is unable to harness the energy from the reaction for any practical use. Even if such a feat is accomplished, the concept of installing a fusion reactor inside a rocket raises another concern. The compactness of the reactor is critical to the rocket’s thrust-to-weight ratio, which determines its overall performance. Considering that the most compact fusion reactor built on Earth weighs about 200 metric tons, minimizing the weight and the size of nuclear engines remains a challenging task.  In short, fusion has a long way to go before it can be used to power rockets. 

        The laser-pushed light sail is a high thrust scheme that relies on the cumulative force exerted by the photons bouncing off the sail to propel the spacecraft. Due to the absence of propellant, the notions of specific impulse and fuel consumption are meaningless in this context. The overall efficiency of this scheme can be assessed by examining the net acceleration of the sail and the corresponding flight duration. A detailed study conducted at Hughes Research Laboratory in California reveals that a light sail accelerated at 0.36 m/s2 by a 65 GW laser setup would reach the exoplanet near Centauri within 40 years (8). This amounts to 360 N of thrust.  For comparison, the highest level of thrust achieved by an ion engine is 5.4 N.  Hence, a light sail surpasses an ion thruster in thrust and speed. Moreover, since the laser propulsion scheme is based on known physics and technology, the engineering challenges involved in the execution of this concept are less complex than the technological limitations of fusion-powered rockets. For instance, one of the major concerns of photonic propulsion is to maximize the reflectivity of the light sail and minimize its absorbance. As depicted in Figure 8, the underlying reason is that a reflected photon transfers two impulses to the sail whereas an absorbed photon gets only one chance to give away its momentum (9).

Figure 8: Momentum Transfer During (a) Absorption (b) Reflection [8]

As a result, these factors, together with the temperature limitations of the light sail, play a critical role in the thickness of the sail and the material selection. Aluminum shows promise due to its desirable melting temperature, tensile strength, and reflectivity. Using a transparent dielectric material instead of a metal film is also a prominent solution since it allows for high reflectance over a wide range of wavelengths. Another important engineering concern of this propulsion scheme is the inverse square law of the intensity of light shown by the mathematical relation (7).

I  = d2                                                                 (7)

The light intensity, I, decreases with the square of the distance, d, which means that it gets harder to focus the beam light energy onto the sail as it moves away from the Earth. Some potential ideas to overcome this challenge is to decrease the wavelength of the laser to reduce the beam spread or use larger sails to increase the surface area. The narrow focus of these problems and the wide range of solutions indicate that these issues are within the scope of current technology and knowledge and hence are likely to be solved shortly. 

        The laser-pushed light sail scheme turns out to be the most viable solution for short-term interstellar flight. While the ion thruster provides great fuel efficiency and is the most feasible solution, it cannot reach the desired destination in a reasonable time interval due to its low-thrust performance. On the other hand, despite their great efficiency, fusion-driven rockets are an unrealistic concept, considering the current technological limitations of fusion control and confinement. The laser propulsion solution strikes the right balance between efficiency and feasibility.

[1] A Novel Approach for Optimization of Nozzle Angle and Thrust Vectoring Controller via a Sub-Mutation Genetic Algorithm – Scientific Figure on ResearchGate. Available from: https://www.researchgate.net/figure/The-parts-of-liquid-rocket-engine_fig1_320628712 [accessed 12 Jun, 2022]

[2] L. C. David, J. T. Varghese and S. R. Nadarajan Assari Syamala, “Ion Propulsion Technology: NASA’s Evolutionary Xenon Thruster (NEXT) Development and Long Duration Tests Results and its Applications,” 2020 Advances in Science and Engineering Technology International Conferences (ASET), 2020, pp. 1-5, doi: 10.1109/ASET48392.2020.9118370.

[3] W. Lockett. “The Fusion Rocket That Could Dominate Interplanetary Travel” Medium. https://medium.com/predict/the-fusion-rocket-that-could-dominate-interplanetary-travel-d4a8234c67cd (accessed May. 27, 2022)

[4]  G. Landis, “Advanced Solar and Laser-Pushed LightSail Concepts,” NASA Institute for Advanced Concepts., Los Angeles, CA, USA,Tech. Rep. TR-0200 (4230-46)-3, Nov. 1999.

[5] Landis, G.A. (1999). Advanced Solar-and Laser-pushed Lightsail Concepts Final Report.

[6] Wurden, G.A., Weber, T.E., Turchi, P.J. et al. A New Vision for Fusion Energy Research: Fusion Rocket Engines for Planetary Defense. J Fusion Energy 35, 123–133 (2016). https://doi.org/10.1007/s10894-015-0034-1

[7] T. Kammash and D. L. Galbraith, “A novel fusion approach to space power and propulsion,” Proceedings of the 24th Intersociety Energy Conversion Engineering Conference, 1989, pp. 2531-2534 vol.5, doi: 10.1109/IECEC.1989.74830.

[8] Forward, R.L. (1984) Roundtrip interstellar travel using laser-pushed lightsails. Journal of Spacecraft, 21, 187- 195. doi:10.2514/3.8632

[9] T. Pultarova, “Ion thruster prototype breaks records in tests, could send humans to Mars,” Space.com, 13-Oct-2017. [Online]. Available :https://www.space.com/38444-mars-thruster-de sign-breaks-records.html#:~:text=%22It%20operated%20at%20a%20huge,at%20the%20University%20of%20Michigan. [Accessed: 13-Jun-2022].

About The Author:

Kisothan Suthakaran is a 3rd-year mechanical engineering student at McGill University with a keen passion for aerospace engineering & modern physics. He is particularly fascinated by rockets, aircrafts & black holes. He likes to work on innovative solutions for challenges in space exploration and ponder deep questions about our Universe.

Contact Kisothan:

Email: kisothan .suthakaran@mail. mcgill.ca

Related Posts

Cybersecurity within the Internet of Things Devices

Unlocking the Creative Mind: Art Therapy for Enhanced Cognition in Seniors

Artificial Intelligence in Finance: Benefits and pitfalls of AI-trading

5 Stem Podcasts to Subscribe to

To create equitable learning opportunities, connections and experiences for students to develop modern skills for the digital age.

Don’t miss out.

© 2022Powered by CreatZi Agency

Privacy Policy

Charity Number: 807174784RR0001