Interplanetary travel refers to spaceflight between planets, and as of the mid-2020s, no human being has done it. Every crewed mission in history has stayed within Earth’s neighborhood, either in low orbit or, during the Apollo program, on brief trips to the Moon. Getting people to Mars and back, the most discussed interplanetary mission, involves solving a tangle of problems that range from propulsion physics and radiation biology to crew psychology and how to grow lettuce in toxic soil. Each challenge has its own body of research, and the solutions interact in ways that make the whole endeavor harder than any single piece suggests.
Getting There and Back
The core engineering problem of interplanetary flight is propulsion. Chemical rockets deliver enormous thrust but burn through fuel fast, which means a spacecraft needs to carry a lot of mass just to push itself. Electrical engines flip that trade-off: they use propellant far more efficiently, meaning less fuel mass, but their thrust is low, which stretches flight times. Research into hybrid approaches tries to find the sweet spot, using chemical thrust during the intense phases near planets and switching to electrical propulsion for the long cruise through open space.
A Mars mission using purely chemical propulsion takes roughly six to nine months one way, depending on the alignment of the two planets. That alignment, called a transfer window, opens only about every 26 months. Miss it and your crew waits on Mars until the geometry works again, turning a round trip into a roughly two-and-a-half-year commitment. Researchers have explored optimal trajectories that combine chemical and electrical propulsion to shave transit time and reduce the mass of propellant needed, since every kilogram saved on fuel is a kilogram available for food, equipment, or shielding.1Acta Astronautica. Computation of optimal Mars trajectories via combined chemical/electrical propulsion, part 1: baseline solutions for deep interplanetary space
Radiation in Open Space
Inside Earth’s magnetic field, astronauts on the International Space Station get a moderate dose of radiation, manageable with mission-length limits. Step outside that magnetic cocoon and the picture changes dramatically. Two sources of radiation dominate in deep space: galactic cosmic rays, which are high-energy particles streaming in from outside the solar system, and solar energetic particle events, which are bursts of radiation hurled out by the Sun during flares and coronal mass ejections. Galactic cosmic rays are a steady background hazard, while solar events are unpredictable spikes that can deliver a dangerous dose within hours.
The primary long-term concern is cancer. Radiation exposure from galactic cosmic rays and solar events damages DNA, and the types of heavy ions found in deep space are more biologically destructive than the radiation people encounter on Earth.2PubMed. Getting ready for the manned mission to Mars: the astronauts’ risk from space radiation In the short term, a large solar particle event could cause acute radiation sickness during a Mars transit if astronauts are caught without adequate shelter. Mission planners treat solar events as emergencies requiring immediate action, such as retreating to a heavily shielded section of the spacecraft.3Space Weather. Characterization of Solar Energetic Particle Radiation Dose to Astronaut Crew on Deep‐Space Exploration Missions
Shielding options are limited by weight. You could wrap a spacecraft in lead, but the mass penalty would be enormous. Lighter materials like polyethylene and Kevlar have shown promise. Testing aboard the ISS found that Kevlar performed comparably to polyethylene, reducing the dose-equivalent rate by roughly half for a shield of a given thickness.4Scientific Reports. Performances of Kevlar and Polyethylene as radiation shielding on-board the International Space Station in high latitude radiation environment That helps, but it does not eliminate the problem. Galactic cosmic rays include particles so energetic that no practical amount of passive shielding can fully block them. Active shielding concepts, like generating a magnetic field around the spacecraft, remain theoretical.
What Weightlessness Does to the Body
Radiation is not the only biological threat. Months of weightlessness cause the body to shed bone, especially in the legs, hips, and spine. Without the constant tug of gravity, weight-bearing bones lose mineral density because the body ramps up bone breakdown while formation slows or stalls. Bone resorption spikes in the first two weeks of spaceflight, and markers of bone loss show up in astronauts’ urine almost immediately. Non-weight-bearing bones, like those in the forearm, are largely spared, which tells researchers that the loss is driven specifically by the absence of mechanical loading.5npj Microgravity. The effects of microgravity on bone structure and function
ISS astronauts counteract bone loss with hours of daily exercise, including resistance training that simulates gravitational load. For a six-month stay in orbit, this regimen limits the damage to manageable levels for most crew members. An interplanetary round trip to Mars could mean a year or more in microgravity, with a stay on the Martian surface where gravity is only about 38% of Earth’s. Whether that partial gravity is enough to halt or reverse bone loss is unknown. The recovery period after return to Earth can stretch beyond a year, and some astronauts never fully regain lost bone density.
Other physiological effects compound the picture. Fluid shifts toward the head in microgravity, contributing to vision changes that have been documented in a significant fraction of long-duration ISS crew. Muscles atrophy, cardiovascular fitness drops, and the immune system behaves differently. Each of these problems has its own countermeasures, but none are fully solved for mission durations measured in years.
Crew Psychology and Medical Emergencies
An interplanetary crew faces psychological conditions unlike anything on Earth or even on the ISS. Communication delays to Mars range from about 4 minutes to over 24 minutes one way, depending on orbital positions. That rules out real-time conversation with mission control and forces the crew to handle problems autonomously. Add to that extreme isolation, confinement in a small habitat, and the knowledge that rescue is impossible, and the psychological stakes become clear.
Agent-based modeling of extended Mars missions suggests that team composition matters as much as individual resilience. Personality variation and role specialization within the crew shape stress levels, group cohesion, and overall performance in ways that interact over time.6PubMed Central. Exploring team dynamics and performance in extended space missions using agent-based modeling Getting crew selection wrong could cascade into interpersonal conflict, degraded decision-making, or withdrawal from group tasks during a mission where every person’s contribution is critical.
Medical emergencies add another layer of autonomy pressure. An analysis of potential medical events on deep-space missions identified sudden cardiac arrest, smoke inhalation, toxic exposure, seizures, and penetrating eye injuries as the events with the highest mission impact and the greatest potential for survival if handled well. The study emphasized that non-technical skills like communication, teamwork, and leadership under stress were essential for managing these situations, and recommended prioritizing those skills in pre-mission training.7PubMed. Medical Event Management for Future Deep Space Exploration Missions to Mars On the ISS, a crew member having a heart attack could theoretically be evacuated to Earth within hours. On a Mars transit, the crew is the entire emergency department.
Navigating and Talking Across Millions of Kilometers
Spacecraft currently navigate using ground-based tracking stations that measure signal time delays and Doppler shifts. This works well enough for robotic probes, but for crewed missions where communication delays make ground-based updates slow to arrive, autonomous onboard navigation becomes valuable. One promising approach uses X-ray pulsars, rapidly rotating neutron stars that emit X-ray beams with clock-like regularity. By timing these pulses, a spacecraft can calculate its own position. NASA’s SEXTANT experiment demonstrated real-time onboard navigation using pulsar observations, targeting accuracy of about 10 kilometers.8Chinese Journal of Aeronautics. Review of X-ray pulsar spacecraft autonomous navigation That is not centimeter-precision, but it is good enough for cruise-phase positioning and could supplement traditional tracking during communication blackouts.
Data transmission is another bottleneck. Current deep-space communication relies on radio frequencies, which work but offer limited bandwidth at interplanetary distances. Laser-based optical communication has the potential to deliver much higher data rates. Reviews of deep-space optical communication development indicate that optical links could extend functional ranges beyond 100 astronomical units while supporting high signal-to-noise data transfer, a leap that would transform how much science data and video an interplanetary mission could send home.9International Journal of Satellite Communications and Networking. Review of Deep Space Optical Communications NASA’s Deep Space Optical Communications experiment, flown on the Psyche mission launched in 2023, tested this concept at planetary distances for the first time.
Powering a Mission Far From the Sun
Solar panels work well in Earth orbit and even at Mars, where sunlight is roughly 43% as intense as at Earth. But solar power has limits as you move farther from the Sun, and even at Mars it poses challenges during dust storms that can blanket the planet for weeks. Robotic Mars missions have died from dust-covered solar panels.
Radioisotope thermoelectric generators, the plutonium-powered devices that run rovers like Curiosity and Perseverance, provide reliable but small amounts of electricity. They are excellent for robots but impractical for powering a crewed base. Their low specific power and the shielding mass they require make them a poor fit for large-scale energy needs.10npj Space Exploration. Lunar surface energy infrastructure: from in-situ resource utilization to deep space exploration Nuclear fission reactors offer far more power per kilogram and can run continuously regardless of sunlight or dust. Analysis comparing solar-plus-storage systems to fission systems shows that as continuous power demand grows, fission becomes decisively lighter, requiring on the order of a hundred kilograms per kilowatt versus roughly a thousand for solar-with-battery systems.11npj Space Exploration. Lunar surface energy infrastructure: from in-situ resource utilization to deep space exploration NASA’s Kilopower project has tested small fission reactors designed for surface operations, though flight-ready versions remain in development.
Making Fuel and Materials on Mars
Carrying all the propellant for a return trip from Mars would require an impractically large spacecraft. The alternative is manufacturing fuel on Mars itself, a concept called in-situ resource utilization. The Martian atmosphere is about 96% carbon dioxide, which is the feedstock for a process known as the Sabatier reaction. In a proposed system, water mined from Martian soil is electrolyzed to produce hydrogen and oxygen. The hydrogen reacts with captured atmospheric carbon dioxide to yield methane and water vapor, with the water recycled back into the process. The methane becomes rocket fuel and the oxygen serves as the oxidizer.12iScience. Thermodynamic modeling of in-situ rocket propellant fabrication on Mars NASA tested a small-scale version of the oxygen-production step with the MOXIE experiment aboard the Perseverance rover, which successfully extracted oxygen from Martian carbon dioxide.
Beyond fuel, surface habitats could potentially be built from local materials rather than shipped from Earth. Research into 3D printing with regolith, the loose rocky material covering planetary surfaces, has explored how to deposit layers of regolith mortar that cure in place. The engineering challenge is that as layers stack up, the weight of the structure can exceed the yield stress of the uncured material below, causing collapse. Controlling the curing rate of the mortar is critical for large-scale printing.13Acta Astronautica. A virtual 3D printing framework for off-Earth construction Some designs propose coaxial printing, depositing a structural regolith shell and a core of phase-change material simultaneously. The phase-change core absorbs and releases heat as temperatures swing, helping regulate interior temperatures passively without heavy mechanical systems.14Microgravity Science and Technology. Numerical Analysis of Coaxially 3D Printed Lunar Habitats: Integrating Regolith and PCM for Passive Temperature Control
Growing Food on Martian Soil
A Mars crew cannot rely on resupply from Earth for every meal, so growing food locally is part of most long-term mission architectures. The obvious question is whether Martian soil can support crops. Studies using Mars regolith simulants, Earth-made soils designed to mimic Martian dirt, paint a complicated picture. None of the tested simulants supported plant growth on their own. With added nutrients, some could grow plants like lettuce and the model plant Arabidopsis. But one simulant with a highly alkaline chemistry above pH 9 resisted even nutrient supplementation; acidifying it roughly doubled plant longevity, suggesting the soil could eventually be coaxed into usefulness with enough modification.15Icarus. Challenging the agricultural viability of martian regolith simulants
The most sobering finding involved perchlorate. Mars is known to have perchlorate salts in its soil, and when calcium perchlorate was added to the simulants at concentrations matching those observed on the real Martian surface, every simulant became incapable of supporting plant growth regardless of nutrient supplements.16Icarus. Challenging the agricultural viability of martian regolith simulants Perchlorate is toxic to humans too, so any agricultural scheme on Mars would first need a way to wash or break down perchlorate in the soil. Bioengineering perchlorate-reducing bacteria or developing chemical remediation processes are active areas of research, but neither has been tested under actual Martian conditions.
Keeping Other Worlds Clean
Interplanetary missions face a biological constraint that has nothing to do with crew health: planetary protection. If Earth microbes hitch a ride to Mars on a spacecraft and contaminate the surface, they could compromise the search for indigenous Martian life, one of the primary scientific motivations for going there in the first place. NASA maintains strict requirements for reducing the bioburden on spacecraft headed to destinations that might harbor life.
Meeting those requirements involves creative engineering. Research into rocket motor insulation adhesives found that certain adhesive compounds have sporicidal properties, killing bacterial spores on contact. Tests showed reductions in viable spores of common contaminant species by 75 to 89 percent when applied to insulation substrates, with the kill attributed to toxic compounds in the adhesives, the physical binding capacity of the insulation, and mechanical damage during processing.17PubMed. Solid rocket motor insulation adhesives with sporicidal activity promote planetary protection for deep space missions The finding is particularly relevant for missions targeting the icy moons of Jupiter and Saturn, like Europa and Enceladus, where subsurface oceans might host conditions for life. A single contaminating Earth bacterium on those worlds could undermine billions of dollars of science.
Protecting the Spacecraft Itself
Micrometeoroids and orbital debris are a hazard in Earth orbit, but interplanetary space has its own population of tiny high-speed particles. A grain of dust traveling at several kilometers per second packs enough energy to puncture a spacecraft wall. Detecting and characterizing impacts in real time matters for long-duration missions where gradual structural damage could go unnoticed until it becomes critical. Researchers have developed flexible impact sensors that can be integrated into spacecraft shielding. These screen-printed sensors, tested with hypervelocity impacts at roughly 7 km/s, successfully determined impact size, location, and energy, offering a way to monitor hull health throughout an extended mission.18Aerospace Science and Technology. Design, fabrication, and hypervelocity impact testing of screen-printed flexible micrometeoroid and orbital debris impact sensors for long-duration spacecraft health monitoring
Supply Chains, Law, and the Economics of Going Interplanetary
Terrestrial supply chains assume that if something breaks, you can order a replacement. Interplanetary logistics cannot make that assumption. A conceptual framework for interplanetary supply chains identifies four design drivers that distinguish them from Earth-based systems: redundancy, because replacement parts cannot arrive quickly; closed-loop sustainability, because waste must be recycled into usable resources; autonomous and adaptive operations, because communication delays prevent real-time management from Earth; and hub-based network architecture, because staging depots in orbit or at intermediate points reduce the cost of individual missions.19Transportation Journal. Interplanetary Logistics: Transportation Challenges and Supply Chain Lessons From Space Exploration Every one of these drivers pushes mission design toward self-sufficiency, which is fundamentally different from how we build systems on Earth.
The legal framework for interplanetary activity is still catching up. The 1967 Outer Space Treaty established that no nation can claim sovereignty over a celestial body, but it said little about resource extraction. As asteroid mining and surface resource utilization become technically plausible, the question of who owns what you dig up has become urgent. Some nations have passed domestic laws granting property rights over extracted space resources, while others argue for benefit-sharing frameworks rooted in the common heritage principle, the idea that space resources belong to all of humanity and their exploitation should be equitably shared.20Global Policy. Sharing the Benefits of Asteroid Mining How this tension resolves will shape whether interplanetary resource use follows a model closer to deep-sea mining treaties or to the free-for-all of historical terrestrial colonization.
The economics of interplanetary missions remain daunting. Launch costs have dropped dramatically with reusable rockets, but the total mass that must reach Mars orbit for a crewed mission still runs into hundreds of metric tons by most estimates. In-situ resource utilization cuts the return-fuel portion of that mass, and local manufacturing of habitats and consumables could eventually reduce it further. But for the first missions, nearly everything goes on the ship. The interplay between propulsion efficiency, mission duration, crew size, and payload mass creates a design space where improving one variable often worsens another, and mission planners spend careers searching for the combination that makes the whole thing barely feasible.