Mars

Mars has an atmosphere that is too thin to be useful and too thick to ignore. Almost every difficulty of Mars exploration follows from that one fact.

Getting there

Transfers depart during a window that recurs at the synodic period of 780 days, about every 26 months, with a cruise of six to nine months. Departure energy varies between windows because Mars’s orbit is eccentric, so some opportunities carry appreciably more payload than others.

A missed window costs 26 months, longer than many programmes can absorb, which makes Mars campaigns unusually schedule-driven. See Trajectory optimization.

The atmosphere problem

Surface pressure is about 610 Pa, 0.6% of Earth’s. The composition is 95% carbon dioxide, and gravity is 3.72 m/s².

That figure sits in the worst possible place for landing.

On the Moon, with no atmosphere, arrival velocity is removed entirely by propulsion. It is expensive but straightforward, and the answer scales: a bigger lander carries more propellant.

On Earth, the atmosphere removes essentially all of it. A capsule enters at 7.8 km/s and is subsonic before parachutes deploy.

On Mars, the atmosphere removes most but not all. An aeroshell decelerates a vehicle from 6 km/s to a few hundred metres per second, but the air runs out before the vehicle stops, so a propulsive terminal phase is mandatory. The vehicle therefore needs a heat shield and parachutes and descent engines, all of which are mass that does not do science.

Worse, parachutes on Mars must deploy supersonically, around Mach 2, into extremely thin air. This is a regime with little terrestrial experience and it has been a persistent source of programme risk; supersonic parachute qualification has required dedicated high-altitude test campaigns.

The whole sequence takes about seven minutes and is executed autonomously, because light time makes intervention impossible. The ground learns the outcome after it has already happened.

The mass limit

Every Mars landing so far has delivered on the order of one tonne to the surface. Perseverance, the heaviest, was about 1025 kg.

Human missions need 20 to 40 tonnes per landing. That is more than an order of magnitude beyond demonstrated capability, and the techniques do not scale smoothly.

Parachutes do not scale

Drag scales with area, and a canopy large enough for tens of tonnes in thin air is beyond what can be packed, deployed and survive supersonic inflation.

Aeroshells are fairing-limited

A larger vehicle needs a larger heat shield, which must fit inside a launch fairing unless it is deployable or inflatable, both of which are immature.

Supersonic retropropulsion

Firing engines into a supersonic oncoming flow is the leading candidate. The aerodynamics of the interaction are complex, and terrestrial booster landings have provided the first substantial flight data.

This is the binding constraint

Not propulsion, not life support, not radiation. Landing mass is the specific technical gap between robotic and human Mars missions.

What the surface missions found

Successful landers include Viking 1 and 2, Pathfinder, Spirit, Opportunity, Phoenix, Curiosity, InSight, Perseverance and Zhurong.

The scientific arc across them is coherent. Viking found a chemically reactive soil and no unambiguous biology. Pathfinder and the rovers established mobility and found mineralogical evidence of past liquid water. Phoenix touched subsurface water ice directly at high latitude. Curiosity confirmed an ancient habitable lakebed environment in Gale crater, with the right chemistry and neutral pH for life as we understand it. InSight measured the interior through seismology, constraining core size and crustal structure. Perseverance is caching samples for return.

The consensus that emerged is that early Mars was warm and wet enough to be habitable. Whether it was ever inhabited is a different question, and answering it is what motivates sample return.

Two secondary results are worth noting. Ingenuity demonstrated powered flight in an atmosphere with 1% of Earth’s density, flying 72 times against a planned five, and opened aerial mobility as a mission architecture. MOXIE produced oxygen from atmospheric carbon dioxide at 5 to 10 g per hour, demonstrating in-situ resource utilisation on another planet for the first time.

Dust

Mars has global dust storms, typically once every three Mars years, that can envelop the planet for months.

The effect on solar-powered missions is severe. Opportunity operated for fourteen years and was ended by the 2018 global storm, which cut insolation below what its arrays could supply. Nuclear-powered missions are unaffected, which is a significant argument for radioisotope power despite its cost and plutonium supply constraints. See Power systems.

Dust also accumulates on arrays continuously between storms. Some missions have been rescued by wind gusts clearing panels, which is not a design feature.

In-situ resource utilisation

The mass argument for ISRU is decisive. Propellant for the return trip is the single largest mass item in a human Mars architecture, and manufacturing it locally removes it from the launch mass entirely.

The Sabatier reaction combines Martian atmospheric carbon dioxide with hydrogen:

Electrolysing the water yields oxygen and returns the hydrogen. The products are methane and oxygen, which is exactly the propellant combination that recent launch vehicle designs have converged on, and that is not a coincidence. See Chemical propulsion.

Hydrogen must be either brought from Earth or extracted from Martian water ice. The second route closes the loop completely and is why subsurface ice distribution is a mission-planning question rather than a purely scientific one.

The human mission problem

The radiation figure comes from the MSL RAD measurements discussed in Space radiation, and it converts a shielding problem into a trip-time problem, which is the strongest case for nuclear propulsion.

References

• Braun, R. D. and Manning, R. M. “Mars Exploration Entry, Descent and Landing Challenges”, Journal of Spacecraft and Rockets, 44:310, 2007. The clearest statement of the landed-mass problem.
• Grotzinger, J. P. et al. “A Habitable Fluvio-Lacustrine Environment at Yellowknife Bay, Gale Crater, Mars”, Science, 343:1242777, 2014.
• Hoffman, J. A. et al. “Mars Oxygen ISRU Experiment (MOXIE)”, Space Science Reviews, 218:43, 2022.
• Zeitlin, C. et al. “Measurements of Energetic Particle Radiation in Transit to Mars on the Mars Science Laboratory”, Science, 340:1080, 2013.
• Balaram, J., Aung, M. and Golombek, M. P. “The Ingenuity Helicopter on the Perseverance Rover”, Space Science Reviews, 217:56, 2021.