Humanity is heading to the moon again. NASA's Artemis II was launched on the 1st local time with four astronauts on board, succeeded in a lunar flyby, and on the 6th set a new record for the farthest crewed spaceflight in human history. The first crewed lunar flight in about half a century has become a symbol that humanity is once again moving into deep space.
Going forward, NASA plans to use the moon as a base to explore Mars. On the surface, both the moon and Mars missions are journeys to other celestial bodies beyond Earth. But in astronautical engineering, the nature of the two missions is entirely different.
Kang Kyung-in, head of the space science exploration division at the Korea AeroSpace Administration, said on the 13th, "The basic concept of sending a probe is similar, but from the perspective of actual exploration, Mars becomes so complex across the entire process—orbital insertion, atmospheric entry, landing, ascent and return—that it is hard to compare with the moon."
◇ Harder because it's farther… the challenges of crewed Mars exploration
Mars is much farther from Earth than the moon. The distance from Earth to the moon is about 380,000 km, reachable in a few days. But the distance to Mars averages about 225 million km, taking 7–10 months one way. Factoring in the round trip and a stay, the mission becomes a 2–3 year long expedition. NASA says a crewed Mars mission could total more than 1.6 billion km (1 billion miles). That means food, water, oxygen, medicine, spare parts, power and waste management must be built to last that long.
The problem is that the farther you go, the less cargo you can send. To reach Mars, a greater change in velocity is needed to escape Earth's gravity well, which increases the propellant fraction. As a result, even with the same launch vehicle performance, the available payload margin for life support, equipment and supplies shrinks.
On top of that, the window to go to Mars does not open often. According to NASA and the Jet Propulsion Laboratory (JPL), launch opportunities when Earth and Mars are aligned to travel with energy efficiency generally come once every 26 months. If a launch is delayed by technical issues, you wait not weeks but more than two years, driving up expense and verification burdens.
According to NASA's assessment materials, communications delay for lunar missions is about 3–14 seconds one way, but for Mars missions, depending on the Earth-Mars distance, the delay can reach up to 22 minutes one way and 44 minutes round trip. That means the approach of reporting a situation to Earth and receiving immediate instructions is effectively unworkable. On the moon, even if ground control's help is late, signals arrive relatively quickly, but on Mars, the crew must judge and fix failures on site for much longer stretches.
As challenging as the technical hurdles are the risks to the human body. Lunar missions end within days and emergency return is relatively quick, but Mars presumes a long stay in deep space. Astronauts are exposed for extended periods to space radiation outside Earth's magnetic field. The European Space Agency (ESA) explained that even a journey of six months to Mars and six months back could expose astronauts to at least 60% of the recommended career limit for radiation. To this are added muscle and bone loss from microgravity and stress from confined spaces.
◇ Thin air and intense heat… why landing on Mars is hard
The second difference is landing and return. Among these, the landing segment is called EDL—entry, descent and landing—in astronautical engineering. It refers to the entire process in which a spacecraft enters a planet's atmosphere, slows down, and finally sets down safely on the surface.
The moon has almost no atmosphere, so parachutes cannot be used, and landing must be done by reducing speed with retropropulsion. Mars, by contrast, has an atmosphere, but it is so thin that it cannot sufficiently slow a spacecraft, while still generating intense heat when the spacecraft enters. Thus, a Mars lander must withstand entry heating with a heat shield and further reduce speed with a parachute and a powered descent system.
In a white paper released last year on Mars EDL, NASA described Mars landing as a high-difficulty task where thin atmosphere, intense heat and limited deceleration methods coincide. In fact, there have been 19 Mars landing attempts by robotic probes so far, 12 of which succeeded. Because a crewed lander must be much larger and heavier than a robotic probe, the difficulty increases further.
A bigger problem is going back up than going down. The moon's surface gravity is about one-sixth of Earth's, so a relatively small ascent vehicle can reach lunar orbit and dock with an orbiter. Mars, by contrast, has gravity about one-third of Earth's, stronger than the moon's, so after descending once, much more propellant and a more complex system are needed to carry people and equipment back to orbit.
◇ Fuel is the key to Mars exploration… technology validation and astronomical expense are variables
Kang, the division head, cited "how much propellant can be transported" as one of the core technologies for crewed Mars exploration. The reason SpaceX emphasizes on-orbit refueling in its Starship architecture is that it is more realistic to top up fuel near Earth than to send a massive spacecraft to Mars in one go.
That is why the concept discussed alongside it is in-situ resource utilization (ISRU). Kang said, "If propellants such as methane can be produced on Mars, mass can be greatly reduced, and Mars exploration would become dramatically advantageous." NASA is also treating this as an active research topic, having conducted a demonstration with equipment on the Perseverance rover to produce oxygen from carbon dioxide in the Martian atmosphere.
Kang also noted, "Artemis already has much of the technology, but from the actual crewed flight and docking in 2026 to the landing in 2028, it is proceeding step by step to validate procedures," adding, "Mars will require far more technology demonstrations, so integrating individual technologies and repeatedly verifying them will take a long time."
Analysts say that if aggressive technology development and massive budgets like SpaceX's are sustained, crewed Mars exploration could proceed in the mid-2030s, but technology maturity, the verification process and astronomical expense will be variables.