Humanity has been sending robotic explorers to Mars since the 1970s, and for our efforts we’ve been rewarded with not only incredible imagery of the Red Planet’s magnificent desolation, but a wealth of information about the alien world and how it’s changed over the last several billion years. But comparatively, we know almost nothing about the planet’s two moons, Phobos and Deimos. In fact, as recently as the 1960s, there were some scientists that believed the unusual shape, mass, and orbit of Phobos hinted at the possibility that it was no moon at all, and was in actuality a derelict spacecraft of some type.

Once the first Viking probes arrived at Mars, we had the photographic evidence and up-close measurements to prove that Phobos wasn’t artificial. But even today, we still don’t know exactly what it is. The data suggests that it’s not solid rock, and that large voids may be hiding under its surface. While we can be fairly sure there won’t be any mummified remains of Martian astronauts to be found within Phobos, vast deposits of ice are a tantalizing possibility.

With the launch of the Martian Moons eXploration (MMX) mission earlier this week, the Japan Aerospace Exploration Agency (JAXA) hopes to bring us one step closer to understanding the formation and structure of Phobos. Should everything go to plan, MMX will deploy a small rover to its tenuous surface in 2029, and by 2031, a sample return capsule will bring pieces of the mysterious moon back to Earth for further study.

A Delicate Operation

It’s only natural that we’d imagine Phobos and Deimos to be similar to Earth’s moon, but the reality of the situation is very different. For one thing, the Martian moons are absolutely dwarfed by our nearest celestial neighbor. Phobos has a diameter of just 22.2 kilometers (13.8 miles), and Deimos is even smaller at roughly 12.6 km (7.8 miles). For reference, Luna is approximately 3,474 km (2,159 miles) across.

Due to their low mass, putting a spacecraft into a stable orbit around either of the Martian moons isn’t really an option. Some clever maneuvering will put the MMX mission within range of both Phobos and Deimos at different points during the mission, but it won’t be able to stay around for long.

As such, final approach to Phobos will look more like a slow-motion “crash” than a traditional landing. A close comparison would be the OSIRIS-REx mission, where the spacecraft effectively “bumped” into the asteroid Bennu while collecting its samples. The impact velocity will need to be very low to prevent the spacecraft from bouncing off, and even then, some reports indicate that JAXA expects the craft to make at couple of hops before finally settling down.

The situation is compounded by the fact that we still don’t know what the surface conditions are like on Phobos. Based on previous observations, it’s expected that the moon is covered with a layer of fine regolith that could make traversal difficult for the mission’s Idefix rover. Indeed, one of the rover’s primary mission goals is to determine if wheeled motion in such an environment is even viable.

Tiny Moon, Huge Potential

Landing a crewed vehicle on Mars and successfully bringing it back home afterwards is perhaps the most daunting engineering challenge ever conceived. There’s even a solid argument to be made that it’s currently beyond our technological capabilities. The energy requirements of making a propulsive landing on the planet and then lifting off and getting back into orbit are enormous, and will very likely require in situ propellant production to refuel the craft between the two phases of the mission.

Landing on Phobos or Deimos however would require a small fraction of that energy. Moreover, due to their low orbital altitude, the moons would make ideal outposts from which to make detailed observations of Mars. This close proximity to the planet also means human operators on Phobos could directly command robotic vehicles on the Martian surface without the transmission delays that make real-time telerobotics from Earth impossible. While such a mission might not have quite the same symbolic impact of actually putting boots on Mars, it could be a valuable stepping stone towards that eventual goal.

The viability of such a landing may well depend on the findings of Japan’s Martian Moons eXploration mission. Not only in terms of the actual mechanics of rendezvousing with Phobos and touching down on it, but should the mission succeed in returning surface samples, we may be able to confirm whether or not the moon is made from ejected Martian material. Should that be the case, there will likely be even greater interest in studying the planet’s strange little moons up close.