Gravity both too strong and too weak: landing on the Martian moons

To successfully sample the rocks of Phobos or Deimos, the Martian Moon eXploration (MMX) spacecraft must be able to secure a position on the surface of the moon.

It is a challenge that places the development of the landing technology among the top technical issues for the mission, alongside the sample return capsules and sampling devices. This equipment will become part of the body of the spacecraft and a very high success rate is required of the design.

In the case of the Hayabusa mission, which collected a sample from the asteroid Itokawa, a “touch n’ go” system was adopted to gather material without a prolonged stay on the asteroid surface. However, MMX will remain on the Martian moon for a longer period in order to gather more than 10g of material (link to previous article).

This is where the gravity of the Mars’s two small moons presents unique challenges. The two satellites have a strong gravitational pull (about 1/2000 G^[*^] for Phobos) compared to other small celestial bodies such as Itokawa (about 1/100000 G). Far more fuel is therefore required to slow the spacecraft as it descends to the moon’s surface to avoid a dangerously high landing speed. On the other hand, the gravity of the Martian moons is much smaller than the Earth’s Moon (about 1/6 G) or Mars (about 1/3 G) and therefore cannot be relied upon to keep the spacecraft secured to the surface after landing. Neither the gear developed for touching down on Itokawa nor equipment for a lunar mission is therefore guaranteed to be suitable for Phobos and Deimos.

The movie shows a simulation of a probe landing on a slope in two different gravitational environments. On the left panel, the gravity is the same as the Earth’s Moon, while the right panel uses gravity equivalent to Phobos. As you can see, the Marian moon landing is a difficult business!

In 2014, the European Space Agency “Rosetta” mission attempted to land a small probe, Philae, on the surface of a comet. Philae was equipped with dampers (to dissipate the energy of impact), harpoons, reverse injection thrusters (that push down) and a drill. Despite this, the probe rebounded twice from the comet surface and ultimately settled far from the target landing site. Overshadowed in this new location, Philae was unable to secure the necessary electric power to continue its mission once its onboard batteries ran down.

Philae touched the comet surface with an initial kinetic energy (energy of motion) of about 50 Joules. 90% of this was successfully dissipated by the dampers inside the landing gear and friction with the comet surface. Despite this, the remaining 10% of energy allowed the probe to rebound twice and move substantially from the planned landing spot. For a small celestial body without strong gravitational forces to hold a probe down, 5 Joules of energy is a large amount.

Unlike the Philae lander, the entire MMX spacecraft plans to land on the moon’s surface. The bigger probe can descend more easily in the weaker gravitational field, but then risks falling or shifting horizontally away from the landing site. Figure 1 shows how the kinetic energy at the time of landing is related to the collisional acceleration (rebound force) from the surface. From the plot, we can see that MMX is closer to the conditions experienced by Hayabusa and Philae when landing on smaller bodies, than were the lunar missions of Apollo, Surveyer and Japan’s planned SLIM. MMX will therefore need to absorb or dissipate a larger amount of kinetic energy than the spacecraft that touched down on our Moon’s surface. This means that MMX will need new landing technology, rather than replicating the successes of previous missions. However, the challenges of developing MMX will increase the success of all missions to the multitude of small bodies in our Solar System!

[* 1 G is the strength of the Earth’s gravity. ]

(Based on an article in Japanese by Dr. Masatsugu Otsuki from the ISAS Spacecraft Applied Engineering Research Team.)