The Japan Aerospace Exploration Agency (JAXA) is preparing to launch an interplanetary station to Mars, but it will land not on the planet itself but on Phobos, the larger of its two small moons. The launch of the Martian Moons Exploration (MMX) mission is scheduled for Monday, 19 October, at 20:41 British time. An H3 rocket will lift off from the Tanegashima space center in southern Japan.
The five-year program calls for orbital studies of Phobos, a landing by the craft, and collection of soil and small rocks. For the first time, samples from a Martian moon are to be brought to Earth for laboratory study. The project is led by JAXA together with the United States, France, Germany and the European Space Agency (ESA).
The main scientific goal is to understand the origin of Phobos and Deimos. Scientists do not agree: the moons could be captured asteroids that flew past Mars, or they could have formed from debris ejected when an asteroid struck the planet. The chemical composition of Phobos's soil will help choose between these versions. In addition, researchers want to study the evolution of Mars and its space environment.
Collecting material in weak gravity is a difficult task. In 2011 the Russian station "Phobos-Grunt" successfully reached near-Earth orbit but failed to fire its engines for the flight to Mars. JAXA has experience in returning samples: the Hayabusa (2003-2010) and Hayabusa 2 (2018-2020) missions have already delivered soil from asteroids.
Gravity on Phobos is about 0.1% of Earth's - a person could jump there roughly 1.5 km in length without much effort. The station will test sampling methods designed for such conditions and also check communications with a new ground station.
The journey to Mars will take about a year: the craft will reach the planet's vicinity in August 2027, first enter orbit around Mars, and about a month later move into orbit around Phobos. The station consists of three modules: a propulsion module, a research module with landing and sampling equipment, and a return module.
There are 13 scientific instruments on board. The high-resolution cameras Tengoo and Orochi will map the surface, and a lidar (a laser rangefinder that measures distance to objects with pulses of light) will map the terrain. The MIRS infrared spectrometer will study minerals, and the Megane gamma-ray and neutron detector will determine elemental composition. From afar the station will also collect data on Deimos.
In early 2029 the research module will drop a small rover, Idefix, the size of a microwave oven (a joint development of JAXA, France's CNES and Germany's DLR) onto Phobos from an altitude of 40-100 m. The craft will operate for about 100 days and, using a Raman spectrometer and a radiometer, will select the landing site for the main module.
In 2030 the module will land on Phobos for about 2.5 hours. Samples can be taken once or twice, with a second landing at another location. Two instruments are available for this: the C-sampler, able to drill a core from a depth of up to 2 cm (such material is unaltered by solar radiation), and a backup P-sampler, which blows soil away with a jet of nitrogen - but in weak gravity there is a risk of tipping the craft over.
After collecting at least 10 grams of material, the station will leave Phobos and return to Earth in 2031. On approach it will release a capsule with the samples, which will descend by parachute in Australia and be taken from there to Japan.
A possible bonus: particles of Martian soil that the moon constantly "picks up" in orbit may be found in Phobos's soil. The mission could thus become a lightweight version of the Mars Sample Return program, effectively cancelled because of budget constraints. If all goes well, scientists will get a consolation prize.