Building a Capsule to Return Home

After three years exploring Mars’s environment, the Martian Moons eXploration (MMX) spacecraft will be ready to return home. But the mission science will be far from over. Onboard the spacecraft will be the Sample Return Capsule (SRC), containing material from the Martian moon.

Welcome everyone, to our first engineering post!

Once the our spacecraft arrives back at the Earth, the SRC will detach from the main craft and enter the Earth’s atmosphere. In the same way falling meteors turn into fireballs as our air compresses and shock heats around them, the SRC will have to withstand an intense rise in temperature in order to safely reach the ground.

Figure 1: Model of the sample return capsule (SRC) for the Martian Moons eXploration mission.

Designing such a resilient container is an engineering challenge, but one JAXA ISAS engineers have tackled before. The SRC for the Martian Moons eXploration mission will be based on the capsule for Hayabusa; the asteroid mission that was the first to return a sample from beyond the Moon. The Hayabusa SRC successfully brought a sample of the asteroid Itokawa back to Earth on 13 June 2010; seven years ago last week.

Since the Martian Moons eXploration mission plans to gather a larger sample that Hayabusa, the SRC also needs to be enlarged. The most fundamental question when considering a redesign is to ask what needs to fit inside the capsule.

The diagram in Figure 1 shows an exploded view of the necessary components and their arrangement within the SRC. In total, there are six main features, (1) the sample container itself, (2) the heat shield that protects the capsule from the blistering aerodynamical heat as it enters the Earth’s atmosphere, (3) the electronic circuits that controls the decent sequence, (4) a transmitter beacon to alert the recovery team to the capsule’s position, (5) the power supply needed to run the electronic components and (6) a parachute so the capsule can slow and land gently on the ground.

All of these must be carefully arranged to fit inside the compact SRC. A particular difficulty is estimating the space for the parachute, as the soft cloth is folded by hand and inserted into the capsule in the final stage. Until this is actually done, it is not clear if it will fit properly!

Figure 2: Mock-up model of the sample return capsule (SRC).

Three dimensional designs can be be constructed on a computer using software like 3D-CAD. This works well for assessing the storage of the capsule’s rigid hardware, but not for soft, foldable items like the parachute. The ISAS engineering team therefore construct transparent models of the SRC like the one shown in Figure 2. The arrangement of all the equipment be seen inside the SRC, including the positioning of the parachute.

When the Martian Moons eXploration spacecraft completes its long journey from Phobos and Deimos, the work of the SRC will begin. To ensure the mission end is a huge success, the research and development team here at ISAS will be making steady progress on perfecting the Sample Return Capsule design.