Mars has been the destination for so many of our spacecraft, and for good reason, it’s probably the most Earthlike place in the Solar System, with water ice on its surface and reservoirs of the liquid beneath the surface. If we’re going to find life, Mars might be the place.

But the tiny moons orbiting Mars, Phobos and Deimos, are scientifically fascinating on their own, and so far, a mission has never reached them.

Last week, Japan announced that they’ve greenlit their Martian Moon eXploration mission, or MMX, which will launch an orbiter, lander and maybe even a rover to Phobos in 2024, returning samples back to Earth by the end of the decade.

We’ve known about the potential for this mission for a few years now, with Japan working in the pre-project phase for several years to reduce the technical uncertainty, but now it’s moved to the actual development phase, and engineers will start working on the hardware and software. The estimated budget is about $417 million 

This mission is inspired by Hayabusa2, which just wrapped up its mission at asteroid Ryugu, sending a variety of landers to explore the surface and retrieving a sample from the surface. Hayabusa2 is on its way back home to Earth. Near the end of 2020, it’ll fly past the Earth and release a re-entry capsule carrying its asteroid samples.

If all goes well, MMX will launch to Mars in 2024, taking almost a year to cross the gulf between our planets, arriving at the Red Planet in 2025. It will then go into a “quasi-satellite orbit”, making regular flybys of the moons for about three years.

Once the mapping stage was completed, it will spiral down to the surface of Phobos and touch down on the surface extracting a sample. It might even deploy a rover that would use a corer-type drill to capture at least one sample that’s more than 10 grams from a depth of at least 2 cm below the surface of Phobos. 

Hayabusa2 pulled its samples from the surface of Ryugu, and from the debris of its impactor. To actually drill out a core sample from beneath the surface of Phobos is a serious engineering challenge.

Then a propulsion module would lift off from Phobos, putting the sample into an interplanetary return trajectory that brings it back to Earth in September 2029. It’s not a sample from the surface of Mars itself, but it’s pretty close.

In order to complete this mission, MMX will need to break entirely new ground in landing and navigating around an asteroid-sized moon while it’s in the larger gravitational influence of Mars. ESA’s Rosetta mission taught us just how difficult it is to land on a comet, and the original Hayabusa mission had its own technical challenges landing on an asteroid. 

This gets much more complicated when you’re trying to complete these maneuvers while also inside the much larger gravitational well of Mars. And yet, Phobos and Deimos could serve as valuable waystations for future human missions to Mars, so it’s useful to master these techniques.

According to NASA Chief Scientist Jim Green, there are only a few places that humans can realistically explore, and Phobos and Deimos are on that list. It makes sense to start with robotic exploration first.

The mission will also measure the ongoing radiation environment in the orbit around Mars, allowing scientists to compare its measurements with the amount of radiation experienced by Curiosity down on the surface of Mars. 

The spacecraft will be equipped with 11 instruments, 4 of which will be provided by international partners like NASA and ESA. It’ll have a variety of telescopes, imaging systems, as well as a LIDAR altimeter, dust monitor and a mass spectrum analyzer to study charged ions.

NASA will be supplying a gamma-ray and neutron spectrometer to study the surface chemicals of the moons, and a pneumatic sampling device, similar to what OSIRIS-REx is using to sample the surface of asteroid Bennu. The German Aerospace Center might be providing the rover, similar to how they contributed a lander to the Hayabusa2 mission.

Planetary exploration is about trying to answer key scientific questions, and when it comes to the Martian moons, there are two this mission will help answer.

How did they form? Were they captured asteroids from the relatively nearby asteroid belt? Or did they form out of material from Mars when a huge meteor struck the planet in the ancient past, and they coalesced into the moons we see today?

Second, they’d like to understand the surface evolution of the Martian moons as well as the Red Planet itself. It’s believed that Phobos and Deimos will have collected debris ejected from Mars from billions of years of meteor bombardments. Digging through that material will help tell the history of the red planet.

Even though Deimos is actually easier to reach and requires less propellant, mission planners chose Phobos as the target for a sample return. It has a closer orbit to Mars, which means there’ll be more of that ejected material on its surface, giving planetary scientists a more useful core sample.

If the moons are captured asteroids, their composition will help scientists understand how water and other volatiles might get transported from the asteroid belt to the inner Solar System. But if they’re chunks of Mars itself, we’ll see a snapshot of Mars at the time that the impact happened.

Timing is everything, though. The orbit of Phobos is deteriorating, causing it to slowly orbit closer and closer to Mars by about 1.8 meters every 100 years. In about 50 million years or so, it’ll cross within the Martian Roche limit and get torn into a ring. Then the debris from the former moon will crash into the surface of Mars, and Phobos will be gone forever.

So, for the engineers working on this mission, no pressure. You’ve got a few million years to get this mission completed.

This isn’t the first time anyone has tried to explore Phobos and Deimos. Other spacecraft have taken a few bonus snapshots while doing their main jobs. A Russian mission was supposed to make the journey back in 2011, and we’ll get to that in a second, but first I’d like to thank:

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Bend their minds:
This won’t be the first time that a country has tried to send a mission to Phobos. Back in 2011, Russia launched its Fobos-Grunt mission, which translates to Phobos-Ground. It was supposed to fly to Phobos, collect 200 grams of regolith from the Martian moon, and then return it back to Earth. 

Unfortunately, it failed to fire its rockets on an interplanetary trajectory to take it to Mars. It was stuck in Earth orbit, and re-entered the atmosphere in 2012, falling into the Pacific Ocean.

Although Phobos and Deimos haven’t been explored directly, we’ve seen a few pictures of the moons captured at high resolution. 

For starters, here’s a series of images showing Phobos in orbit around Mars captured by the Hubble Space Telescope. It’s a timelapse composed of 13 separate images, watching its orbit for 22 minutes.

And in 2019, NASA’s Mars Odyssey spacecraft observed Phobos with its Thermal Emission Imaging System to see it in infrared. These images helped to reveal some of its surface composition, including metals like iron and nickel. But obviously, MMX will be able to take much better images up close.

One of the best images of Phobos comes from the Mars Global Surveyor spacecraft, which made a pass over the moon in 1998 and took high-resolution images. You can see boulders scattered across the surface of Phobos, and their long shadows, hurled out by a more recent impact. 

And one of my favorites was actually captured from the surface of Mars by Curiosity, watching as Phobos crossed in front of the Sun from its perspective. You can see what it would look like to experience an eclipse from the surface of Mars.
Conclusion:
I’m really glad to hear that Japan has decided to move forward with its mission to Phobos and Deimos. So much emphasis on Mars itself, but what about its moons? The more we can learn about how they formed, the more we can learn about Mars and the history of the Solar System. 

I also really appreciate the ingenuity and boldness of the JAXA missions. Although they’ve had some failures, they’ve also had some incredible successes, testing really clever and risky ideas to get more science done than you’d expect from smaller missions. 

What do you think? Let me know your thoughts in the comments. 

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Not only is Phobos scientifically interesting, it’s an important waypoint for the exploration of the surface of Mars itself. We did a whole video about why this makes sense, and what kinds of missions would be required to set up on Phobos. You can watch that video now.