90 minutes on the clock! Discussing the MMX sampling device with JAXA robotic specialist, Hiroki Kato

“The mission is to collect a sample of at least 10 grams of sample from a depth of more than 2 cm within 90 minutes after landing on the Martian moon Phobos.”

Dr Hiroki Kato of JAXA started working on the development of a sampling device around 2015 to meet this difficult task. He is now focusing hard in order to “make this happen” in preparation the launch in FY2024.  In this interview, Kato tells us why it is necessary to collect samples in 90 minutes, what kind of methods are used for sample collection, the difficulties that exist, and the kind of future is in sight.

(Interview and text by Kimiyo Hayashi, translated by Ayumu Tokaji with Elizabeth Tasker)

Dr Hiroki Kato, Senior Research Scientist, Research and Development Directorate at JAXA, was born in Kanazawa in Japan in 1977. He received his M.S. from Carnegie Mellon University and Ph.D. from the University of Tokyo, before joining JAXA in 2007. As a robotics specialist, he has been involved in the Martian Moons eXploration (MMX) Mission, the International Space Station robotics project, and space debris removal.

The goal is to bring home a sample about 100 times the size of Hayabusa2

How long have you been working for the MMX project, Dr Kato?

Hiroki Kato (hereafter, Kato): I joined the MMX project team very early, although the project itself had already begun.

When you joined the team, were there any specific requirements for the mission regarding sample collection? What were your assignments?

Kato: It had been decided that MMX would return a sample from one of the Martian moons. The asteroid explorer Hayabusa2 had not even collected samples yet, but the science team told me, “Let’s increase the amount of sample we bring back (more than Hayabusa2) with MMX. The discussion therefore began with the question, “How do we increase the sample volume?”

So, you were involved in the process from the beginning, deciding what kind of method should be used to collect the sample from Phobos. What were the requirements and restrictions?

Kato: We were required to gather a sample of at least 10g at a time, from a depth of at least 2cm below the surface. However, there is not much time to take samples after landing. So, we were asked to “develop a method to ensure that a sample is taken within 90 minutes”.

A 1.5m-long manipulator (robotic arm) is attached to the underside of the MMX spacecraft and can be moved within a radius of 1m from the arm attachment point after landing. It is capable of detecting and autonomously moving away stones as small as 1cm using sensors attached to its hand.

The target sample volume for Hayabusa2 was initially 0.1g, so the target volume for MMX is 100 times larger. And why was such a short time required?

Kato: It is because we have to perform landing operations during the daytime on Phobos. We would need to bring a large battery in order to work at night, but due to the weight constraints of the spacecraft, we can’t bring such a large battery. So, MMX lands at sunrise and takes off by sunset, about 2.5 hours later. In that time, 90 minutes was assigned for sample collection.

The corer has a double structure, with the outer part serving as a shovel and the inner tube containing the sample. The manipulator pulls out only the inner tube and stores it in the Sample Return Capsule.

The 90-minute time limit is a strict limit. What kind of ideas for sampling were considered to meet this target?

Kato: There were many ideas, such as digging with a shovel or turning a spiral drill. We chose manipulators [robotic arm] because of the time limit and the fact this allowed us to decide where to collect samples within the area where we landed. We decided to attach a “corer mechanism”, a cylindrical device for collecting samples, to the tip of the manipulator. The corer is ejected to penetrate under the surface of Phobos to collect as much sample as possible.

One of the prototypes of the corer mechanism for sample collection verification introduced during the interview. This prototype has a lid that closes at the end of the inner cylinder (inner diameter 21 mm x height 75 mm) when the tip is pulled out.

What kind of mechanism does the corer use to take samples?

Kato: The corer mechanism has an ejection device that pushes out a shovelled corer. The mechanism is equipped with a special shape memory alloy that stretches a little when heated. This causes a bolt to break, releasing a spring that pops the corer out with a snap.  The corer has a double structure, and the outer cylinder has a sharp tip (like a shovel) that can slip into the ground of Phobos. As this happens, the sample enters the inner tube of the corer. The corer has to then be pulled out of the ground, and when a ball lock separation mechanism is driven, the lid on the tip of the corer inner cylinder closes, separating the outer cylinder shovel at the same time. The manipulator pulls out the inner tube of the corer and stores it in the return capsule.

When there is an obstructing rock, the spacecraft can decide on its own to move to another location.

This is very different from the “Hayabusa” method using bullets and sampler horn. Can you tell us about the sampling sequence currently planned?

Kato: After the MMX spacecraft lands on Phobos, we will wait for about 30 minutes for the shaking of the spacecraft to settle down. Then we will be ready and can start taking samples.   First, we use the terrain measurement sensor on the underside of the spacecraft to obtain a topographic image with 3D information. An area of approximately 2m square can be captured.

The acquired images are sent to the ground [Earth]. It takes about 15 minutes to communicate from Phobos’s position near Mars to the Earth. The 3D information received on the ground is reconstructed in the control room, and science and engineering personnel decide on two candidate locations for sample collection from amongst the flat areas.

The MMX spacecraft’s Exploration Module has multiple sensors. The terrain measurement sensor, which measures the topography of the Phobos surface in 3D, is mounted on the underside of the MMX spacecraft, while a camera and a tube-shaped sensor called a “pick” are mounted on the tip of the robotic arm. The pick pokes at the ground to check for rocks under the ground.

How long does it take to make a selection?

Kato: Within 10 minutes. We have trained in advance so that we can say “if we receive this type of image, we will choose this location”, and select that point as quickly as possible. We then send that information back to the spacecraft. That is why it takes about 30 minutes for the round-trip communication time.

A terrain measurement sensor is mounted on the underside of the MMX spacecraft with a camera that can acquire raw images of the terrain. At the same time, a mesh pattern is projected onto the ground using LEDs, which is then photographed by the camera. If there is nothing on the ground, the undisturbed mesh pattern projected by the LEDs appears as expected, but if there are stones or hollows, the mesh pattern is distorted. In this way, 3D topographic information is obtained.

An unavoidable time unique to deep space exploration. What is the sequence of events after that?

Kato: Once the spacecraft receives the information, the manipulator moves to the first candidate sampling point. The manipulator has a camera at its end that automatically detects rocks larger than 1cm in diameter and makes the decision to move away from them. The tip of the manipulator also has a tube called a pick, which pokes at the ground and if it bounces off a hard rock, the manipulator decides not to eject the corer at that location and moves to the second sampling location. If there are no rocks, the corer is ejected.

Even if the camera does not detect any rocks on the surface, the manipulator is smart enough to check underground with the pick and make the decision by itself to stop ejecting the corer if there are any rocks.  How long does it take you to pull out the corer after ejecting it?

Kato: It is one to two minutes.

Left: When the pick detects a rock, it makes the decision to stop ejecting the corer. Right: If no rocks are detected, corer will be ejected!

That seems short, but didn’t Hayabusa2 only take a few seconds after touchdown?

Kato: In terms of being fast, Hayabusa2’s sample collection was excellent. But with Hayabusa2, the target site is fully selected prior to landing and you can’t afterwards choose precisely where on the ground to hit. With MMX, we can observe and select a sample collection site after landing. And most importantly, MMX’s sample volume is large.

What will the spacecraft do after sample collection?

Kato: To prevent the sample from spilling, the manipulator will be used to point the corer in the opposite direction of the Phobos surface and the spacecraft will take off.

After the corer is inserted into the ground, the corer inner cylinder is pulled out of the ground and the tip lid simultaneously closed. The manipulator then tips the corer in the opposite direction to the Phobos surface to prevent the sample spilling before take-off.

How do you confirm whether the MMX spacecraft was able to collect a sample?

Kato: While on the Phobos surface, we will only receive information that “the corer has been launched”. After lifting off from Phobos and returning to orbit, we will use the camera on the terrain measurement sensor to check through the window of the corer mechanism to see if the sample is contained. It will then be stowed in the capsule for return to Earth. Landing on Phobos and sample collection will be done twice.

We can’ t tell if the Phobos surface will be fluffy or hard!

It is a tremendously difficult challenge to communicate with the Earth and collect samples from a rock-free area within a time limit of 90 minutes. There must be many challenges. Which is the hardest part?

Kato: The most difficult of all is the inability to test and verify in a microgravity environment. Phobos is in a microgravity environment of 1/2000 G [Earth’s gravity = 1 G], and we don’t know whether the moon’s surface material is fluffy or hardened. Even if we were to make soft, fluffy sand-like material on Earth and conduct tests with this, we would not be able to create the conditions on Phobos. To estimate the reality, we have to do a long series of verification experiments, to try to find an answer to the question; “How many grams of sample are going to fit in the corer?

Corer experiment using simulated Phobos material, with a 25mm diameter pick to a depth of 80mm below ground at a constant penetration rate of 0.5mm/s.

We’re unsure of the condition of the surface material on Phobos?

Kato: We have more information on Phobos than we had about asteroid Ryugu, which was explored by Hayabusa2, but we do not have as much information as for the Earth’s Moon.  No spacecraft has ever landed on Phobos, nor have any orbiting spacecraft explored Phobos. We do have data provided by scientists on the estimated radius, density, and porosity of the surface particles on Phobos. “Porosity” represents the gap between the grains, and the higher the porosity number, the more space there is between the grains. Since Phobos is a microgravity environment, it is possible that the porosity is high and the surface material is fluffy and piled up. The University of Tokyo’s Hideaki Miyamoto Laboratory has made a simulated sample material for Phobos called “regolith simulant” and we are conducting experiments, but a sample with too many gaps cannot be made on the ground due to the effects of Earth’s gravity.

Are you able to conduct experiments in a microgravity environment?

Kato: We are conducting experiments in drop towers to replicate what will happen when the corer is ejected in microgravity on Phobos.  Uematsu Electric’s Cosmotorre drop tower in Hokkaido is 57m high and can create a microgravity environment for 2 seconds, while the drop tower in Bremen, Germany, where MMX has international cooperation with DLR, is 146m high and can create a microgravity environment for 9.3 seconds.  We placed a set of corer ejection mechanisms and simulated Phobos material in a vacuum chamber, set up a high-speed camera, and then dropped this set-up, measuring everything in 2 seconds.

Photo from a September 2019 experiment at the ZARM drop tower in Bremen, Germany (from ZARM Twitter), front row, left side is Dr. Kato.

In two seconds! What did you find out?

Kato: We wanted to verify if the corer sticks firmly in the surface material of Phobos, and another important thing, to see how much material would be scattered and in what way when the corer gets stuck. From this experiment, we were able to verify where to mount the cameras and other important equipment to protect them from surface particles. We had to do the drop experiment many times until we could get the necessary data in a limited time, so we were all screaming as we did it (laughs).

I want to contribute to the understanding of the process of human evolution.

MMX is scheduled to launch in FY2024, isn’t it?  What stage is MMX at now?

Kato: We cannot afford any delays, and we have a critical design review in the spring of 2022, so we are working hard to get everything done. If the project passes the review, we will start building the flight hardware that will actually fly.

As a robotics expert, what are your feelings about MMX?

Kato: Originally, I was working on an autonomous vehicle project in the US, but now I’m in the middle of testing in the sand (laughs). What I think makes MMX a very good mission is that it will be the first in the world to bring material back from the Martian sphere. The beauty of a sample return mission is that it will come back to Earth.  

Phobos could be a candidate for a future base for crewed Mars exploration. Once landed on Mars, astronauts would have to launch back to Earth, shaking free of the Martian gravity. But return to Earth from Phobos would be easier because of the moon’s microgravity. In this sense, I strongly believe that this project will leave a mark on human history and contribute to the making of history, and I find that very rewarding. On the other hand, failure is not an option.

That is why I do my best.  It’s a high-profile mission, so if the spacecraft returns and there’s no sample, everyone is going to say, “Who made that?” and I’m going to say, “I did” (laughs).  So, I think we have to be able to say, “It worked, cool!”

That’s a lot of responsibility. Once again, as a developer, what are the selling points and innovations of your sample collection system?

Kato: The most difficult part is that the landing time is limited and there is a communication delay. If the spacecraft is in front of me and I can take time to complete the landing, it’s not that difficult at all. If it were that situation, we could consider the spacecraft as a robot that just moves the manipulators and ejects (the corer). But because of the time limit, there are difficulties. Like being able to eject the corer quickly, or even choose a location with the manipulators in a couple of minutes. We don’t know what’s going on with the surface material on Phobos and the lack of gravity makes it really difficult. But even if we lose communication, we are doing our best to implement it so that we can pick up some samples and come back.

That’s great!  Did you join JAXA after studying at a US university because you wanted to do space missions?  How will MMX lead to the future?

Kato: I hope to contribute to the understanding of the process of human evolution, even if only in a small way. My personal desire is that various robotics missions will be launched in the future, and robots with intelligence will also be launched into space. I think MMX is a good step in that direction. The range of human activity will expand.  If MMX succeeds in going to the Mars sphere and returning, there will be more and more crewed Mars exploration missions. I hope that both space agencies and the private sector can work together. We are now at the equivalent point of trying to go around the Cape of Good Hope in preparation for the Space Grand Voyage.