Phobos has resisted every attempt to bring a piece of it home. In 1988 the Soviet Union launched two spacecraft toward Mars, each carrying a Phobos lander, and ground teams lost contact with both before either finished its mission. Russia tried again in 2011 with Phobos-Grunt, which failed shortly after launch and never left for Mars.
Now it’s Japan’s turn. The Martian Moons eXploration mission, or MMX, cost $578 million and lifted off from Tanegashima Space Center in southern Japan at 3:41 pm EDT Monday (4:41 am Japan Standard Time Tuesday). The launch begins a five-year expedition to land on Mars’ largest moon and return samples to Earth.
Hiroshi Yamakawa, president of the Japan Aerospace Exploration Agency (JAXA), called the launch a success. “A long journey of five years has just started,” he said.
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A beefed-up H3 for Japan’s heaviest probe yet
The rocket was a 207-foot-tall (63-meter) H3, Japan’s flagship launcher, flying in a configuration that had never been used before. It carried four strap-on solid rocket boosters, ran its main engines at full throttle and used an extended payload shroud. Japan needed all of that for the heaviest and most ambitious interplanetary mission it has ever attempted.
MMX weighs nearly 9,900 pounds (4.5 metric tons), and propellant makes up two-thirds of that. The H3’s upper stage pushed the probe past 24,000 mph, enough speed to escape Earth’s gravity and start the 10-month trip to Mars. Separation from the rocket came roughly 71 minutes after liftoff.
Early operations went smoothly. Soon after separation the spacecraft made contact with ground controllers, deployed its solar panels and settled into a “very stable condition,” according to Yasuhiro Kawakatsu, JAXA’s project manager for the mission. “So far, so good.”
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Japan tells NASA and Europe it’s the safer bet
JAXA has returned samples from deep space before, though never from Mars. Hayabusa and Hayabusa 2 were the first two spacecraft in the world to bring pristine asteroid specimens back to Earth. Kawakatsu said those missions turned deep-space sample return into a “speciality” for Japan’s space program.
NASA and European space agencies contributed to MMX because of that history, and Kawakatsu didn’t soften the reason.
“We have a proven track record of success in these areas,” Kawakatsu said. “Because of this, I believe they are participating with the understanding that if they want to conduct celestial body sample returns, joining a mission led by Japan is more certain to succeed than the US or Europe launching their own missions.”
Yoshihiro Seki, Japan’s science minister, described the mission as a world first. “The MMX [mission] we launched today is a challenging mission aiming for the world’s first sample return from the Martian moons,” Seki said. “We hope that the results obtained through this mission will contribute to the development of our nation’s space science and exploration technology.”
Captured asteroid or crash debris?
Of Mars’ two moons, Phobos is the bigger one. It’s a lumpy, potato-shaped body with an average diameter of 14 miles (22 kilometers) and roughly the surface area of Houston. Its origin is still unknown.
“One of the most intriguing questions about Phobos and Deimos is simply why these two little moons are there in the first place,” said Tom Statler, NASA's lead scientist for solar system small bodies. “In some ways, they resemble asteroids from the outer part of the asteroid belt, so they could be asteroids that were captured by Mars’s gravity. But there are other lines of evidence pointing to these moons being remnants of a giant impact early in Mars's history.”
One idea is that Phobos is an asteroid that Mars pulled into orbit billions of years ago. Checking that theory is among MMX’s science goals, and Statler said samples in a lab are what will answer it.
“By going to the Martian moons, studying them up close, and bringing samples back to Earth, we’ll be able to determine whether Phobos material is more consistent with an asteroid composition or with debris created by a collision involving Mars,” Statler said.
The mission could also return something extra. Phobos sits roughly 40 times closer to Mars than our Moon does to Earth, so material from the red planet may have landed on Phobos after it formed or was captured, and MMX could pick some of it up. That would give scientists an early look at Mars surface material. Returning samples from Mars itself is a far harder job than returning them from Phobos.
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Circling a moon too small to orbit
MMX reaches Mars next August and will stay in the neighborhood for about three years, making repeated close passes of Phobos and its smaller companion, Deimos.
After arrival the probe jettisons its propulsion module, which is dead weight once it has burned all its fuel to enter Mars orbit. The remaining main spacecraft and its four-legged landing gear, together called the exploration module, then moves closer to Phobos.
A conventional orbit around the moon isn’t possible because Phobos’ gravity is too weak and Mars pulls too hard. Instead MMX will use a quasi-satellite orbit, which keeps it bound to Mars while letting it maneuver around Phobos.
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A shoebox-sized rover goes down first
Reconnaissance ends in late 2028 or early 2029. At that point the mothership releases a small four-wheeled rover called Idefix onto Phobos. The French and German space agencies built it.
Idefix is about as big as a large shoebox. It will attempt to drive across Phobos for up to 100 days, coping with rough terrain and gravity below one-thousandth of Earth’s.
Two landings, two sampling methods
Later in 2029 comes the main event: the spacecraft will try up to two landings on Phobos. Because the moon’s gravity field is weak and uneven, there’s a real risk of the probe tipping over or bouncing back into space, so the landing gear is designed to soak up the energy of each touchdown.
MMX will spend several hours on the surface during each landing and gather material in two independent ways. A coring drill at the end of its robotic arm will extract a cigar-shaped specimen. A pneumatic sampler from NASA, mounted on one landing leg, will fire a cartridge of high-pressure nitrogen gas to blow bits of Phobos into a collection canister.
Scientists expect a combined haul of at least 10 grams, or one-third of an ounce. Once back in space, MMX will shed its landing gear and fire its engines in 2030 for the trip to Earth. In 2031 it will steer a sample return capsule toward a remote landing zone in South Australia.
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Laboratories in Japan will analyze the samples. In return for NASA’s hardware, US researchers will receive 15 percent of the Phobos material. NASA supplied one of the samplers plus a gamma-ray and neutron spectrometer that measures the elemental makeup of the Martian moons.
Kawakatsu said what MMX learns about Phobos could help future planners judge whether the moon is suitable for human exploration, possibly as a stopover between Earth and Mars.
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Decades of studying them from a distance
Scientists have only ever studied these moons from far away. MMX is designed to fix that.
“Basically, these objects look more like primitive asteroids than anything else, but the data is coming from remote sensing from far away,” said Patrick Michel, a French asteroid scientist who co-leads the Idefix rover project. “That cannot really tell us exactly what are the properties, as we can do with a sample return in the lab.”
“The goal of the mission is effectively to reveal the origin of the Martian moons,” Michel said. “If we understand the origin, that will tell us something about the early phases of the Solar System.”