Phobos: Japan Launches a Sample-Return Mission to the Moon That's Slowly Falling Into Mars

Phobos is falling.

Every year, Mars's inner moon drifts 1.8 centimetres closer to the planet below it. Slowly. Inevitably. In roughly 50 million years, tidal forces will either pull it apart into a debris ring — or slam it into the Martian surface.

In six days, a Japanese spacecraft launches to study it up close. What it finds could rewrite everything we think we know about where Mars came from.

What makes Phobos so strange?

Phobos is not a normal moon. It's roughly 26 km long and shaped like a battered potato. It orbits Mars so close and so fast that from the Martian surface you'd watch it cross the sky in just four hours — and it rises in the west, not the east, because it orbits faster than Mars itself rotates.

That's already unusual. But here's the measurement that keeps planetary scientists up at night.

For comparison: water is 1.0. Most rocky asteroids measure 2.5 to 3.5. Phobos is extraordinarily light for its size. Either it's riddled with internal cavities, or it's a loosely bound rubble pile — essentially a bag of gravel barely held together by its own gravity. Scientists call it "porous." Nobody agrees on why.

Where did Phobos come from?

There are three competing theories. None of them fully works.

Captured asteroid. Phobos's dark, carbon-rich surface looks like a C-type asteroid. Maybe it drifted too close to Mars and got gravitationally trapped. But the physics resist: a captured object should have a tilted, eccentric orbit. Phobos's orbit is nearly circular and almost perfectly aligned with Mars's equator. That doesn't fit a random capture.

Giant impact. Same origin story as Earth's Moon: something large hit Mars, debris flew into orbit, debris clumped together. This explains the neat equatorial orbit. But Phobos's composition doesn't obviously match Martian surface rocks — which might mean the impactor was very unlike Mars, or might mean this theory is wrong.

In-situ formation. Phobos formed directly from a debris disk around early Mars. Tidy in theory. But how did the disk form in the first place? Another impact. Another unknown.

What is the MMX mission launching October 19?

The Martian Moons eXploration mission — MMX — is JAXA's most ambitious deep-space project to date. It lifts off on Japan's H3 rocket from Tanegashima Space Center at 04:41 JST on October 20 (that's 19:41 UTC on October 19 for most of the world). Six days from now.

The landing plan: a pneumatic sampler fires a gas jet at the surface and captures the material that bounces back. Simple in principle. Nobody has ever done it on a body this small, this remote. MMX will attempt the landing multiple times, then fly past Deimos — Mars's other tiny moon — before beginning the long journey back to Earth, arriving around 2031.

If the Phobos sample matches C-type asteroid chemistry: captured asteroid. If it matches Martian material: giant impact. If it matches neither, that conversation gets very complicated, very fast.

What is the Stickney Crater — and why does it terrify scientists?

Phobos has a wound. Stickney Crater is 9 km across on a moon that's only 22 km wide. Whatever hit Phobos hit it hard enough that it should have shattered the entire body. Somehow, Phobos survived. That tells us something about its internal structure — though scientists disagree on exactly what.

Radiating outward from Stickney across the entire moon's surface are long, parallel grooves. For decades they were explained as impact fractures — cracks propagating through the body after the collision.

Then a 2015 paper proposed something more unsettling: the grooves might be tidal stress fractures. As Phobos spirals closer to Mars, the gravitational forces stretching and squeezing it intensify. The grooves could be the visible signature of a moon being slowly torn apart in real time. The MMX spacecraft carries geophysical instruments specifically designed to test which story the data supports.

The Soviet mission that went silent — 50 km from Phobos

Here's the part people tend to screenshot.

In 1988–89, the Soviet Union sent two spacecraft to Phobos. Phobos 1 was lost in transit — a single errant command from Earth accidentally shut down its attitude thrusters. Phobos 2 made it all the way to Mars orbit.

On March 27, 1989, just 50 km from its target, Phobos 2 went silent. Forever.

Soviet mission controllers attributed the loss to a computer failure causing the spacecraft to tumble and lose antenna lock — a plausible, mundane explanation for 1989-era deep-space hardware. However, some of the final thermal infrared images Phobos 2 transmitted appeared to show an elongated elliptical shadow on Mars's surface that mission scientists could not immediately explain. These images were published in Soviet scientific literature but were never subjected to a full independent public analysis before the program dissolved.

Is the Phobos Monolith real?

Yes. NASA photographed it.

The Mars Global Surveyor imaged a rectangular structure on Phobos's surface roughly 90 metres long and 45 metres wide, standing upright and casting a distinct shadow. Photographed from orbit in 1998, at a resolution where individual large boulders are just barely discernible.

NASA's position: it's a boulder — most likely a fragment ejected from the Stickney impact and later settled on the surface at an angle. "There are many angular rocks on planetary surfaces that appear striking from orbit," the agency noted. Geometry, they point out, is not evidence of construction.

Buzz Aldrin mentioned the monolith in his 2009 interview, saying humanity should investigate it. He did not claim it was artificial. The media ecosystem did the rest.

MMX will photograph Phobos's surface at far higher resolution than any previous mission. If the monolith is a natural boulder, we'll see that clearly. If it's something stranger — the conversation changes. We should know around 2027. Follow the SkyLens blog as the mission unfolds, and explore the live orbital data for context — all 15,968 objects currently tracked above you right now.