Japan's MMX probe is due to launch Oct. 19 to fetch Phobos soil that tests two origin theories
Japan's MMX probe is due to launch on an H3 rocket on Oct. 19 to bring back more than 10 grams of Phobos soil. A sample rich in Martian material would favor a giant impact, and one matching known meteorites would favor capture, phys.org reports.
The Scientist · Science desk
Key figures: MMX aims to collect more than 10 grams of Phobos soil; Phobos has a mean diameter of 22.2 km; MMX's round trip to Mars takes five years in total.
| Figure | Value | Note | Claim |
|---|---|---|---|
| Phobos soil MMX aims to collect, more than | 10 grams | 3 | |
| Phobos mean diameter | 22.2 km | 6 | |
| MMX round trip, in total | 5 years | 4 |
What happened
- The round trip takes five years in total, with arrival at Mars in late 2027 and return of the sample to Earth in 2031.
- Simulations by Pascal Rosenblatt of the LPG laboratory in Nantes and colleagues leave Phobos and Deimos as the only moons to survive a giant impact.
- The capture idea requires an extremely dense Martian atmosphere to trap the asteroids, and Mars today has a thin one made mainly of carbon dioxide.
- The spacecraft has three modules for travel, landing and return, and 11 instruments, including a gamma-ray and neutron spectrometer for Phobos's chemistry.
Why it matters
- What's now possible capability Orbit-based arguments have not separated the theories because each has a stated weak point. A sample gives laboratories a different kind of evidence, a direct composition measurement.
- What it limits constraint The composition test is laid out only for the two leading theories. The proto-moon and ring-cycle ideas have no stated prediction, so a sample cannot yet be read for or against them.
- What it limits constraint A Phobos result speaks first to Phobos. Deimos is smaller and farther out, and extending the answer to it would need an assumption that the two moons formed the same way.
The test rests on two predictions about composition. If the soil holds a large share of Martian material along with material from the colliding asteroid, that favors the giant impact [13]. If its composition resembles meteorites from known asteroid categories, that favors capture [14]. The phys.org account uses the verb "favor" for both outcomes. In our view that is the right strength, because a sample that fits neither pattern cleanly would leave the question where it is.
Orbits have not settled it. The account says there are several theories and no clear front-runner [11]. Capture is the older idea, going back to the 1960s [8]. But captured asteroids tend to have highly eccentric orbits, and the orbits of Phobos and Deimos are almost circular [10].
The denominator matters here. Phobos has a mean diameter of 22.2 km and a mass of 10,637 million million kg [6]. A sample of more than 10 g [3] is about one part in a billion billion of that mass: 0.01 kg divided by roughly 1.06 x 10^16 kg is 9.4 x 10^-19 [15]. Laboratories can read a lot from a few grams of soil.
The account does not say how the landing site will be chosen. That gap matters, because the thing a 10 g scoop cannot tell you is whether the rest of Phobos looks the same.
What to watch
- Whether MMX lifts off from Tanegashima on an H3 on Oct. 19 as scheduled.
- The Phobos landing after arrival at Mars in late 2027, and whether the mission reaches its 10 g sample target.
- The first laboratory composition results after the 2031 return: how much Martian material the soil holds, and whether it matches known meteorite classes.
Claim ledger
Ranked by verification strength, evidence, and original report placement.
- [1]
MMX is due to lift off on an H3 rocket from Tanegashima Space Center in southern Japan on Monday, Oct. 19.
- [2]
The mission will aim to land on Phobos, collect a sample of soil, and send the payload to Earth for study in laboratories.
- [3]
MMX aims to collect more than 10g of material from Phobos, the world's first sample return mission from a Martian moon.
- [4]
MMX's round trip to Mars will take five years in total; it will arrive at the red planet in late 2027 and return to Earth in 2031.
- [5]
The spacecraft consists of three modules (one to get to Mars, one to land on Phobos, one to return the sample) and carries 11 scientific instruments, including a gamma-ray and neutron spectrometer for measuring the chemical composition of Phobos.
- [6]
Phobos has a mean diameter of 22.2km and a mass of 10,637 million million kg; Deimos has a mean diameter of 12.5km and a mass of 1,480 million million kg.
- [7]
In the giant impact theory, ejected material forms a single giant moon plus smaller satellites that each break up and fall to Mars; simulations by Pascal Rosenblatt of the LPG laboratory in Nantes, France, and colleagues show Phobos and Deimos would have been the sole moons to survive.
- [8]
The captured asteroid theory goes back to the 1960s and suggests Phobos and Deimos were originally asteroids that wandered so close to Mars they were captured by its gravity.
- [9]
The captured asteroid hypothesis requires an extremely dense Martian atmosphere to explain how the asteroids were trapped by tidal forces; Mars today has a thin atmosphere mainly composed of carbon dioxide.
- [10]
Asteroids captured by planets tend to have highly eccentric orbits, whereas the orbits of Phobos and Deimos around Mars are almost circular.
- [11]
There are several theories on how Mars got its moons, but no clear front-runner; two main hypotheses dominate discussion.
- [12]
Other proposed origins, such as debris from the destruction of a single proto-moon and billion-year cycles in which rings of rocky material coalesce into moons, command lower levels of support among scientists.
- [13]
If the samples contain a large amount of material from Mars itself, along with material from the asteroid that collided with the red planet, that would favor the giant impact theory.
- [14]
If the composition of Phobos is similar to that of meteorites from known categories of asteroid, it would favor the asteroid capture theory.
- [15]
A 10 g sample is about 9.4 x 10^-19 of Phobos's mass, roughly one part in a billion billion.
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Sources
1 independent publisher whose own reporting we read for this story.
- phys.orgHow did Mars get its moons? Japan's MMX space mission aims to solve the puzzle
1 article · October 15, 2026