Can a Japanese Spacecraft Unlock the Mystery of Mars' Twin Moons?

Searching for clues on Phobos and Deimos. 

Mars has been studied and scrutinized more than any alien world. Since 1964, some 40 expeditions have been dispatched to the Red Planet. Orbiting spacecraft have snapped photos of vast canyons and rust-colored sand dunes, while robotic rovers have endured dust storms and freezing temperatures to drill into rock formations and search for vestiges of life beneath ancient riverbeds.

Yet two of the Red Planet’s most enduring mysteries have been largely unexplored: its twin moons, Phobos and Deimos. While Mars orbiters have paid periodic calls on them, these spacecraft have recorded only slim dossiers on both. To be fair, the two moons, which are large potato-shape chunks of rock, lack the spectacular charisma of the moons that orbit our solar system’s outer giant planets. That would include Titan, with its methane seas; Io, with its 400 active volcanoes; and Europa and Enceladus, with liquid oceans locked beneath frozen ice.

Appearances aside, the moons of Mars have an epic story to tell. “The origins of these moons are one of the solar system’s great mysteries,” says Abigail Fraeman, a research scientist at NASA’s Jet Propulsion Laboratory (JPL) in southern California.

For decades, the leading theory has posited that the moons are asteroids captured by Mars during the solar system’s infancy. More recent research has suggested that Phobos and Deimos are made from debris dislodged when a giant asteroid slammed into Mars. The two moons might even be remnants of a larger body that was blasted apart in its own giant impact. “There are all kinds of scenarios,” says Bob Craddock, a geologist at the National Air and Space Museum’s Center for Earth and Planetary Studies and the author of the giant-impact hypothesis.

On the other hand, if the moons are captured asteroids, “then they are a record of the material that moved through the early solar system,” says Japanese scientist Yasuhiro Kawakatsu. “It’s suspected that ices and organics needed for the origin of life may have formed in the outer solar system and been scattered inward toward the terrestrial planets. Sitting at the gateway between the inner and outer solar system, the Martian moons could be evidence of this process, helping us understand the starting conditions for how life began.”

Soon, researchers might finally have some answers and—as is often the case when opening new frontiers of science—enough data to point them toward even more questions and avenues for exploration. In November 2026, the Japan Aerospace Exploration Agency (JAXA), with some assistance from NASA and Europe, plans to launch the $417 million Mars Moons eXploration (MMX) mission. 

Upon its arrival at the Red Planet next year, the ambitious spacecraft will study both moons from close range—relying on a phalanx of instruments. Then, in 2028, the mission’s grand finale will begin: A small rover named Idefix will be deployed to study the surface of Phobos in preparation for the mission’s primary lander, which will capture a few grams of pebbles and dust grains for return to Earth in 2031. For the first time in history, scientists will get to study a piece of a moon other than our own. 

Fear and fate

Mars, appearing as a wandering red star in the night skies of Earth, has captivated observers for millennia. But the Red Planet’s moons—Phobos (fear) and Deimos (fate), the mythological attendants to the war god, Mars—weren’t discovered until the late 19th century. Their detection was delayed because both moons are small, and they are concealed by the light reflecting from Mars. Positioned close to the planet’s surface, Phobos and Deimos complete their orbits in a matter of hours. 

On the night of August 11, 1877, American astronomer Asaph Hall caught a glimpse of a tiny star-like point of light that could have been a moon of Mars. He was scanning the planet with the 26-inch refracting telescope—then, the world’s largest—at the U.S. Naval Observatory in Washington, D.C. Earlier searches for moons by other astronomers had been unsuccessful. “When he looked at those previous attempts, he was willing to say that maybe they were looking too far out,” says Kimberly Rupley, the observatory’s public affairs specialist. “He opted to look really close in.”

During Hall’s observations, Mars was passing especially close to Earth, making it easier to scan the space just beyond the planet’s bright surface. But the observatory was located at the appropriately named Foggy Bottom, on the banks of the Potomac River, a swampy mosquito-infested spot that offered few good nights of skywatching, says Rupley. Another week passed before Hall could confirm his initial discovery—and find a second moon even closer to the planet. He had almost abandoned his search, but his wife and former math tutor, Chloe Angeline Stickney, had encouraged him to persevere. Fittingly, the largest feature on Phobos, an impact crater, is named in her honor.

In the century after Hall’s discoveries, the moons of Mars appeared as nothing more than dots in a telescope. The first close-range examinations of the two moons didn’t come until 1972, when Mariner 9, the first Mars orbiter, turned its cameras toward them. Additional missions have provided a few more details.

Phobos, the larger moon, is an irregularly shaped chunk of rock with an average diameter of 14 miles. It orbits just 3,700 miles from the surface of Mars, so it’s not visible from the planet’s polar regions. It rises in the west, then sets in the east less than four and a half hours later, making two or three appearances every Martian day. 

Deimos is some eight miles in diameter and about three times farther from Mars, so its transit is slower. It takes about two and a half days for Deimos to cross the sky from east to west. It’s also much fainter than Phobos, and it has a distinctive large dimple on one side.

The surfaces of both moons are quite dark, reflecting only a small fraction of the sunlight that strikes them. This characteristic is much like the most common class of asteroids, known as carbonaceous chondrites: ancient carbon-rich bodies with high concentrations of frozen water.

“They’re nothing like Mars,” says JPL’s Fraeman, a veteran of several Mars missions who will analyze Mars Moons eXploration observations as part of NASA’s contribution to the project. “Their texture, density—they’re really like asteroids.” 

Astronomers had suspected the Martian moons were captured asteroids soon after their discovery. When the solar system was young, it was populated by many “planetesimals,” large blocky leftovers from the birth of the planets. At the same time, the young Mars was surrounded by a haze of gas and dust from its own birth. Astronomers theorized that, as Phobos and Deimos passed through this cloud, it acted as a brake, slowing them enough for Mars’ gravity to snag them. “They’re asteroid size, asteroid shape, asteroid composition, so the thinking was they were captured asteroids,” says Craddock.

But it’s not easy for a planet to capture an asteroid, and the odds of catching two of them and having them assume the present-day orbits of Phobos and Deimos would be like winning the lottery—twice.

The captured asteroids of the giant outer planets follow highly elongated and tilted orbits, which are far different from those of the Martian moons. “Both of them are in nearly perfect circular orbits, and the inclination of the orbits is nearly perfectly aligned with the equator of Mars,” says Craddock. “It’s possible to capture a single object and put it into an orbit like that. But getting two objects aligned like that is almost statistically impossible.” 

“It’s hard enough when you build a spacecraft and fly it to Mars to get it captured into orbit,” says David J. Lawrence, a planetary scientist at the Johns Hopkins University Applied Physics Laboratory and lead investigator for one of two NASA-contributed Mars Moons eXploration instruments. “The idea that you’d have two asteroids captured in just that way is very strange.” 

In the early 2000s, Craddock proposed another explanation: Phobos and Deimos were produced by a giant impact. Scientists have theorized that Earth’s moon was also born this way, when a Mars-size object slammed into our young planet, blasting Earth’s nascent crust and part of its mantle into space. Much of this material quickly cooled and coalesced to form one or more moons; the present-day moon is the lone survivor. This scenario is supported by the composition of lunar samples returned by Apollo astronauts. It also explains Earth’s unexpectedly high rotation speed and the moon’s slow retreat from Earth.

Craddock believes that a giant impact could likewise explain the orbits of Phobos and Deimos. A large planetesimal could have slammed into Mars, shooting material into orbit and forming a ring. The ring settled around the young planet’s equator, with rock and dust coming together to form small moons. As with Earth, some of the moons fell back onto the Red Planet, which would explain several large impact craters around its middle. Phobos and Deimos, however, remained in orbit.

“If the moons formed in the wake of a giant impact with Mars, then they are material carved from an early Mars during an era when the planet might have been habitable,” says Kawakatsu, who is also the project manager of the Mars Moons eXploration mission. “The composition of the moons would hold clues to conditions on this young Mars, and whether life could have existed.”

But other scientists have suggested additional scenarios. In one, Mars captured a large asteroid: It passed too close to the planet and was then sheared apart by gravity, creating a ring from which Phobos and Deimos emerged. “This has the same advantage of fitting the orbits of the moons we see today as a giant impact, while also leading to moons made from pure asteroid-building blocks instead of a mix of Martian material,” says Jacob Kegerreis, a planetary scientist at Imperial College London and one of the researchers who proposed the idea. 

Beyond observation

As telescopes improved over the centuries, so too did our understanding of our closest celestial neighbor, Earth’s moon. Galileo scrutinized the moon’s rugged surface of mountains, valleys, and craters. Analyzing the shadows cast by such features enabled him to make the first scientific estimates of their height. Nearly three centuries later, American geologist Grove Karl Gilbert used the Naval Observatory’s 26-inch refracting telescope to study the moon. He noted key differences between terrestrial volcanoes and lunar craters, and he suggested that impact events, not volcanism, were responsible for most of the moon’s craters. 

But with geology, telescopes will only get you so far. Lunar rock and soil specimens, returned primarily by NASA’s crewed Apollo missions, continue to offer insights into our moon’s composition, age, and thermal history that are not possible through remote observation alone. So it is with Phobos and Deimos. Geological samples will be key to deciphering their origins. Are the tiny moons made of material from Mars? Or are they more akin to the rocky flotsam and jetsam of the asteroid belt?

By examining meteorites found on Earth, scientists have a good understanding of the compositions of asteroids. The ones that formed closer to the sun contain less water and other volatile substances (compounds that boil off easily in high temperatures) and a higher proportion of heavier materials. Asteroids born farther out, under colder conditions, retain more water and volatiles. Martian moon profiles that match that of the asteroids from a particular region would confirm the capture hypothesis.

The composition of Mars itself is well understood. Over the years, laboratories aboard landers and rovers have analyzed materials from the surface and just below it. Scientists have also recovered dozens of meteorites that were propelled to Earth from Mars by strong impacts. The ratio of different forms of key elements in the meteorites, especially oxygen, matches that found on Mars and is different from what is seen in asteroids. 

The payoff from the Mars Moons eXploration mission is that it will definitively reveal the elements of the Martian moons, first from afar and then in greater detail with the samples it collects.

The 9,000-pound craft will enter an orbit around Mars that enables it to keep company with Phobos without actually orbiting the rocky moon. For roughly a year, the spacecraft’s instruments will scan the surface and the space around Phobos in far greater detail than any previous robotic mission. A NASA-contributed instrument, MEGANE (Mars-moon Exploration with Gamma rays and Neutrons), which means “eyeglasses” in Japanese, will measure the chemical composition of Phobos down to a depth of a foot or more.

Cosmic rays—which originate from sources such as the sun, supernovae, and distant galaxies—are continuously bombarding Phobos with energetic particles, triggering nuclear reactions within the surface rocks and soil. The subatomic particles generated by these reactions then collide with other atomic nuclei, causing them to emit gamma-ray photons as they return to a stable state. 

“The energy of the gamma rays from each element are unique, kind of like a fingerprint,” says Lawrence, who leads the Mars Moons eXploration instrument team. “So the energy of the gamma rays tells us which element produced it, and the number of gamma rays tells us how much of that element is there.” The technique is especially good at detecting hydrogen, which helps pinpoint deposits of frozen water. (The same method was used to confirm ice at the poles of Earth's moon and Mercury.) 

Another instrument aboard the spacecraft is the MIRS (Mars Moons eXploration Infrared Spectrometer), which was developed in collaboration with France. By examining Phobos at infrared wavelengths, scientists can analyze the structure of minerals on the moon’s surface. “That tells you a lot more about the history of that body than just the elemental composition does,” says Christopher Edwards, a professor of planetary science at Northern Arizona University in Flagstaff, who will study the instrument’s readings. “The structure of the minerals tells you their cooling history, their alteration history, and the evolution of the body.” 

Data acquired with these instruments will assist the mission scientists in picking the best spots for the mission’s next phase: safely placing two robotic explorers on Phobos.

First penguin

“One of the greatest challenges of visiting Phobos is that no one has ever done this before,” says Kawakatsu. “This means that the surface of Phobos is very much an unknown. For example, how hard are the Phobos surface and subsurface? Will it be easy or difficult to core down into the moon and gather material?”

The first answers will come from the Mars Moons eXploration mission’s boxy 55-pound rover, Idefix, which will gently drop to the surface of Phobos after being released from the main spacecraft. “Idefix will be the first risk taker, commonly known as the ‘first penguin’ in Japan,” says Kawakatsu.

Idefix—which is named after a dog in Asterix, the popular French comic series—is a collaborative project between the German Aerospace Center and France’s national space agency, Centre National d’Études Spatiales. After Idefix is deployed, the rover will trundle across Phobos for about three months. Its instruments will analyze rocks and dirt and snap pictures of the rover’s surroundings. Once the scouting mission is complete, the Mars Moons eXploration’s landing craft will descend to the surface of Phobos to gather material for return to Earth.

Sampling, after all, is the mission’s main event. And it’s a duet: The lander has two sampling instruments. One of them, the pneumatic sampler, was developed by Honeybee Robotics, a subsidiary of Blue Origin, in Altadena, California. A version of the instrument, known as PlanetVac, was successfully tested on Earth’s moon aboard the robotic Blue Ghost 1 lander in March of last year. 

“Fundamentally, we’re sending a vacuum cleaner to Phobos, except it works a bit in reverse,” says Kris Zacny, a vice president at Honeybee and one of the system’s designers. The pneumatic sampler, which is mounted on one of the landing craft’s legs, will place what looks like a small, upside-down salad bowl on the surface, then squirt pressurized nitrogen gas into the enclosure. That will stir up dirt and pebbles, a few grams of which will then be drawn into a sample container.

Engineers had to compensate for the moon’s extremely low gravity. “I weigh 200 pounds, but on Phobos I’d weigh one ounce, so I could jump very high,” says Zacny. “So every time we shoot gas down, we’ll also shoot it up to balance the reaction force” and keep the lander from toppling over or high-jumping into space.

The other instrument, the coring sampler, will be deployed by a robotic arm. It will drill about an inch deep, where it can extract materials that have undergone less processing (the constant churning of the surface by micrometeorite impacts and bombardments of charged particles).

Regardless of Phobos’ origins, the samples will likely contain some pieces of the Red Planet itself. “There will certainly be some small amount of Mars material that’s been kicked out from the planet by small impacts throughout the moons’ histories, just like we get some landing on Earth as meteorites,” says Kegerreis. “It could be a cool opportunity to study some material originally from Mars as well as from the moons.” Scientists’ familiarity with the composition of Mars would tell them which samples have a Martian origin and which are native to Phobos.

After the samples have been stowed, scientists might decide to stage another landing. That could solve another intriguing Phobos puzzler: Why does the small moon have a two-tone appearance? Color-enhanced images show that some of the surface is red and some is blue; scientists would like to gather bits of both.

“The redder stuff might have been exposed to space longer, so it’s been exposed to more radiation, while the bluer material has been more recently exposed,” says Fraeman. “Another idea is that the two regions are completely different, and the moon is made of different kinds of materials. We don’t really know—which is the exciting part.”

When the expedition is over, the lander will rocket away from Phobos and head outward to Deimos. Because Deimos is farther away from Mars, most orbiters have had to study it from greater distances, so the images have been fuzzier and the readings less precise. 

The Mars Moons eXploration mission won’t put a lander on Deimos, but the main spacecraft will get within 60 miles of the small moon, which mission scientists regard as a safe distance. 

“If we could get close enough, we could determine whether the composition of Deimos is the same as Phobos or not,” says Lawrence. “If we can establish that they’re the same, that would have an impact on its origin. But what if they’re different? That would be fascinating as well. But you can imagine, if you’re doing these flybys of Deimos with the samples of Phobos aboard, and you’re getting really close, you don’t want to hit it. That would be a bad day.”

Once the Phobos samples have been secured (and, hopefully, not strewn across Deimos), the Mars Moons eXploration return vehicle will head for Earth in 2030. If all goes well, the sample capsule will parachute into the Australian outback in February 2031, and its precious cargo will be doled out to researchers around the world.

“Whatever it finds will be really interesting,” says Fraeman. “It’s going to answer one of the big questions we have, and I’m confident that it will raise 50 more questions that we don’t even know to ask yet.” 

Damond Benningfield is a freelance science writer and audio producer in Austin, Texas. He is a frequent contributor to Air & Space Quarterly.

This article, originally titled "Martian Moon Mysteries," is from the Winter 2026 issue of Air & Space Quarterly, the National Air and Space Museum's signature magazine that explores topics in aviation and space, from the earliest moments of flight to today. Explore the full issue.

Want to receive ad-free hard copies of Air & Space Quarterly? Join the Museum's National Air and Space Society to subscribe.