NASA is developing a key instrument for a daring mission to the Martian moons, Phobos and Deimos, including a sample return from Phobos.

MMX mission
An artist's conception of MMX at a Martian moon

Amid the flurry of missions set to head towards Mars during the next few launch windows, there's one ambitious plan to go where no spacecraft has gone before: the Martian moons.

NASA recently announced that it will join the Japan Aerospace Exploration Agency  (JAXA) and Japan's Institute of Space and Aeronautical Science (ISAS) by contributing a crucial instrument to the Martian Moons eXploration (MMX) mission.

Set for launch in 2024, MMX will explore the tiny Martian moons Phobos and Deimos close up, then touch down on Phobos for a sample collection for return to Earth. The sample return capsule will then arrive back on Earth with its precious cargo in 2029.


NASA's contribution to the MMX mission is MEGANE, the Mars moon Exploration with GAmma-rays and NEutrons instrument, a sophisticated spectrograph. “Megane,” pronounced meh-gah-nay, also means “eyeglasses” in Japanese.

Cosmic rays and solar particles continually bombard Phobos and Deimos, causing them to emit particles and radiation. MEGANE will analyze these emissions to determine the elemental composition of the moons’ surfaces.

“We'll see the composition of the region from which MMX collects its sample,” says Thomas Statler (NASA HQ) in a recent press release. “This will help us better understand what we discover in the laboratory when the mission returns the sample to Earth for analysis.”

MEGANE will be devloped under NASA's Discovery Program, which provides low-cost access to space on planetary science missions.

MMX spacecraft
The MMX spacecraft

The primary goal of the MMX mission is to understand the origin of the Martian moons — are they captured asteroids or material left over from an ancient impact on Mars? A secondary goal is to characterize the global dynamics of the Martian atmosphere and the orbital environment the moons occupy.

Due to the moons' suspected origins, a sample return might even return a piece of Mars itself. Both moons also probably get pelted by ejecta from Mars impacts from time to time, so there's a good chance that MMX could find a Martian rock on Phobos's surface.

The Strange Worlds of Phobos and Deimos

Astronomer Asaph Hall discovered Phobos and Deimos in 1877 using the U.S. Naval Observatory's 26-inch refractor during a favorable opposition. Deimos, a small rock just 13 kilometers (8 miles) across, orbits Mars every 30 hours at a respectable distance of 23,460 kilometers.

Phobos, on the other hand, is the larger, closer, and quicker of the pair. It circles Mars every 7.7 hours, even faster than it rotates, which means that Mars actually rises in the west and sets in the east from the moon's perspective. No moon in the solar system hugs its planet as close as Phobos, which orbits just 6,000 kilometers above the Martian surface. Its close orbit is just one clue of what recent research has shown: Phobos is a doomed world, destined to either disintegrate into a ring or smash into the surface of Mars in 30 million to 50 million years. The fate of the more distant Deimos is less clear.

Phobos Mars Express
A Mars Express image of rocky Phobos shows grooves, which may be due to tidal stress already stretching out the interior of the close moon.

Curious theories for how these strange moons arose go back to the early Space Age. In 1958 Russian astrophysicist Iosif Samuilovich Shklovsky proposed that the moons might actually be hollowed-out space stations! Surprisingly, this claim gained traction in fringe circles, despite a hoax perpetrated by Sky & Telescope contributor Walter Scott Houston, who claimed the same as a parody on April Fool's Day the following year.

deimos hirise
NASA's Mars Reconnaissance orbiter imaged outermost Deimos in 2008.
NASA / JPL-Caltech / University of Arizona

We obtained our first good looks at the two moons starting with Mariner 7 in 1969, followed by  the Viking missions in the 1970s. These close-ups revealed natural, rocky worlds, featuring long grooves along their surfaces, which are also pockmarked with craters. The European Space Agency's Mars Express, India's Mars Orbiter Mission and NASA's Mars Reconnaissance Orbiter have all imaged the moons during flybys.

Curiosity has even imaged the moons from the surface of Mars, as they complete strange potato-shaped annular eclipses in front of the Sun (see below). All of these observations have enabled astronomers to refine the orbits of Phobos and Deimos with greater precision.

Phobos eclipse
Curiosity nabs a unique "Phobos eclipse."
NASA / JPL-Caltech/Malin Space Science Systems / Texas A&M University

A landing on Phobos would be a first, but it would actually be easier than landing on Mars itself due to the moon's negligible surface gravity.

Humans have tried to visit the Martian moons before: NASA proposed a mission named the Phobos and Deimos Mars Environment explorer (PADME) in 2014, but it lost to Psyche and Lucy for Discovery Program funding. The Russians also attempted a sample return mission mission named Phobos-Grunt in 2011, but it failed to leave Earth orbit and re-entered over the South Pacific on January 15, 2012.

Though it's unclear whether Russia will attempt another mission like Phobos-Grunt, Roscosmos did contribute hardware to the ExoMars Trace Gas Orbiter and failed EDM Schiaparelli lander, and the country's contributions will also play a role in ESA's ExoMars Rover, set to launch in July 2020.

To date, JAXA has only attempted one mission to Mars: the ill-fated Nozomi mission, which failed to enter orbit around the Red Planet in 2003 due to a malfunctioning valve, which resulted in insufficient fuel for braking. However, JAXA does have experience with sample returns from the Hayabusa 1 mission to 25143 Itokawa, as well as Hayabusa 2, which is set to arrive at 162173 Ryugu in June 2018.

Interest in Mars is sure to spike next summer as we head towards an opposition on July 27, 2018 that's nearly as favorable as the historic 2003 passage. This time will also present an ideal opportunity to hunt for the moons through your eyepiece. The trick is knowing when the moons will be at greatest elongation and hiding dazzling Mars just out of view or behind an occulting bar eyepiece.

Seeing Edgar Rice Burrough's “hurtling moons of Barsoom” for yourself is a thrill, and a mission to explore Phobos and Deimos will surely be an epic odyssey of exploration.


Image of Aqua4U


November 24, 2017 at 11:43 pm

Wow! I've never seen those Phobos Eclipse images before, they are way double cool & thanx! "...opposition on July 27, 2018 that's nearly as favorable as the historic 2003 passage..." Is really great news. I didn't know about that. Remind of '03 and what I saw on the Mars surface with my 4". Now I have a 12 1/2" and can hardly wait!

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November 27, 2017 at 7:58 pm

More likely that the April Fool's prankster was Walter Scott Houston aka Scotty. I think Walter Scott Hudson was an insurance salesman from Yonkers.

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Monica Young

November 28, 2017 at 9:43 am

Yes, typo fixed, thanks!

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