Late on October 19, Moscow time, the Martian Moons eXploration (MMX) spacecraft, developed by the Japan Aerospace Exploration Agency (JAXA), launched toward Mars. Its mission is to study the Red Planet’s moons, Phobos and Deimos, and bring back a soil sample from one of them. The mission could mark a major milestone in space exploration: Scientists have never recovered a soil sample from a Martian moon. Samples from Phobos could contain a substantial proportion of material from the Red Planet itself. Roscosmos planned a similar mission in 2011, but a launch error thwarted Russian scientists’ ambitious plans, and the Phobos-Grunt spacecraft burned up in the atmosphere.
The biggest mystery about Phobos and Deimos (Fear and Terror) is their origin. Astronomers once thought they were alien spacecraft
Jonathan Swift is often credited with being the first to write about Mars’ moons, 150 years before scientists actually discovered them. This is also often cited as the reason craters and other features on Phobos are named after characters in “Gulliver’s Travels.” In the book, published in 1726, Swift writes of astronomers in the flying city of Laputa:
They have likewise discovered two lesser stars, or satellites, which revolve about Mars; whereof the innermost is distant from the centre of the primary planet exactly three of his diameters, and the outermost, five […].
Galileo has also been credited with discovering Phobos and Deimos. In 1610, he observed Saturn through a telescope and discovered its rings. To avoid revealing his discovery prematurely, he published this coded anagram:
smaismrmielmepoetaleumibuvnenugttaviras
When Johannes Kepler tried to crack the code, he came up with salve umbistineum geminatum martia proles — Latin for “Greetings, twins, offspring of Mars.” But Kepler had gotten it wrong, both in deciphering the message and in identifying the “twins.” What Galileo had actually written was altissimum planetam tergeminum observavi — “I observed the most distant planet in triple form.”
It wasn’t until 1877 that American astronomer Asaph Hall identified Phobos and Deimos as moons of Mars. British educator Henry Madan suggested naming the moons after Phobos and Deimos, the sons of Ares and Aphrodite, whose names mean Fear and Terror.
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For a long time, little was known about Phobos and Deimos. But in the late 1950s, Soviet astronomer Iosif Shklovsky unexpectedly brought them to public attention. He was trying to explain why Phobos’ orbit, for reasons no one understood, was decaying noticeably over time. After considering every possibility, Shklovsky concluded that friction with Mars’ atmosphere was the only possible explanation. This seemingly reasonable hypothesis posed an even bigger problem: For the Red Planet’s thin atmosphere to slow the moon so noticeably, the moon’s mass would have to be exceptionally small. That would require a density so low that the observations could be explained only by assuming Phobos was completely hollow.
This reasoning led Shklovsky to suggest that Mars’ moons were artificial objects built by aliens. He first proposed the hypothesis in an interview with Komsomolskaya Pravda, and in 1966 it appeared in “Intelligent Life in the Universe,” the book he co-wrote with Carl Sagan.
Further research showed that the orbits of Mars’ moons were changing far more slowly than early observations had suggested. But the idea of enormous, hollow objects built by aliens so captured the public imagination that it spread widely through science fiction. For example, a character in the Strugatsky brothers’ short story “Noch na Marse” (“Night on Mars”) mentions “two giant satellites.”
Today, the biggest mystery about the “satellites” is their origin. Their relatively small size (Phobos is 13 kilometers in diameter and Deimos 7), irregular shapes, and Mars’ proximity to the main asteroid belt led astronomers in the 1960s to suggest that they had formed very differently from Earth’s moon. The hypothesis was that they were former asteroids captured by the Red Planet’s gravity, rather than bodies ejected from a protoplanet by a collision.
Even then, however, there were objections to this idea. Both bodies had nearly circular orbits that lay almost in the planet’s equatorial plane. This was more consistent with the hypothesis that the moons had formed through accretion, as debris from proto-Mars clumped together. Captured asteroids would surely have had much more elongated and tilted orbits.
Soviet and Russian scientists have a long history of studying Phobos and Deimos. In July 1988, the Soviet Union launched Phobos 1 and Phobos 2 to study the moon as part of its final interplanetary mission.
The first spacecraft was lost just two months later, in September. The probe first lost attitude control, leaving its solar panels unable to recharge its batteries. Eventually, contact was lost. The cause, it later emerged, was a software error: A single character had been changed in the control program transmitted to the spacecraft.
The second spacecraft fared better. It reached Mars, entered a circular orbit, and made a series of close approaches to Phobos. Phobos 2 took 37 pictures of the moon but also lost contact shortly before it was scheduled to deploy a lander designed for extended operations and a rover onto Phobos.
The next attempt to explore Phobos came in post-Soviet Russia, when Phobos-Grunt launched toward Mars in 2011. It was an extremely ambitious project, especially considering that it was Roscosmos’ first interplanetary program in 15 years.
Phobos-Grunt was meant to put a lander on the moon’s surface, collect about 200 grams of soil, and bring it back to Earth. The mission failed: Shortly after launch, the probe suffered a malfunction that left it in low Earth orbit. Scientists and engineers around the world tried to establish contact with the spacecraft but never succeeded. A few months later, Phobos-Grunt burned up in the atmosphere. The failure forced Roscosmos to rethink its entire interplanetary program and, in particular, led to cooperation with the European Space Agency.
In Japan, the idea of sending a probe to Phobos emerged in the mid-2010s. Masaki Fujimoto, director of ISAS/JAXA and the driving force behind MMX, recalled how the project began:
During one of our informal Friday discussions, someone asked, “What kind of solar system research could we do with an Epsilon launch vehicle?” And Phobos was suggested as a destination. The project then began to gather momentum, largely driven by questions about what would come after Hayabusa2 (which at that point hadn’t even reached its target, the asteroid Ryugu), as well as discussions about how to approach a Mars landing mission. And all this was happening at a time when many people feared Japan might abandon such ambitious plans altogether.
From the outset, Japanese engineers understood that landing on Mars would require extensive research and new engineering solutions. A soft landing, for example, would be impossible without them. But JAXA wasn’t starting from scratch. The agency had successfully explored asteroids with its Hayabusa and Hayabusa2 missions, and Phobos resembles an asteroid in some respects. JAXA also believed that operating spacecraft and conducting research in orbit around Mars would provide valuable experience for a larger mission to the planet. The agency therefore saw MMX as a bridge between asteroid exploration and the study of the Red Planet.
Like FC Barcelona’s crest, Phobos has blue and red sections. But no one knows where these stripes came from
Scientists are still asking whether Phobos and Deimos are asteroids captured by Mars or formed from debris thrown into orbit after the planet collided with another object in space. That second scenario — the giant-impact hypothesis — is now considered the most plausible explanation for the formation of Earth’s moon.
There are variations on these hypotheses, too. Some researchers, for example, suggest that the moons may have broken apart under tidal forces and then reassembled from the debris. It is also possible that Phobos and Deimos formed in different ways.
To determine which explanation is correct, or at least narrow the possibilities, scientists need to analyze soil samples from Phobos. This will allow them to determine its mineral composition, establish whether it contains water or ice, assess its porosity, and look for organic matter. It will also help them study its overall geological structure: Many asteroids are more like piles of rubble than solid bodies.
Scientists also hope to understand why Phobos has distinct “blue” and “red” regions. These are the names scientists have given to two areas of the moon’s surface with very different spectral properties. The “red” region covers most of the surface, while the “blue” region lies around Stickney crater. The blue region extends across both the floor and rim of Stickney crater. The spectral differences suggest that Phobos is composed of different kinds of material, rather than having distinct surface and subsurface layers.
One of Phobos’ most striking features is the long, parallel grooves that cover much of its surface. To understand the moon’s geological history, scientists need to determine what caused them. Cracks? Marks left by falling debris? Something else? Finding the answer will require detailed imaging from orbit.
The mission will also help scientists learn more about Mars. They believe the samples from Phobos may contain a significant share of Martian material thrown onto the moon by events such as meteorite impacts. Calculations suggest this material could account for up to 1% of the samples’ total volume.
If so, the samples from Phobos will be the first Martian material available to scientists on Earth. They might even contain traces of Martian life. Among those who have raised this possibility is the mission’s lead scientist, Kiyoshi Kuramoto, a planetary scientist at Hokkaido University: “I do think there’s a possibility that chemical traces of biological activity might be contained within that sand sample.”
The Martian Moons eXploration spacecraft has three modules. The mission includes a microwave-sized rover
The MMX spacecraft consists of three separate modules:
- A propulsion module: It will control the spacecraft and adjust its orbit during the journey to Mars and subsequent maneuvers
- A lander: It is intended to descend to the surface of Phobos
- A return module: It is intended to leave the surface of Phobos and fly back to Earth
Most of the scientific instruments are aboard the lander. The most important are the samplers, which are intended to collect about 10 grams of soil for scientists on Earth to study.
One of these instruments, the P-sampler, is designed to collect particles directly from the surface. It works much like a vacuum cleaner: First, a jet of nitrogen lifts the soil, and then the instrument sucks it inside. The second sampler, the C-sampler, is a miniature drill intended to obtain a sample from a depth of several centimeters.
The module also carries infrared and gamma-ray spectrometers to study the Martian moons’ mineral composition and search for ice and organic matter, along with high-resolution and wide-angle cameras to investigate the “red” and “blue” regions. MMX also carries a laser altimeter and cameras designed to capture images of Mars and its moons in 4K and even 8K.
A radiation monitor and instruments for studying dust will operate during the flight. The dust instruments will search for traces of comets and dust rings around the Red Planet.
Another important part of the mission is IDEFIX, a French-German four-wheeled mini-rover the size of a small microwave, weighing about 25 kilograms (55 pounds). Its job is to scout the landing area. It has its own instruments for this: a spectrometer to identify minerals, a radiometer to measure temperature and thermal properties, and stereo navigation cameras.
The mission is expected to last several years. If it succeeds, the return to Earth is scheduled for 2031
The spacecraft launched aboard an H3-24L rocket from Japan’s Tanegashima Space Center at 4:41 a.m. local time on October 20 (10:41 p.m. October 19 in Moscow).
The rest of the mission is expected to follow this schedule:
- August 2027: Arrival in the Mars system. MMX is to enter orbit around Mars, after which the propulsion module will separate. The spacecraft will then approach Phobos, enter its orbit, and begin imaging. At the same time, it will map spectral properties and study mineral composition to find a safe, scientifically interesting landing site.
- December 2028: Landing of the IDEFIX rover. This will be followed by about 100 Earth days of operations, including imaging and studies of minerals, temperature, and the soil’s mechanical properties. These data will help prepare for the larger spacecraft’s landing. Once mission planners select and assess a landing site, MMX will begin its descent. After touching down, it will collect samples from the surface and below it, then transfer them to a container in the return capsule. A second landing is planned to collect material from a different location. However, if the first sampling succeeds, the second landing may be canceled to avoid exposing the spacecraft and the samples already collected to further risk. After the landing operations, MMX will continue studying Phobos, Mars, and the surrounding space. It will then raise its orbit and make a series of flybys of Deimos.
- November 2030: Departure of the return module for Earth, carrying the sample capsule.
- July 2031: Arrival at Earth and landing in Australia.
Before arrival, the sample return capsule (SRC) will separate from the return module, enter the atmosphere, and descend by parachute. The spacecraft itself will fly past Earth.
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