Why Phobos? Japan Targets Tiny Moon Over Mars

The Martian Moons eXploration (MMX) spacecraft was launched aboard the 10th H3 rocket from the Tanegashima Space Center in Kagoshima Prefecture at 4:41 a.m. on October 20. Approximately one hour and 11 minutes after liftoff, the spacecraft separated successfully from the rocket and entered a trajectory toward Mars.

Its destination is Phobos, one of Mars’ two natural satellites, which scientists believe could hold valuable evidence about the evolution of the planet and its surrounding environment.

Mars has two moons, Phobos and Deimos, both of which have irregular, potato-like shapes. Although much smaller than Earth’s moon, they have attracted scientific interest because their composition and origins remain poorly understood.

The central question behind Japan’s mission is why scientists would travel hundreds of millions of kilometers toward Mars, only to land on one of its moons rather than the planet itself.

The answer lies largely in the formidable technical challenges of landing on Mars and then launching a spacecraft back into space.

Mars has an atmosphere, meaning an approaching spacecraft must survive intense heating during atmospheric entry while slowing down sufficiently to land safely. The planet’s gravity, approximately 2.3 times stronger than that of Earth’s moon, also makes launching a return vehicle from its surface extremely demanding.

A mission designed to collect samples from Mars and transport them back to Earth therefore requires sophisticated landing systems, substantial quantities of fuel and enormous financial resources.

Phobos offers a significantly simpler alternative.

The moon has virtually no atmosphere, eliminating the need to protect a spacecraft from the intense heat associated with atmospheric entry. Its gravity is only about one-650th that of Mars, dramatically reducing the energy required to land and depart.

These characteristics make Phobos a particularly attractive destination for a sample-return mission, allowing Japan to pursue an objective that remains one of the most difficult challenges in planetary exploration.

The choice also plays directly to Japan’s strengths.

The country has developed internationally recognized expertise in collecting material from small celestial bodies through its Hayabusa asteroid exploration program.

The original Hayabusa spacecraft returned microscopic particles from asteroid Itokawa despite losing communication and encountering serious technical problems during its mission.

Hayabusa2 subsequently collected approximately 5.4 grams of sand and other material from asteroid Ryugu and successfully transported the samples back to Earth.

The technologies developed through these missions, particularly precision navigation, controlled landing and sample collection in extremely weak gravitational environments, have been passed on to MMX.

However, the scientific value of Phobos extends beyond the relative ease of reaching its surface.

Researchers believe the moon could serve as a natural repository of material originating from Mars itself.

Over billions of years, meteorite impacts may have blasted soil and rock fragments from the Martian surface into space. Some of this material could subsequently have fallen onto Phobos, becoming mixed with the moon’s own surface deposits.

If MMX successfully retrieves samples containing such fragments, scientists may be able to study evidence from Mars without having to land directly on the planet.

Yasuhiro Kawakatsu, project manager of MMX, said exploring Phobos could provide important clues about the history of the entire Martian system.

“Phobos is a place humanity has never reached. We may be able to discover how the Martian system evolved and where the water and organic materials found on Earth and elsewhere originally came from,” Kawakatsu said.

The mission could also help researchers better understand the formation of Mars’ moons and the processes that shaped the inner solar system.

Despite its advantages, Phobos is far from an easy destination.

Its surface is uneven, with numerous ridges, depressions and other obstacles that complicate the search for a safe landing site.

MMX will need to achieve landing accuracy within approximately 10 meters of its intended target to reach terrain suitable for collecting samples.

Navigation presents another major challenge. Because Phobos orbits close to Mars, the planet’s gravitational influence creates complicated forces that continuously affect the spacecraft, making stable positioning and precise maneuvering difficult.

Successfully landing, collecting material and departing will therefore demand extremely sophisticated spacecraft control.

The mission is also taking place during an intensifying international competition to retrieve samples from Mars and its surroundings.

The United States has already collected samples from the Martian surface using spacecraft that remain operational on the planet. However, plans to return those samples to Earth have faced major obstacles because of their enormous projected costs.

China is meanwhile planning a launch in 2028 for its own Mars sample-return program, raising the prospect of competing missions seeking to achieve historic firsts in planetary exploration.

Japan’s strategy differs from those of its rivals by targeting a smaller celestial body within the Martian system rather than attempting the more demanding task of returning directly from Mars.

If successful, MMX would become the first spacecraft in human history to return samples from the Martian system, extending Japan’s record of achievements in asteroid exploration.

The spacecraft is scheduled to return to Earth in 2031, bringing back material that could help scientists reconstruct the history of Mars and investigate the origins of substances essential to life.

For Japan, the decision to target Phobos is therefore more than a way to simplify an extraordinarily difficult mission. It represents an effort to combine established technological strengths with a scientific opportunity that could reveal some of Mars’ secrets without ever setting foot on the planet itself.

Source: TBS