Phobos
Phobos is the larger and closer of Mars' two moons. It is a small, irregularly shaped body that is slowly spiraling toward Mars and will eventually be destroyed.
Phobos is the larger and closer of Mars’ two moons. This small, irregularly shaped body is believed to be a captured asteroid from the asteroid belt, and it has a fascinating and doomed fate ahead of it.
A Captured Asteroid
Phobos is thought to be a D-type asteroid that was captured by Mars’ gravity billions of years ago. Its composition is similar to asteroids found in the outer asteroid belt, and its irregular shape suggests it has not been significantly altered since its capture.
The moon orbits Mars at a very close distance—closer than any other moon to its planet in the solar system. Phobos completes an orbit around Mars in just 7 hours and 39 minutes, which is faster than Mars rotates on its axis.
Doomed Fate
Phobos is slowly spiraling inward toward Mars due to tidal forces. In approximately 30-50 million years, the moon will either collide with Mars or be torn apart by tidal forces to form a ring system around the planet.
This orbital decay is causing Phobos to rise in Mars’ sky over time. Currently, Phobos appears about one-third the size of Earth’s Moon as seen from Mars, but this will increase as it gets closer.
Surface Features
Phobos has a heavily cratered surface with one dominant feature: the large Stickney Crater, which is about 9 km wide—nearly half the width of Phobos itself. This crater was formed by a massive impact and has grooves radiating outward from it.
The surface is covered in regolith (loose material) that is so thick that it smothers many smaller craters. Phobos also has several linear grooves that may be fractures from the Stickney impact or tidal stresses.
Exploration
Phobos has been imaged by several Mars-orbiting spacecraft, including Mars Global Surveyor, Mars Express, and Mars Reconnaissance Orbiter. NASA’s upcoming Mars Sample Return mission may include a Phobos sample collection component.
The moon’s proximity to Mars makes it a potential target for future human exploration. Its resources could be mined, and its low gravity (about 0.0058 m/s²) makes it accessible for spacecraft operations.
Phobos represents a unique opportunity to study a captured asteroid in detail and understand the processes that shape small bodies in the solar system.