Phobos and Deimos are small Martian moons with irregular shapes and no hydrostatic equilibrium. Phobos orbits very close to Mars and has strange grooves and a large crater named Stickney. The moons were observed in transit by rovers on the Martian surface.
Phobos and Deimos are Martian moons. They are among the smallest known moons in the solar system, with mean diameters of 22.2 km and 12.6 km, respectively. Phobos orbits very close to the Martian surface, with an altitude of only 6000 km (3728 mi). This is elevated compared to say, the International Space Station, which orbits about 350 km (217 mi) above the Earth’s surface, but is the closest orbiting moon of any major planet known to date. Phobos orbits Mars every seven and a half hours, comparable to the International Space Station orbiting Earth every hour and a half.
From the Martian surface, Phobos can be observed in transit towards the Sun, covering about 20% of the disk. Deimos, similar in size to Phobos but significantly more distant, orbiting 23.460 km (14.577 mi) above the Martian surface, appears only as a black dot transiting towards the Sun, which it does regularly. Both transit events were observed by rovers on the surface of Mars.
Phobos and Deimos are not massive enough to be in hydrostatic equilibrium, i.e. spherical. Phobos is so irregularly shaped and deep in Mars’ gravity well that the gravity on its surface varies by up to 450%. Its escape velocity is only 11 m/s – this is too high for a human to jump from its surface, as the fastest human jumps are only about 2 m/s – but a weak rocket or even a spring spiral would be sufficient. You could even create a ramp and drive a car fast enough to reach escape velocity, as long as you could supply oxygen to your combustion engine.
Phobos is covered in strange grooves and a large crater named Stickney, after the wife of its discoverer, Asaph Hall, chief astronomer of the United States Naval Observatory in the late 19th century. Surveying the Martian moons required a 19cm/66in telescope, the largest in the world at the time. The highest resolution images of Phobos and Deimos to date were taken by the Mars Global Surveyor, which entered Martian orbit in 2020 and lost contact in 1997.
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