Phobos is the larger of the two moons of Mars. It is a dark, lumpy body about 27 kilometers (17 miles) long, measured along its longest axis. It circles Mars three times a day, so close to the planet that it can never be seen from some places on Mars.

Size

Phobos is not round: NASA describes it as about 27 by 22 by 18 kilometers (17 by 14 by 11 miles). These are its length, width and thickness, measured along three lines at right angles to each other. NASA's Mars fact sheet gives slightly different figures, because it treats Phobos as an ellipsoid, a smooth, stretched ball: 26.0 by 22.8 by 18.2 kilometers.

A ball with the same volume as Phobos would be about 22.2 kilometers across, according to NASA's Jet Propulsion Laboratory. Like Earth's Moon, Phobos always keeps the same side turned toward its planet. Its longest axis points toward Mars.

Size comparisons

• Phobos is about three times as long as Mount Everest is tall. Everest rises 8,849 meters (29,032 feet) above sea level.
• A marathon race of 42.195 kilometers (26.2 miles) is about 1.6 times the length of Phobos.
• A neutron star, the collapsed core of a massive star, is about 24–26 kilometers (15–16 miles) across, a little less than the length of Phobos. Yet a typical one has about 260 trillion times the mass of Phobos.
• Phobos is 1.8 times as long as Deimos, the other moon of Mars, which is about 15 kilometers (9 miles) long.
• If Mars were shrunk to the size of a basketball, Phobos would be a speck about 1 millimeter long. It would circle only 21 centimeters (8 inches) above the ball's surface.

A moon moving toward Mars

Phobos orbits about 6,000 kilometers (3,700 miles) above the surface of Mars. It goes around the planet once every 7 hours 39 minutes, about three times in a single Earth day.

Its orbit is slowly shrinking. NASA reports that Phobos moves about 1.8 meters (6 feet) closer to Mars every hundred years. NASA expects that in about 50 million years it will either crash into Mars or break up into a ring.

Stickney crater and the surface

A giant impact nearly shattered Phobos, and thousands of smaller impacts have gouged its surface. The most prominent feature is Stickney crater, about 9 kilometers (6 miles) across, a third of the moon's length. It is named after Chloe Angeline Stickney Hall, a mathematician.

Countless small impacts have pounded the surface into fine dust. That dust probably explains why Phobos loses heat so fast. Temperatures reach about −4 degrees Celsius (25 degrees Fahrenheit) on the sunlit side and fall to −112 degrees Celsius (−170 degrees Fahrenheit) on the dark side.

Discovery and origin

Chloe Stickney Hall's husband, the astronomer Asaph Hall, discovered Phobos on August 17, 1877. He named it after a son of Ares, the Greek counterpart of the Roman god Mars. Phobos means fear.

Like Deimos, Phobos appears to be made of dark rock similar to carbon-rich asteroids. It may be an asteroid that Mars captured, but some scientists have found evidence against this idea.

Sources

• Phobos. NASA Science, 2024.
• Deimos. NASA Science, 2024.
• Mars Fact Sheet. NASA, 2025.
• Planetary Satellite Physical Parameters. NASA Jet Propulsion Laboratory.
• NASA's NICER Probes the Squeezability of Neutron Stars. NASA, 2021.
• The Radius of PSR J0740+6620 from NICER and XMM-Newton Data. Miller et al., The Astrophysical Journal Letters, 2021.
• Sun Fact Sheet. NASA, 2024.
• 8,848.86 metres. That's the height of Mt Everest now. The Kathmandu Post, 2020.
• Marathon. World Athletics.
• Official Basketball Rules 2026: Basketball Equipment. FIBA, 2026.