How big is Deimos?

Deimos is the smaller of the two moons of Mars. It is a small, lumpy body about 15 kilometers (9 miles) long, measured along its longest axis. It is not round, so its width depends on the direction it is measured in.

Size

NASA describes Deimos as about 15 by 12 by 11 kilometers (9 by 7 by 6.8 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. It models Deimos as an ellipsoid, a smooth, stretched ball, of 15.6 by 12.0 by 10.2 kilometers.

A ball with the same volume as Deimos would be about 12.4 kilometers across, according to NASA's Jet Propulsion Laboratory. Like Earth's Moon, Deimos always keeps the same side turned toward its planet. Its longest axis points toward Mars.

Size comparisons

Deimos is about 1.7 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 2.8 times the length of Deimos.
The other moon of Mars, Phobos, is about 27 kilometers (17 miles) long, 1.8 times as long as Deimos.
If Mars were shrunk to the size of a basketball, Deimos would be a speck about half a millimeter long. It would circle about 83 centimeters (33 inches) from the center of the ball.
At that scale, Phobos would be a speck about 1 millimeter long, circling only 33 centimeters (13 inches) from the ball's center.

A smooth, dusty surface

Deimos is dark and heavily cratered, like Phobos. Its largest crater is about 2.3 kilometers (1.4 miles) wide. Some of its craters are partly filled in, which makes Deimos look smoother than Phobos.

The gravity of Deimos is too weak to hold on to the dust and rock thrown up when something strikes it. The gravity of Mars keeps that debris in a ring near the orbit of Deimos, and the moon sweeps it up again as it travels. Meteorite impacts have also crushed its surface into a thick layer of loose rock and dust, called regolith, perhaps as deep as 100 meters (328 feet).

Orbit and discovery

Deimos circles Mars once every 30 hours, about 23,500 kilometers (14,600 miles) from the planet's center. Asaph Hall discovered it on August 11, 1877, six days before he found Phobos. He named the two moons after the sons of Ares, the Greek counterpart of the Roman god Mars. Deimos means dread.

Both moons appear to be made of material like that of the dark, carbon-rich asteroids in the outer asteroid belt. Scientists think they may be captured asteroids, or debris left over from early in the formation of the solar system.