Named for the Greek god of fear, Phobos is one of Mars' two moons (Deimos, named for the god of terror, is the other), and it's only about 13 miles (21 kilometers) across. Stickney Crater, the indentation on the moon's lower right, is about 5.6 miles (9 kilometers) wide in this HiRISE image. Despite its small size, Phobos is of great interest to scientists: Is it a captured asteroid, or a chunk of Mars that broke off after a massive impact? A Japanese mission is scheduled to launch to Phobos in the near future, and the moon has been proposed as a staging ground for astronauts before they go to Mars.
NASA