This image from NASA's Mars Reconnaissance Orbiter shows one of the best views ever obtained of Mars' tiny moon Phobos. Phobos is the larger of the two moons of Mars. Both are small and irregular in shape. Astronomers believe that they were probably asteroids that were captured by the gravity of Mars. The large indentation on the right side of the moon in this image is known as Stickney crater. Phobos is only about 13 miles (21 kilometers) in diameter. It orbits so close to Mars that it will one day crash into the planet.