The inner satellite of Mars, 9380 km from the planet's centre and only 6000 km above its surface. Phobos orbits in 0.319 days, faster than Mars rotates on its axis, keeping one face turned permanently towards the planet. Phobos is irregularly shaped, 26 × 23 × 18 km in size, and is covered with impact craters, the largest, Stickney, being 9 km in diameter. There are also several families of parallel grooves or chains of craters, possibly secondary craters from large impacts on Mars. Phobos was discovered in 1877 by A. Hall, and may be a captured asteroid.
Subjects: astronomy and cosmology.