Δείμο is one of the two moons of the planet Mars, the other being Phobos. It was discovered in 1877 by American astronomer Asaph Hall. Δείμο is smaller than Phobos, with a diameter of about 12.4 kilometers (7.7 miles). Its surface is covered in a layer of dust and rocks, giving it a dark appearance. The name Δείμο comes from Greek mythology, where he is the personification of terror and dread. The moon orbits Mars at a distance of approximately 23,460 kilometers (14,580 miles) and takes about 30.3 hours to complete one orbit, which is longer than a Martian day.