Moons of Mars

Discovery

The two moons of Mars, Phobos and Deimos, were discovered by the American astronomer Asaph Hall in August 1877. He found them at the United States Naval Observatory in Washington, D.C., using a large refracting telescope during a close approach of Mars to Earth.

Hall discovered Deimos first on 12 August 1877, followed by Phobos on 18 August 1877. He had been carefully searching for moons around Mars, as it had long been suspected that the planet might have satellites, but none had been confirmed before.

The moons are very small and orbit close to Mars, which makes them difficult to see because they are often hidden in the planet’s bright glare. By carefully tracking their movement over several nights, Hall was able to confirm that they were natural satellites orbiting Mars.

Names and meaning

The moons of Mars, Phobos and Deimos, are named after figures from Greek mythology. They are the sons of Ares, the Greek god of war (known as Mars in Roman mythology), who often accompanied him into battle.

The name Phobos means “fear” or “panic”, while Deimos means “terror” or “dread”. In mythology, they were seen as personifications of the emotions that soldiers feel during war.

Both names were suggested shortly after their discovery by Asaph Hall in 1877. The suggestion came from the British schoolmaster Henry Madan, who based them on a passage from Homer’s Iliad, where Ares calls upon Fear and Terror.

Orbits and motions

Phobos and Deimos both orbit Mars in nearly circular paths and travel close to the planet's equator. Like Earth's Moon, they are tidally locked, which means they always keep the same side facing Mars.

Phobos is much closer to Mars than Deimos. It orbits the planet about once every 7.7 hours, making it the fastest-known natural satellite in the Solar System in relation to the rotation of its planet. Because it moves around Mars faster than Mars rotates, Phobos appears to rise in the west and set in the east. It can cross the sky in about four hours and may be seen twice during a Martian day.

Deimos is farther from Mars and takes about 30.3 hours to complete one orbit. Since it orbits more slowly than Mars rotates, it appears to rise in the east and set in the west, like Earth's Moon. However, because its orbit is only slightly longer than a Martian day, it moves very slowly across the sky.

Phobos is gradually moving closer to Mars because of tidal forces. Scientists think that, in the distant future, it may break apart and form a ring around Mars or crash into the planet. Deimos is slowly moving farther away from Mars.