Deimos was discovered by Asaph Hall at the United States Naval Observatory in Washington, D.C., on 12 August 1877, at about 07:48 UTC.

[a] Hall, who also discovered Phobos shortly afterwards, had been specifically searching for Martian moons at the time.

[11] The name was suggested by academic Henry Madan, who drew from Book XV of the Iliad, where Ares (Greek counterpart of the Roman god Mars) summons Dread (Deimos) and Fear (Phobos).

Because of the postulated similarity to the composition of C- or D-type asteroids, one hypothesis is that the moons may be objects captured into Martian orbit from the asteroid belt, with orbits that have been circularized either by atmospheric drag or tidal forces,[20] as capture requires dissipation of energy.

[19] Geoffrey Landis has pointed out that the capture could have occurred if the original body was a binary asteroid that separated due to tidal forces.

Another hypothesis is that Mars was once surrounded by many Phobos- and Deimos-sized bodies, perhaps ejected into orbit around it by a collision with a planetesimal.

The craters Swift and Voltaire are named after writers who speculated on the existence of two Martian moons before Phobos and Deimos were discovered.

Deimos is possibly an asteroid that was perturbed by Jupiter into an orbit that allowed it to be captured by Mars, though this hypothesis is still controversial and disputed.

[19] Both Deimos and Phobos have very circular orbits which lie almost exactly in Mars's equatorial plane, and hence a capture origin requires a mechanism for circularizing the initially highly eccentric orbit, and adjusting its inclination into the equatorial plane, most likely by a combination of atmospheric drag and tidal forces;[20] it is not clear that sufficient time was available for this to have occurred for Deimos.

[41] In 2008, NASA Glenn Research Center began studying a Phobos and Deimos sample-return mission that would use solar electric propulsion.

Recently, it was proposed that the sands of Deimos or Phobos could serve as a valuable material for aerobraking in the colonization of Mars.

[citation needed] In April 2023, astronomers released close-up global images, for the first time, of Deimos that were taken by the Mars Hope orbiter.

[48][49] Observations reported by this mission contravene the captured asteroid hypothesis and indicate basaltic planetary origin of Deimos.

[51] It will make flybys of Deimos to investigate its composition and structure, as well as performing a sample return on Phobos and placing a rover on that moon.