Short answers, pulled from the story.
Deimos was discovered by Asaph Hall at the United States Naval Observatory in Washington, D.C., on the 12th of August 1877, at about 07:48 UTC. Hall was specifically searching for Martian moons at the time and also discovered Phobos shortly afterward.
Deimos orbits Mars at a distance of 23,460 kilometers, significantly farther than Mars's other moon, Phobos. Its nearly circular orbit takes 30.3 hours to complete.
The name Deimos was suggested by academic Henry Madan, who drew from Book XV of the Iliad, where Ares summons Deimos (Dread) and Phobos (Fear). Because Ares is the Greek counterpart of the Roman god Mars, naming the Martian moons after Ares's companions was considered fitting.
Deimos is composed of rock rich in carbonaceous material, similar in composition to C-type asteroids and carbonaceous chondrite meteorites. Its surface has a mean diameter of 12.5 kilometers and features craters partially filled with porous regolith that has a radar-estimated density of 1.471.
The main hypotheses are capture from the asteroid belt, in-place accretion, and disruption of a shared parent body with Phobos. In 2021, researchers from ETH Zurich and the US Naval Observatory proposed, based on Mars InSight Mission data, that Phobos and Deimos formed from a common parent body shattered between 1 and 2.7 billion years ago. Images from the Mars Hope orbiter released in April 2023 indicate a basaltic planetary origin for Deimos, contradicting the captured-asteroid hypothesis.
No landings on Deimos have been made. Several spacecraft have photographed it during Mars missions, including the Emirates Mars Mission in March 2023 and the ESA's Hera spacecraft in March 2025, which passed within 300 kilometers. The JAXA MMX mission, planned for launch in October 2026, will conduct flybys of Deimos to study its composition and structure.