Deimos (moon)

• 1. Introduction

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  Deimos is the smaller and outer of the two moons of Mars, and it carries the name of dread itself. In Greek mythology, Deimos personified terror, a companion to Ares on the battlefield. The moon orbiting Mars today is small enough that a person jumping straight up could theoretically reach its escape velocity of 5.6 meters per second. That figure raises a question worth sitting with: what exactly are we dealing with when we look at Deimos? How did it get there? Who found it, and when? And what would it look like from the rusty plains below, rising in the east against a salmon sky?

• 2. Discovery and Naming

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  Asaph Hall spotted Deimos on the 12th of August 1877, at around 07:48 UTC, from the United States Naval Observatory in Washington, D.C. He had set out deliberately to find Martian moons, and he succeeded twice in quick succession, locating Phobos shortly after. The name Deimos was not Hall's suggestion. Henry Madan, an academic, proposed it by drawing on Book XV of the Iliad, the ancient Greek epic, where Ares calls upon Deimos (Dread) and Phobos (Fear) as companions. Because Ares is the Greek counterpart of the Roman god Mars, the pairing was fitting. A software engineer named Denis Moskowitz, who designed most of the dwarf planet symbols, later proposed a symbol for Deimos: a Greek delta combined with Mars's spear. The symbol has not seen wide adoption.

• 3. A Moon Written About Before It Was Found

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  Two craters on Deimos are named for writers who guessed Mars had two moons before anyone confirmed it. The crater Swift honors Jonathan Swift, the Irish writer who lived from 1667 to 1745, and the crater Voltaire honors the French writer Voltaire, who lived from 1694 to 1778. Both craters received their names in 1973. Swift is 1 kilometer in diameter; Voltaire is 1.9 kilometers. The fact that the only named features on Deimos commemorate literary speculation rather than scientific discovery says something about how the moon captured the imagination long before any telescope brought it into focus.

• 4. Physical Shape and Surface

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  Deimos has a mean radius of 6.2 kilometers, giving it a mean diameter of 12.5 kilometers. That makes it roughly 57% the size of Phobos. Its shape is highly non-spherical, with triaxial dimensions that set it apart from a simple ball. The surface is gray and composed of rock rich in carbonaceous material, similar in composition to C-type asteroids and carbonaceous chondrite meteorites. Craters mark the surface, but it is noticeably smoother than Phobos. The reason is regolith, the loose surface material that has partially filled in those craters over time. That regolith is highly porous, with a radar-estimated density of only 1.471. The apparent magnitude of Deimos from a distance is 12.45.

• 5. Orbit and What Mars Sees

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  Deimos orbits Mars at a distance of 23,460 kilometers, completing one orbit in 30.3 hours. Its path is nearly circular and sits close to Mars's equatorial plane. Unlike Phobos, which races around Mars so quickly that it rises in the west and sets in the east, Deimos rises in the east and sets in the west. Its orbital period barely exceeds the Martian solar day of about 24.7 hours, which means an equatorial observer on Mars would wait 2.48 days between its rising and its setting. From Deimos-rise to the next Deimos-rise takes 5.466 days. Because it cannot be seen from Martian latitudes greater than 82.7 degrees, large stretches of the poles would never glimpse it at all. At its brightest, from the Martian surface, Deimos would appear about as bright as Venus looks from Earth. Observers with a small telescope could watch its phases cycle over 1.2648 days. When it passes in front of the Sun, it does not block enough light to cause a total eclipse; it appears only as a small black dot, with an angular diameter about 2.5 times that of Venus during a transit of Venus as seen from Earth. Mars rovers Opportunity and Spirit both photographed transits of Deimos, on the 4th and the 13th of March 2004 respectively. Deimos's orbit is slowly expanding. It is far enough from Mars that tidal acceleration is pushing it outward, and it is expected to eventually escape Mars's gravity entirely.

• 6. Disputed Origins

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  Where Deimos came from is a genuinely open question, and the leading hypotheses disagree sharply. One possibility is capture: Deimos may have been an asteroid perturbed by Jupiter into an orbit that Mars's gravity then claimed. Its composition, similar to C- or D-type asteroids, supports this idea. The problem is that the current Martian atmosphere is too thin to slow down a body the size of Phobos through atmospheric braking. Geoffrey Landis has suggested that capture could have worked if the original object was a binary asteroid that was torn apart by tidal forces. The other main hypothesis is that Deimos accreted where it is, forming in place. A third possibility holds that Mars was once surrounded by many moon-sized bodies, knocked into orbit by a collision with a planetesimal. In 2021, researchers Amirhossein Bagheri and Amir Khan of ETH Zurich, together with Michael Efroimsky of the US Naval Observatory and their colleagues, offered a new analysis. Working from seismic and orbital data gathered by the Mars InSight Mission and other missions, they proposed that Phobos and Deimos both originated from the disruption of a single parent body between 1 and 2.7 billion years ago. Something struck that common progenitor and shattered it. Images released in April 2023 from the Mars Hope orbiter added a complication: observations from that mission indicate that Deimos may have a basaltic, planetary origin rather than an asteroid origin, which contradicts the captured-asteroid hypothesis.

• 7. Missions and Future Plans

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  Deimos has been photographed close-up by several spacecraft sent primarily to study Mars. In March 2023, the Emirates Mars Mission made a rare close encounter with Deimos, capturing global images of the moon that were released in April 2023. The Indian Space Research Organisation's Mars Orbiter Mission photographed the far side of Deimos, a face rarely seen because most Mars missions orbit closer to Mars than Deimos does and therefore only observe the Mars-facing side. The highly elliptical orbit of MOM was the exception that made those images possible. In March 2025, the European Space Agency's Hera spacecraft observed Deimos during a gravity assist from Mars on its way to the asteroid 65803 Didymos, passing within 300 kilometers. Proposals for a dedicated sample-return mission have circulated for decades. In 1997 and 1998, a mission called Aladdin reached finalist status in the NASA Discovery Program. The plan called for launching projectiles at both Phobos and Deimos, then collecting the debris during a slow flyby at roughly 1 kilometer per second, and returning those samples to Earth three years later. The principal investigator was Carle M. Pieters of Brown University. Total mission cost, including launch vehicle and operations, was estimated at $247.7 million. The mission chosen to fly instead was MESSENGER, the probe to Mercury. A separate concept called Gulliver would have returned 1 kilogram of Deimos material to Earth. The JAXA MMX mission, planned for launch in October 2026, will make flybys of Deimos to study its composition and structure while also placing a rover on Phobos.