Deimos (moon)
Asaph Hall III stood at the United States Naval Observatory in Washington, D.C. on the 12th of August 1877. He spotted Deimos at about 07:48 UTC that morning. Hall had been searching specifically for Martian moons when he found this small object. He discovered Phobos shortly after finding its sibling moon. The name came from a suggestion by academic Henry Madan. Madan drew inspiration from Book XV of the Iliad where Ares summons Dread and Fear. These figures correspond to the Greek names Deimos and Phobos. No other planetary moons received symbols until Denis Moskowitz proposed one decades later. His design combined a Greek delta with Mars's spear. This symbol remains rarely used today.
Scientists debate how these two moons formed around Mars. One theory suggests they are captured asteroids from the asteroid belt. C-type or D-type asteroids share similar compositions with Deimos. Atmospheric drag might have circularized their orbits over time. Geoffrey Landis proposed that binary asteroids separated due to tidal forces during capture. Another hypothesis states the moons accreted in their current positions. A collision with a planetesimal could have ejected many bodies into orbit. In 2021 Amirhossein Bagheri and his colleagues analyzed seismic data from the Mars InSight Mission. They proposed a common parent body shattered 1 to 2.7 billion years ago. This disruption created both Phobos and Deimos simultaneously.
Deimos measures approximately 15 kilometers across its longest dimension. It appears gray and highly non-spherical with triaxial dimensions. The moon is about 57% the size of Phobos. Its surface contains rock rich in carbonaceous material like C-type asteroids. Regolith covers much of the terrain and fills some craters partially. Radar estimates show regolith density at only 1.8 grams per cubic centimeter. Escape velocity reaches just 5.6 meters per second. A human could theoretically jump off this small world easily. Apparent magnitude sits at 12.45 making it faint to naked eyes.
Only two features bear official names on Deimos. Crater Swift spans one kilometer in diameter while Voltaire measures 1.9 kilometers wide. Both received approval in 1973 by international astronomical authorities. Jonathan Swift wrote Gulliver's Travels where he speculated about Martian moons. Voltaire also wrote essays predicting two satellites orbiting Mars before discovery. These literary figures inspired the naming convention for these geological scars. No other craters or mountains carry formal designations yet.
Deimos rises in the east and sets in the west unlike Phobos. Its orbital period takes 30.3 hours around Mars. This exceeds the Martian solar day of about 24.7 hours significantly. An equatorial observer sees 2.48 days pass between rising and setting events. Five point four six six days elapse from one rise to the next. The moon cannot be seen from latitudes greater than 82.7 degrees north or south. Tidal acceleration causes its orbit to slowly expand outward. Eventually Deimos will escape Mars gravity entirely. Its path lies nearly circular within Mars's equatorial plane.
Mars rover Opportunity photographed Deimos crossing the Sun on the 4th of March 2004. Spirit captured another transit event thirteen days later on the 13th of March 2004. The moon appears as a small black dot against the bright disk. It never causes total eclipses due to its tiny angular diameter. Venus transits appear roughly 2.5 times wider than this shadow. A Martian observer would see phases lasting 1.2648 days per cycle. Brightness reaches levels comparable to Vega during quarter phases. These visual events provide rare opportunities for scientific observation.
The Emirates Mars Mission sent close-up images in March 2023. April 2023 brought global views taken by the Hope orbiter for the first time. ISRO's Mars Orbiter Mission captured far side imagery using highly elliptical orbits. ESA's Hera spacecraft approached within 300 kilometers in March 2025. JAXA plans to launch the MMX mission in October 2026. This project includes flybys of both moons and sample return from Phobos. Conceptual missions like Hall and Gulliver aim to bring material back to Earth. Aladdin was selected as a finalist in NASA Discovery Program during 1997. No landings have occurred on Deimos despite multiple proposals.
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Common questions
Who discovered the moon Deimos and when did the discovery occur?
Asaph Hall III discovered the moon Deimos on the 12th of August 1877 while working at the United States Naval Observatory in Washington, D.C. He spotted the object at about 07:48 UTC that morning after searching specifically for Martian moons.
What is the origin of the name Deimos according to historical records?
The name came from a suggestion by academic Henry Madan who drew inspiration from Book XV of the Iliad where Ares summons Dread and Fear. These figures correspond to the Greek names Deimos and Phobos as described in classical literature.
How large is the moon Deimos compared to other celestial bodies?
Deimos measures approximately 15 kilometers across its longest dimension and appears gray with highly non-spherical triaxial dimensions. It is about 57% the size of Phobos and has an escape velocity reaching just 5.6 meters per second.
Which craters bear official names on the surface of Deimos?
Only two features bear official names on Deimos including Crater Swift which spans one kilometer in diameter and Voltaire measuring 1.9 kilometers wide. Both received approval in 1973 by international astronomical authorities based on literary figures Jonathan Swift and Voltaire.
When did Mars rovers capture images of Deimos transiting the Sun?
Mars rover Opportunity photographed Deimos crossing the Sun on the 4th of March 2004 while Spirit captured another transit event thirteen days later on the 13th of March 2004. The moon appears as a small black dot against the bright disk during these rare visual events.
What are the future plans for exploring the moon Deimos?
JAXA plans to launch the MMX mission in October 2026 which includes flybys of both moons and sample return from Phobos. ESA's Hera spacecraft approached within 300 kilometers in March 2025 and Aladdin was selected as a finalist in NASA Discovery Program during 1997.