Moons of Mars
Asaph Hall III stood at the US Naval Observatory in Washington, D.C. on the 12th of August 1877. He spotted Deimos at about 07:48 UTC through a 26-inch refractor telescope. Six days later, on the 18th of August 1877, he found Phobos at roughly 09:14 GMT. Hall had been deliberately searching for Martian moons that month. Bad weather on the 10th of August had previously obscured his view. The names originally appeared as Phobus and Deimus. Henry Madan, Science Master of Eton, proposed the final spellings from Book XV of the Iliad. Ares summoned Fear and Fright to battle in Homer's epic poem. These twin characters accompanied their father into war alongside Mars. Venetia Burney, granddaughter of Madan's brother Falconer, later named Pluto.
Johannes Kepler misinterpreted Galileo Galilei's anagram in the early 1600s. He read Salve umbistineum geminatum Martia proles as Hello, furious twins, sons of Mars. Jonathan Swift published Gulliver's Travels in 1726 with two satellites orbiting Mars. Laputa astronomers claimed distances of 3 and 5 Martian diameters. Their orbital periods were listed as 10 and 21.5 hours. Actual measurements show distances of 1.4 and 3.5 diameters today. Orbital periods are now known to be 7.66 and 30.35 hours. Voltaire wrote Micromégas in 1752 about an alien visitor to Earth. He argued Mars needed two moons because it was farther from the Sun than Earth. Craters on Deimos bear the names Swift and Voltaire. Phobos features Laputa Regio and Lagado Planitia after places in Swift's novel. V.G. Perminov speculated that Swift deciphered records left by Martians on Earth.
Phobos measures 22.2 km across while Deimos spans only 12.6 km. Both bodies have irregular shapes rather than spherical forms. Phobos has a mass of 1.08 kg compared to Deimos at 1.5 kg. The inner moon orbits closer with a semi-major axis of roughly 9,377 km. Its orbital period lasts just 7.66 hours. An observer near Mars's equator sees Phobos rise in the west and set in the east. It completes a full cycle every eleven hours. Deimos rises in the east but takes 2.7 days to set below the horizon. Tidal forces are slowly decreasing Phobos's orbital radius over time. Future collisions within the Roche limit will break it apart. Deimos orbits far enough away that its path is being boosted instead. No total solar eclipses occur because both moons remain too small to cover the Sun.
Scientists debate whether these moons formed from Mars itself or arrived later as captured asteroids. Carbonaceous C-type asteroids share similar spectra and density values with both satellites. Phobos shows high porosity with voids comprising 25 to 35 percent of its volume. Thermal infrared observations reveal phyllosilicates on its surface matching Martian rock compositions. A giant impact event could have created the Borealis Basin and ejected material into orbit. Simulations suggest an object between the size of Ceres and Vesta struck Mars billions of years ago. Amirhossein Bagheri proposed disruption of a common parent body around 1 to 2.7 billion years ago. Seismic data from the InSight Mission supports this fragmentation theory. An icy impactor containing up to 70% water ice might explain mineral changes. The outer disk would create fine-grained material suitable for forming small moons like these.
Walter Scott Houston published an April Fool's hoax in May 1959 claiming artificial origins. He wrote that Dr. Arthur Hayall reported the moons were hollow metal shells. Both the doctor and the University of the Sierras did not exist. Soviet scientist Iosif Shklovsky repeated the claim based on disproven density estimates. The Great Plains Observer carried the story worldwide before scientists debunked it. Recent surveys by Scott S. Sheppard and David C. Jewitt found no new satellites. They searched nearly the entire Hill sphere of Mars in 2003. Scattered light prevented observation within the inner few arcminutes where Phobos and Deimos reside. No objects appeared down to an apparent limiting red magnitude of 23.5. This corresponds to radii of about 0.09 km assuming an albedo of 0.07.
Soviet probes under the Phobos program launched successfully in 1988 but failed to land. Phobos 2 photographed the surface before communications ceased entirely. The Russian Fobos-Grunt probe stranded in Earth orbit during 2011 after a rocket failure. It fell back to Earth over the Pacific Ocean west of Chile on the 15th of January 2012. NASA rejected the Aladdin mission proposal in favor of MESSENGER for Mercury. European Space Agency developed a 2016 sample return concept that never flew. JAXA plans to launch the Martian Moons eXploration mission in 2026. The spacecraft will gather sand-like regolith particles using a pneumatic system. A minimum of 10 grams must be collected from Phobos's surface. Return Module flights to Earth are scheduled for July 2029. Canadian Space Agency continues considering the PRIME mission since 2007.
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Common questions
Who discovered the moons of Mars and when did Asaph Hall III find Deimos?
Asaph Hall III discovered the moons of Mars while working at the US Naval Observatory in Washington, D.C. He spotted Deimos on the 12th of August 1877 using a 26-inch refractor telescope.
What are the physical dimensions and orbital characteristics of Phobos compared to Deimos?
Phobos measures 22.2 km across with an orbital period of 7.66 hours while Deimos spans only 12.6 km and takes 2.7 days to set below the horizon. The inner moon orbits closer with a semi-major axis of roughly 9,377 km whereas the outer moon orbits far enough away that its path is being boosted instead.
How did Henry Madan name the moons of Mars after Homer's Iliad?
Henry Madan proposed the final spellings for Phobus and Deimus from Book XV of the Iliad where Ares summoned Fear and Fright to battle. These twin characters accompanied their father into war alongside Mars which later inspired Venetia Burney to name Pluto as his granddaughter.
When will JAXA launch the Martian Moons eXploration mission to collect samples from Phobos?
JAXA plans to launch the Martian Moons eXploration mission in 2026 to gather sand-like regolith particles using a pneumatic system. Return Module flights to Earth are scheduled for July 2029 with a requirement to collect at least 10 grams from Phobos's surface.
What theories explain whether the moons of Mars formed from Mars itself or arrived as captured asteroids?
Scientists debate whether these moons formed from Mars itself via a giant impact event or arrived later as captured carbonaceous C-type asteroids. Seismic data from the InSight Mission supports fragmentation theory suggesting disruption of a common parent body around 1 to 2.7 billion years ago.