Phobos (moon)

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Phobos colour 2008.jpg
Enhanced-color image of Phobos from the Mars Reconnaissance Orbiter with Stickney crater on the right
Discovered by Asaph Hall
Discovery date17 August 1877
Mars I
Adjectives Phobian, Phobosian
Orbital characteristics
Epoch J2000
Periapsis 9234.42 km [1]
Apoapsis 9517.58 km [1]
9376 km [1] (2.76 Mars radii)
Eccentricity 0.0151 [1]
0.31891023  d
(7 h 39.2 min)
Average orbital speed
2.138 km/s [1]
Inclination 1.093° (to Mars's equator)
0.046° (to local Laplace plane)
26.04° (to the ecliptic)
Satellite of Mars
Physical characteristics
Dimensions27 × 22 × 18 km [1]
Mean radius
11.2667 km
(1.76941  mEarths)
1548.3 km2 [1]
(3.03545 µEarths)
Volume 5783.61 km3
(5.33933  nEarths)
Mass 1.0659×1016 kg [1]
(1.78477 nEarths)
Mean density
1.876 g/cm3 [1]
0.0057 m/s2 [1]
(581.4 µ g)
11.39 m/s
(41 km/h) [1]
Equatorial rotation velocity
11.0 km/h (6.8 mph) (at longest axis)
Albedo 0.071±0.012 [2]
Temperature 233 K
11.8 [3]

    Phobos ( /ˈfbəs/ FOH-bəs, /-bɒs/ -boss, [4] from Greek Φόβος'; systematic designation: Mars I) is the innermost and larger of the two natural satellites of Mars, [5] the other being Deimos. Both moons were discovered in 1877 by American astronomer Asaph Hall.

    Greek language language spoken in Greece, Cyprus and Southern Albania

    Greek is an independent branch of the Indo-European family of languages, native to Greece, Cyprus and other parts of the Eastern Mediterranean and the Black Sea. It has the longest documented history of any living Indo-European language, spanning more than 3000 years of written records. Its writing system has been the Greek alphabet for the major part of its history; other systems, such as Linear B and the Cypriot syllabary, were used previously. The alphabet arose from the Phoenician script and was in turn the basis of the Latin, Cyrillic, Armenian, Coptic, Gothic, and many other writing systems.

    In ancient times, only the Sun and Moon, a few stars, and the most easily visible planets had names. Over the last few hundred years, the number of identified astronomical objects has risen from hundreds to over a billion, and more are discovered every year. Astronomers need to be able to assign systematic designations to unambiguously identify all of these objects, and at the same time give names to the most interesting objects and, where relevant, features of those objects.


    Phobos is a small, irregularly shaped object with a mean radius of 11 km (7 mi) [1] and is seven times as massive as the outer moon, Deimos. Phobos is named after the Greek god Phobos, a son of Ares (Mars) and Aphrodite (Venus) and the personification of fear (cf. phobia).

    Greek mythology body of myths originally told by the ancient Greeks

    Greek mythology is the body of myths originally told by the ancient Greeks. These stories concern the origin and the nature of the world, the lives and activities of deities, heroes, and mythological creatures, and the origins and significance of the ancient Greeks' own cult and ritual practices. Modern scholars study the myths in an attempt to shed light on the religious and political institutions of ancient Greece and its civilization, and to gain understanding of the nature of myth-making itself.

    Phobos (mythology) personification of fear in Greek mythology

    Phobos is the personification of fear in Greek mythology. He is the offspring of Aphrodite and Ares. He was known for accompanying Ares into battle along with the ancient war goddess Enyo, the goddess of discord Eris, and Phobos' twin brother Deimos (terror).

    Ares Ancient Greek god of war

    Ares is the Greek god of war. He is one of the Twelve Olympians, the son of Zeus and Hera. In Greek literature, he often represents the physical or violent and untamed aspect of war, in contrast to his sister, the armored Athena, whose functions as a goddess of intelligence include military strategy and generalship.

    Phobos orbits 6,000 km (3,700 mi) from the Martian surface, closer to its primary body than any other known planetary moon. It is so close that it orbits Mars much faster than Mars rotates, and completes an orbit in just 7 hours and 39 minutes. As a result, from the surface of Mars it appears to rise in the west, move across the sky in 4 hours and 15 minutes or less, and set in the east, twice each Martian day.

    Primary (astronomy) main physical body of a gravitationally bound, multi-object system

    A primary is the main physical body of a gravitationally bound, multi-object system. This object constitutes most of that system's mass and will generally be located near the system's barycenter.

    Natural satellite astronomical body that orbits a planet

    A natural satellite or moon is, in the most common usage, an astronomical body that orbits a planet or minor planet.

    Phobos is one of the least reflective bodies in the Solar System, with an albedo of just 0.071. Surface temperatures range from about −4 °C (25 °F) on the sunlit side to −112 °C (−170 °F) on the shadowed side. [6] The defining surface feature is the large impact crater, Stickney, which takes up a substantial proportion of the moon's surface. In November 2018, astronomers concluded that the many grooves on Phobos were caused by boulders ejected from the asteroid impact that created Stickney crater that rolled around on the surface of the moon. [7] [8]

    Solar System planetary system of the Sun

    The Solar System is the gravitationally bound planetary system of the Sun and the objects that orbit it, either directly or indirectly. Of the objects that orbit the Sun directly, the largest are the eight planets, with the remainder being smaller objects, such as the five dwarf planets and small Solar System bodies. Of the objects that orbit the Sun indirectly—the moons—two are larger than the smallest planet, Mercury.

    Albedo ratio of reflected radiation to incident radiation

    Albedo is the measure of the diffuse reflection of solar radiation out of the total solar radiation received by an astronomical body. It is dimensionless and measured on a scale from 0 to 1.

    Impact crater Circular depression on a solid astronomical body formed by a hypervelocity impact of a smaller object

    An impact crater is an approximately circular depression in the surface of a planet, moon, or other solid body in the Solar System or elsewhere, formed by the hypervelocity impact of a smaller body. In contrast to volcanic craters, which result from explosion or internal collapse, impact craters typically have raised rims and floors that are lower in elevation than the surrounding terrain. Impact craters range from small, simple, bowl-shaped depressions to large, complex, multi-ringed impact basins. Meteor Crater is a well-known example of a small impact crater on Earth.

    Images and models indicate that Phobos may be a rubble pile held together by a thin crust, and that it is being torn apart by tidal interactions. [9] Phobos gets closer to Mars by about 2 meters every one hundred years, and it is predicted that within 30 to 50 million years it will either collide with the planet, or break up into a planetary ring. [6]

    Rubble pile celestial body that is not a monolith

    In astronomy, a rubble pile is a celestial body that is not a monolith, consisting instead of numerous pieces of rock that have coalesced under the influence of gravity. Rubble piles have low density because there are large cavities between the various chunks that make them up.

    Crust (geology) The outermost solid shell of a rocky planet, dwarf planet, or natural satellite

    In geology, the crust is the outermost solid shell of a rocky planet, dwarf planet, or natural satellite. It is usually distinguished from the underlying mantle by its chemical makeup; however, in the case of icy satellites, it may be distinguished based on its phase.

    Tidal acceleration

    Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite, and the primary planet that it orbits. The acceleration causes a gradual recession of a satellite in a prograde orbit away from the primary, and a corresponding slowdown of the primary's rotation. The process eventually leads to tidal locking, usually of the smaller first, and later the larger body. The Earth–Moon system is the best studied case.


    Phobos was discovered by astronomer Asaph Hall on 18 August 1877, at the United States Naval Observatory in Washington, D.C., at about 09:14 Greenwich Mean Time (contemporary sources, using the pre-1925 astronomical convention that began the day at noon, give the time of discovery as 17 August at 16:06 Washington mean time). [10] [11] [12] Hall had discovered Deimos, Mars's other moon, a few days earlier on 12 August 1877 at about 07:48 UTC. The names, originally spelled Phobus and Deimus respectively, were suggested by Henry Madan (1838–1901), Science Master of Eton, based on Greek mythology, in which Phobos is a companion to the god Ares. [13] [14]

    Asaph Hall American astronomer

    Asaph Hall III was an American astronomer who is most famous for having discovered the moons of Mars, Deimos and Phobos, in 1877. He determined the orbits of satellites of other planets and of double stars, the rotation of Saturn, and the mass of Mars.

    United States Naval Observatory scientific agency in the United States

    The United States Naval Observatory (USNO) is one of the oldest scientific agencies in the United States, with a primary mission to produce Positioning, Navigation and Timing (PNT) for the United States Navy and the United States Department of Defense. Located in Northwest Washington, D.C. at the Northwestern end of Embassy Row, it is one of the pre-1900 astronomical observatories located in an urban area; at the time of its construction, it was far from the light pollution thrown off by the (then-smaller) city center. Former USNO director Gernot M. R. Winkler initiated the "Master Clock" service that the USNO still operates, and which provides precise time to the GPS satellite constellation run by the United States Air Force. The USNO performs radio VLBI-based positions of quasars with numerous global collaborators, in order to produce Earth Orientation parameters.

    Washington, D.C. Capital of the United States

    Washington, D.C., formally the District of Columbia and commonly referred to as Washington or D.C., is the capital of the United States. Founded after the American Revolution as the seat of government of the newly independent country, Washington was named after George Washington, the first President of the United States and a Founding Father. As the seat of the United States federal government and several international organizations, Washington is an important world political capital. The city is also one of the most visited cities in the world, with more than 20 million tourists annually.

    Physical characteristics

    Phobos-surface temperatures (THEMIS)
    29 September 2017
    130–270 K (−143 – −3 °C; −226–26 °F)
    24 April 2019
    200–300 K (−73–27 °C; −100–80 °F)

    Phobos has dimensions of 27 km × 22 km × 18 km, [1] and retains too little mass to be rounded under its own gravity. Phobos does not have an atmosphere due to its low mass and low gravity. [15] It is one of the least reflective bodies in the Solar System, with an albedo of about 0.071. [2] Infrared spectra show that it has carbon-rich material found in carbonaceous chondrites. Instead, its composition shows similarities to that of Mars’ surface. [16] Phobos's density is too low to be solid rock, and it is known to have significant porosity. [17] [18] [19] These results led to the suggestion that Phobos might contain a substantial reservoir of ice. Spectral observations indicate that the surface regolith layer lacks hydration, [20] [21] but ice below the regolith is not ruled out. [22] [23]

    Phobos is heavily cratered. [24] The most prominent of these is the crater, Stickney, (named after Asaph Hall's wife, Angeline Stickney Hall, Stickney being her maiden name) a large impact crater some 9 km (5.6 mi) in diameter, taking up a substantial proportion of the moon's surface area. As with Mimas's crater Herschel, the impact that created Stickney must have nearly shattered Phobos. [25]

    Phobos in InfraRed
(24 April 2019) PIA23204-MartianMoon-Phobos-Odyssey-20190424.jpg
    Phobos in InfraRed
    (24 April 2019)

    Many grooves and streaks also cover the oddly shaped surface. The grooves are typically less than 30 meters (98 ft) deep, 100 to 200 meters (330 to 660 ft) wide, and up to 20 kilometers (12 mi) in length, and were originally assumed to have been the result of the same impact that created Stickney. Analysis of results from the Mars Express spacecraft, however, revealed that the grooves are not in fact radial to Stickney, but are centered on the leading apex of Phobos in its orbit (which is not far from Stickney). Researchers suspect that they have been excavated by material ejected into space by impacts on the surface of Mars. The grooves thus formed as crater chains, and all of them fade away as the trailing apex of Phobos is approached. They have been grouped into 12 or more families of varying age, presumably representing at least 12 Martian impact events. [26] Nonetheless, in November 2018, astronomers concluded that the many grooves on Phobos were caused by boulders, ejected from the asteroid impact that created Stickney crater, that rolled around on the surface of the moon. [7] [8]

    Faint dust rings produced by Phobos and Deimos have long been predicted but attempts to observe these rings have, to date, failed. [27] Recent images from Mars Global Surveyor indicate that Phobos is covered with a layer of fine-grained regolith at least 100 meters thick; it is hypothesized to have been created by impacts from other bodies, but it is not known how the material stuck to an object with almost no gravity. [28]

    The unique Kaidun meteorite that fell on a Soviet military base in Yemen in 1980 has been hypothesized to be a piece of Phobos, but this has been difficult to verify because little is known about the exact composition of Phobos. [29] [30]

    A person who weighs 68 kg (150 lb) on Earth would weigh about 60 g (2 oz) standing on the surface of Phobos. [31]

    Named geological features

    Geological features on Phobos are named after astronomers who studied Phobos and people and places from Jonathan Swift's Gulliver's Travels . [32]

    Craters on Phobos

    A number of craters have been named, and are listed in the following table. [33]

    Craters Coordinates Diameter
    Eponym RefAnnotated map
    Clustril 60°N91°W / 60°N 91°W / 60; -91 (Clustril) 3.42006Character in Lilliput who informed Flimnap that his wife had visited Gulliver privately in Jonathan Swift's novel Gulliver's Travels WGPSN
    D'Arrest 39°S179°W / 39°S 179°W / -39; -179 (D'Arrest) 2.11973 Heinrich Louis d'Arrest; German/Danish astronomer (1822–1875) WGPSN
    Drunlo 36°30′N92°00′W / 36.5°N 92°W / 36.5; -92 (Drunlo) 4.22006Character in Lilliput who informed Flimnap that his wife had visited Gulliver privately in Jonathan Swift's novel Gulliver's Travels WGPSN
    Flimnap 60°N10°E / 60°N 10°E / 60; 10 (Flimnap) 1.52006Treasurer of Lilliput in Jonathan Swift's novel Gulliver's Travels WGPSN
    Grildrig 81°N165°E / 81°N 165°E / 81; 165 (Grildrig) 2.62006Name given to Gulliver by the farmer's daughter Glumdalclitch in the giants’ country Brobdingnag in Jonathan Swift's novel Gulliver's Travels WGPSN
    Gulliver 62°N163°W / 62°N 163°W / 62; -163 (Gulliver) 5.52006 Lemuel Gulliver surgeon captain and voyager in Jonathan Swift's novel Gulliver's Travels WGPSN
    Hall 80°S150°E / 80°S 150°E / -80; 150 (Hall) 5.41973 Asaph Hall; American astronomer discoverer of Phobos and Deimos (1829–1907) WGPSN
    Limtoc 11°S54°W / 11°S 54°W / -11; -54 (Limtoc) 22006General in Lilliput who prepared articles of impeachment against Gulliver in Jonathan Swift's novel Gulliver's Travels WGPSN
    Öpik 7°S63°E / 7°S 63°E / -7; 63 (Öpik) 22011 Ernst J. Öpik, Estonian astronomer (1893–1985) WGPSN
    Reldresal 41°N39°W / 41°N 39°W / 41; -39 (Reldresal) 2.92006Secretary for Private Affairs in Lilliput; Gulliver's friend in Jonathan Swift's novel Gulliver's Travels WGPSN
    Roche 53°N177°E / 53°N 177°E / 53; 177 (Roche) 2.31973 Édouard Roche; French astronomer (1820–1883) WGPSN
    Sharpless 27°30′S154°00′W / 27.5°S 154°W / -27.5; -154 (Sharpless) 1.81973 Bevan Sharpless; American astronomer (1904–1950) WGPSN
    Shklovsky 24°N112°E / 24°N 112°E / 24; 112 (Shklovsky) 22011 Iosif Shklovsky, Soviet astronomer (1916–1985) WGPSN
    Skyresh 52°30′N40°00′E / 52.5°N 40°E / 52.5; 40 (Skyresh) 1.52006Skyresh Bolgolam High Admiral of the Lilliput council who opposed Gulliver's plea for freedom and accused him of being a traitor in Jonathan Swift's novel Gulliver's Travels WGPSN
    Stickney 1°N49°W / 1°N 49°W / 1; -49 (Stickney) 91973 Angeline Stickney (1830–1892) ; wife of American astronomer Asaph Hall (above) WGPSN
    Todd 9°S153°W / 9°S 153°W / -9; -153 (Todd) 2.61973 David Peck Todd; American astronomer (1855–1939) WGPSN
    Wendell 1°S132°W / 1°S 132°W / -1; -132 (Wendell) 1.71973 Oliver Wendell; American astronomer (1845–1912) WGPSN
    Stickney mro.jpg
    Left: The impact crater Stickney imaged by the Mars Reconnaissance Orbiter in March 2008. The second impact crater inside Stickney is Limtoc. Right: Labeled Map of Phobos – Moon of Mars (U.S. Geological Survey). [34]

    Other named features

    There is one named regio, Laputa Regio, and one named planitia, Lagado Planitia; both are named after places in Gulliver's Travels (the fictional Laputa, a flying island, and Lagado, imaginary capital of the fictional nation Balnibarbi). [35] The only named ridge on Phobos is Kepler Dorsum, named after the astronomer Johannes Kepler.

    Orbital characteristics

    The relative sizes of Deimos and Phobos as might be seen from the surface of Mars, compared to the relative size in the sky of the Moon as seen from Earth PIA17351-ApparentSizes-MarsDeimosPhobos-EarthMoon.jpg
    The relative sizes of Deimos and Phobos as might be seen from the surface of Mars, compared to the relative size in the sky of the Moon as seen from Earth
    Orbits of Phobos and Deimos. Phobos makes about four orbits for every one made by Deimos. Orbits of Phobos and Deimos.gif
    Orbits of Phobos and Deimos. Phobos makes about four orbits for every one made by Deimos.

    The orbital motion of Phobos has been intensively studied, making it "the best studied natural satellite in the Solar System" in terms of orbits completed. [36] Its close orbit around Mars produces some unusual effects. With an altitude of 5,989 km (3,721 mi), Phobos orbits Mars below the synchronous orbit radius, meaning that it moves around Mars faster than Mars itself rotates. [18] Therefore, from the point of view of an observer on the surface of Mars, it rises in the west, moves comparatively rapidly across the sky (in 4 h 15 min or less) and sets in the east, approximately twice each Martian day (every 11 h 6 min). Because it is close to the surface and in an equatorial orbit, it cannot be seen above the horizon from latitudes greater than 70.4°. Its orbit is so low that its angular diameter, as seen by an observer on Mars, varies visibly with its position in the sky. Seen at the horizon, Phobos is about 0.14° wide; at zenith it is 0.20°, one-third as wide as the full Moon as seen from Earth. By comparison, the Sun has an apparent size of about 0.35° in the Martian sky. Phobos's phases, inasmuch as they can be observed from Mars, take 0.3191 days (Phobos's synodic period) to run their course, a mere 13 seconds longer than Phobos's sidereal period. As seen from Phobos, Mars would appear 6,400 times larger and 2,500 times brighter than the full Moon appears from Earth, taking up a quarter of the width of a celestial hemisphere. The Mars–Phobos Lagrangian L1 is 2.5 kilometers (1.6 mi) above Stickney, which is unusually close to the surface.

    Solar transits

    Annular eclipse of the Sun by Phobos as viewed by the Mars Curiosity rover (20 August 2013). PIA17356-MarsCuriosityRover-EclipseOfSunByPhobos.jpg
    Annular eclipse of the Sun by Phobos as viewed by the Mars Curiosity rover (20 August 2013).

    An observer situated on the Martian surface, in a position to observe Phobos, would see regular transits of Phobos across the Sun. Several of these transits have been photographed by the Mars Rover Opportunity . During the transits, Phobos's shadow is cast on the surface of Mars; an event which has been photographed by several spacecraft. Phobos is not large enough to cover the Sun's disk, and so cannot cause a total eclipse.

    Predicted destruction

    Tidal deceleration is gradually decreasing the orbital radius of Phobos by 2 meters every one hundred years. [9] Scientists estimate that Phobos will be destroyed in approximately 30–50 million years, [9] [36] with one study's estimate being about 43 million years. [37]

    Phobos' grooves were long thought to be fractures caused by the impact that formed the Stickney crater. Other modelling suggested since the 1970s support the idea that the grooves are more like "stretch marks" that occur when Phobos gets deformed by tidal forces, but in 2015 when the tidal forces were calculated and used in a new model, the stresses were too weak to fracture a solid moon of that size, unless Phobos is a rubble pile surrounded by a layer of powdery regolith about 100 m (330 ft) thick. Stress fractures calculated for this model line up with the grooves on Phobos. The model is supported with the discovery that some of the grooves are younger than others, implying that the process that produces the grooves is ongoing. [38] [39] [40] [9]

    Given Phobos's irregular shape and assuming that it is a pile of rubble (specifically a Mohr–Coulomb body), it will eventually break up when it reaches approximately 2.1 Mars radii. [41]

    When Phobos is eventually torn apart by tidal forces, a fraction of the debris will likely form a planetary ring around Mars, which may last from one million to one hundred million years. [42] [43]


    Video (01:30/ real-time ): Eclipse of the Sun by Phobos, larger of the two moons of Mars (Curiosity Rover, 20 August 2013).

    The origin of the Martian moons is still controversial. [44] Phobos and Deimos both have much in common with carbonaceous C-type asteroids, with spectra, albedo, and density very similar to those of C- or D-type asteroids. [45] Based on their similarity, one hypothesis is that both moons may be captured main-belt asteroids. [46] [47] Both moons 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 probably by a combination of atmospheric drag and tidal forces, [48] although it is not clear that sufficient time is available for this to occur for Deimos. [44] Capture also requires dissipation of energy. The current Martian atmosphere is too thin to capture a Phobos-sized object by atmospheric braking. [44] Geoffrey A. Landis has pointed out that the capture could have occurred if the original body was a binary asteroid that separated under tidal forces. [47]

    Curiosity's view of the Mars moons: Phobos passing in front of Deimos - in real-time (video-gif, 1 August 2013). PIA17352-MarsMoons-PhobosPassesDeimos-RealTime.gif
    Curiosity's view of the Mars moons: Phobos passing in front of Deimos – in real-time (video-gif, 1 August 2013).

    Phobos could be a second-generation Solar System object that coalesced in orbit after Mars formed, rather than forming concurrently out of the same birth cloud as Mars. [49]

    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 large planetesimal. [50] The high porosity of the interior of Phobos (based on the density of 1.88 g/cm3, voids are estimated to comprise 25 to 35 percent of Phobos's volume) is inconsistent with an asteroidal origin. [51] Observations of Phobos in the thermal infrared suggest a composition containing mainly phyllosilicates, which are well known from the surface of Mars. The spectra are distinct from those of all classes of chondrite meteorites, again pointing away from an asteroidal origin. [52] Both sets of findings support an origin of Phobos from material ejected by an impact on Mars that reaccreted in Martian orbit, [53] similar to the prevailing theory for the origin of Earth's moon.

    Shklovsky's "Hollow Phobos" hypothesis

    In the late 1950s and 1960s, the unusual orbital characteristics of Phobos led to speculations that it might be hollow.

    Around 1958, Russian astrophysicist Iosif Samuilovich Shklovsky, studying the secular acceleration of Phobos's orbital motion, suggested a "thin sheet metal" structure for Phobos, a suggestion which led to speculations that Phobos was of artificial origin. [54] Shklovsky based his analysis on estimates of the upper Martian atmosphere's density, and deduced that for the weak braking effect to be able to account for the secular acceleration, Phobos had to be very light—one calculation yielded a hollow iron sphere 16 kilometers (9.9 mi) across but less than 6 cm thick. [54] [55] In a February 1960 letter to the journal Astronautics, [56] Fred Singer, then science advisor to U.S. President Dwight D. Eisenhower, said of Shklovsky's theory:

    Globe of Phobos at the Memorial Museum of Astronautics in Moscow (19 May 2012). Globe of Phobos.JPG
    Globe of Phobos at the Memorial Museum of Astronautics in Moscow (19 May 2012).

    If the satellite is indeed spiraling inward as deduced from astronomical observation, then there is little alternative to the hypothesis that it is hollow and therefore Martian made. The big 'if' lies in the astronomical observations; they may well be in error. Since they are based on several independent sets of measurements taken decades apart by different observers with different instruments, systematic errors may have influenced them. [56]

    Subsequently, the systemic data errors that Singer predicted were found to exist, and the claim was called into doubt, [57] and accurate measurements of the orbit available by 1969 showed that the discrepancy did not exist. [58] Singer's critique was justified when earlier studies were discovered to have used an overestimated value of 5 cm/yr for the rate of altitude loss, which was later revised to 1.8 cm/yr. [59] The secular acceleration is now attributed to tidal effects, [60] which had not been considered in the earlier studies.

    The density of Phobos has now been directly measured by spacecraft to be 1.887 g/cm3. [61] Current observations are consistent with Phobos being a rubble pile. [61] In addition, images obtained by the Viking probes in the 1970s clearly showed a natural object, not an artificial one. Nevertheless, mapping by the Mars Express probe and subsequent volume calculations do suggest the presence of voids and indicate that it is not a solid chunk of rock but a porous body. [62] The porosity of Phobos was calculated to be 30% ± 5%, or a quarter to a third being empty. [51]


    Launched missions

    Phobos imaged by Spirit rover (first two images) and by Mars Express (last image) in 2005. Phobos Viewed from Mars.jpg
    Phobos imaged by Spirit rover (first two images) and by Mars Express (last image) in 2005.
    Illustration of the Phobos probe Phobos Marte.jpg
    Illustration of the Phobos probe

    Phobos has been photographed in close-up by several spacecraft whose primary mission has been to photograph Mars. The first was Mariner 7 in 1969, followed by Viking 1 in 1977, Mars Global Surveyor in 1998 and 2003, Mars Express in 2004, 2008, and 2010, [63] and Mars Reconnaissance Orbiter in 2007 and 2008. On 25 August 2005, the Spirit rover, with an excess of energy due to wind blowing dust off of its solar panels, took several short-exposure photographs of the night sky from the surface of Mars. [64] Phobos and Deimos are both clearly visible in the photograph.

    The Soviet Union undertook the Phobos program with two probes, both launched successfully in July 1988. Phobos 1 was accidentally shut down by an erroneous command from ground control issued in September 1988 and lost while the craft was still en route. Phobos 2 arrived at the Mars system in January 1989 and, after transmitting a small amount of data and imagery but shortly before beginning its detailed examination of Phobos's surface, the probe abruptly ceased transmission due either to failure of the on-board computer or of the radio transmitter, already operating on the backup power. Other Mars missions collected more data, but the next dedicated mission attempt would be a sample return mission.

    The Russian Space Agency launched a sample return mission to Phobos in November 2011, called Fobos-Grunt . The return capsule also included a life science experiment of The Planetary Society, called Living Interplanetary Flight Experiment, or LIFE. [65] A second contributor to this mission was the China National Space Administration, which supplied a surveying satellite called "Yinghuo-1", which would have been released in the orbit of Mars, and a soil-grinding and sieving system for the scientific payload of the Phobos lander. [66] [67] [68] However, after achieving Earth orbit, the Fobos–Grunt probe failed to initiate subsequent burns that would have sent it off to Mars. Attempts to recover the probe were unsuccessful and it crashed back to Earth in January 2012. [69]

    Missions considered

    Fuel is mined from Phobos with the help of a nuclear reactor. (P. Rawlings, 1986) S86 25375patraw.jpg
    Fuel is mined from Phobos with the help of a nuclear reactor. (P. Rawlings, 1986)

    In 1997 and 1998, the Aladdin mission was selected as a finalist in the NASA Discovery Program. The plan was to visit both Phobos and Deimos, and launch projectiles at the satellites. The probe would collect the ejecta as it performed a slow flyby (~1 km/s). [71] These samples would be returned to Earth for study three years later. [72] [73] The Principal Investigator was Dr. Carle Pieters of Brown University. The total mission cost, including launch vehicle and operations was $247.7 million. [74] Ultimately, the mission chosen to fly was MESSENGER , a probe to Mercury. [75]

    In 2007, the European aerospace subsidiary EADS Astrium was reported to have been developing a mission to Phobos as a technology demonstrator. Astrium was involved in developing a European Space Agency plan for a sample return mission to Mars, as part of the ESA's Aurora programme, and sending a mission to Phobos with its low gravity was seen as a good opportunity for testing and proving the technologies required for an eventual sample return mission to Mars. The mission was envisioned to start in 2016, was to last for three years. The company planned to use a "mothership", which would be propelled by an ion engine, releasing a lander to the surface of Phobos. The lander would perform some tests and experiments, gather samples in a capsule, then return to the mothership and head back to Earth where the samples would be jettisoned for recovery on the surface. [76]

    Proposed missions

    The Phobos monolith (right of center) as taken by the Mars Global Surveyor (MOC Image 55103, 1998). Monolith55103h-crop.jpg
    The Phobos monolith (right of center) as taken by the Mars Global Surveyor (MOC Image 55103, 1998).

    In 2007, the Canadian Space Agency funded a study by Optech and the Mars Institute for an unmanned mission to Phobos known as Phobos Reconnaissance and International Mars Exploration (PRIME). A proposed landing site for the PRIME spacecraft is at the "Phobos monolith", a prominent object near Stickney crater. [77] [78] [79] The PRIME mission would be composed of an orbiter and lander, and each would carry 4 instruments designed to study various aspects of Phobos's geology. [80]

    In 2008, NASA Glenn Research Center began studying a Phobos and Deimos sample return mission that would use solar electric propulsion. The study gave rise to the "Hall" mission concept, a New Frontiers-class mission under further study as of 2010. [81]

    Another concept of a sample return mission from Phobos and Deimos is OSIRIS-REx II, which would use heritage technology from the first OSIRIS-REx mission. [82]

    As of January 2013, a new Phobos Surveyor mission is currently under development by a collaboration of Stanford University, NASA's Jet Propulsion Laboratory, and the Massachusetts Institute of Technology. [83] The mission is currently in the testing phases, and the team at Stanford plans to launch the mission between 2023 and 2033. [83]

    In March 2014, a Discovery class mission was proposed to place an orbiter in Mars orbit by 2021 to study Phobos and Deimos through a series of close flybys. The mission is called Phobos And Deimos & Mars Environment (PADME). [84] [85] [86] Two other Phobos missions that were proposed for the Discovery 13 selection included a mission called Merlin, which would flyby Deimos but actually orbit and land on Phobos, and another one is Pandora which would orbit both Deimos and Phobos. [87]

    The Japanese Aerospace Exploration Agency (JAXA) unveiled in 9 June 2015 the Martian Moons Exploration (MMX), a sample return mission targeting Phobos. [88] MMX will land and collect samples from Phobos multiple times, along with conducting Deimos flyby observations and monitoring Mars's climate. By using a corer sampling mechanism, the spacecraft aims to retrieve a minimum 10 g amount of samples. [89] NASA, ESA, and CNES [90] are also participating in the project, and will provide scientific instruments. [91] [92] The U.S. will contribute the Neutron and Gamma-Ray Spectrometer (NGRS), and France the Near IR Spectrometer (NIRS4/MacrOmega). [89] [93] Although the mission has been selected for implementation [94] [95] and is now beyond proposal stage, formal project approval by JAXA has been postponed following the Hitomi mishap. [96] Development and testing of key components, including the sampler, is currently ongoing. [97] As of 2017, MMX is scheduled to be launched in 2024, and will return to Earth five years later. [89]

    Russia plans to repeat Fobos-Grunt mission in the late 2020s, and the European Space Agency is assessing a sample-return mission for 2024 called Phootprint. [98] [99]

    As part of a human mission to Mars

    Phobos in 1998 Phobosmgs.gif
    Phobos in 1998

    Phobos has been proposed as an early target for a human mission to Mars. The teleoperation of robotic scouts on Mars by humans on Phobos could be conducted without significant time delay, and planetary protection concerns in early Mars exploration might be addressed by such an approach. [101]

    Phobos has also been proposed as an early target for a manned mission to Mars because a landing on Phobos would be considerably less difficult and expensive than a landing on the surface of Mars itself. A lander bound for Mars would need to be capable of atmospheric entry and subsequent return to orbit, without any support facilities (a capacity that has never been attempted in a manned spacecraft), or would require the creation of support facilities in-situ (a "colony or bust" mission); a lander intended for Phobos could be based on equipment designed for lunar and asteroid landings. [102] Additionally, the delta-v to land on Phobos and return is only 80% of that for a trip to and from the surface of the Moon, partly due to Phobos's very weak gravity. [103] [ full citation needed ]

    The human exploration of Phobos could serve as a catalyst for the human exploration of Mars and be scientifically valuable in its own right. [104]

    Most recently, it was proposed that the sands of Phobos could serve as a valuable material for aerobraking in the colonization of Mars. [105] [106] Because the small delta-v budget of Phobos, a small amount of chemical fuel brought from Earth could be transformed in a very large amount of sand lifted from the surface of Phobos -from a permanent outpost, to a transfer orbit. This sand could be released in front of the spacecraft during the descent maneuver and then resulting in a densification of the atmosphere just in front of the spacecraft.

    See also

    Related Research Articles

    Deimos (moon) Moon of Mars

    Deimos is the smaller and outermost of the two natural satellites of the planet Mars, the other being Phobos. Deimos has a mean radius of 6.2 km (3.9 mi) and takes 30.3 hours to orbit Mars. Deimos is 23,460 km (14,580 mi) from Mars, much further than Mars's other moon, Phobos. It is named for Deimos who in Greek mythology is the twin brother of Phobos, and personifies terror.

    Lander (spacecraft) spacecraft which descends toward and comes to rest on the surface of an astronomical body

    A lander is a spacecraft which descends toward and comes to rest on the surface of an astronomical body. By contrast with an impact probe, which makes a hard landing and is damaged or destroyed so ceases to function after reaching the surface, a lander makes a soft landing after which the probe remains functional.

    Exploration of Mars Mars scientific exploration programs

    The planet Mars has been explored remotely by spacecraft. Probes sent from Earth, beginning in the late 20th century, have yielded a large increase in knowledge about the Martian system, focused primarily on understanding its geology and habitability potential. Engineering interplanetary journeys is complicated and the exploration of Mars has experienced a high failure rate, especially the early attempts. Roughly sixty percent of all spacecraft destined for Mars failed before completing their missions and some failed before their observations could begin. Some missions have met with unexpected success, such as the twin Mars Exploration Rovers, which operated for years beyond their specification.

    Discovery Program NASAs space exploration missions

    NASA's Discovery Program is a series of lower-cost, highly focused American scientific space missions that are exploring the Solar System. It was founded in 1992 to implement then-NASA Administrator Daniel S. Goldin's vision of "faster, better, cheaper" planetary missions. Discovery missions differ from traditional NASA missions where targets and objectives are pre-specified. Instead, these cost-capped missions are proposed and led by a scientist called the Principal Investigator (PI). Proposing teams may include people from industry, small businesses, government laboratories, and universities. Proposals are selected through a competitive peer review process. All of the completed Discovery missions are accomplishing ground-breaking science and adding significantly to the body of knowledge about the Solar System.

    Stickney (crater) crater on Phobos

    Stickney is the largest crater on Phobos, which is a satellite of Mars. It is 9 km (5.6 mi) in diameter, taking up a substantial proportion of the moon's surface.

    Moons of Mars natural satellites orbiting Mars

    The two moons of Mars are Phobos and Deimos. Both were discovered by Asaph Hall in August 1877 and are named after the Greek mythological twin characters Phobos (panic/fear) and Deimos (terror/dread) who accompanied their father Ares into battle. Ares, god of war, was known to the Romans as Mars.

    Sample-return mission space mission to retrieve tangible samples from an extraterrestrial location and return with them to Earth for analysis

    A sample-return mission is a spacecraft mission with the goal of collecting and returning samples from an extraterrestrial location to Earth for analysis. Sample-return missions may bring back merely atoms and molecules or a deposit of complex compounds such as loose material ("soil") and rocks. These samples may be obtained in a number of ways, such as soil and rock excavation or a collector array used for capturing particles of solar wind or cometary debris.

    Pascal Lee American planetary scientist

    Pascal Lee is co-founder and chairman of the Mars Institute, a planetary scientist at the SETI Institute, and the Principal Investigator of the Haughton-Mars Project (HMP) at NASA Ames Research Center in Mountain View, California. He holds an ME in geology and geophysics from the University of Paris, and a PhD in astronomy and space sciences from Cornell University.

    Mars has two moons, Phobos and Deimos. Due to their small size, both moons were discovered only in 1877, by astronomer Asaph Hall. Nevertheless, they frequently feature in works of science fiction.

    Mars Fourth planet from the Sun in the Solar System

    Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System after Mercury. In English, Mars carries a name of the Roman god of war, and is often referred to as the "Red Planet" because the iron oxide prevalent on its surface gives it a reddish appearance that is distinctive among the astronomical bodies visible to the naked eye. Mars is a terrestrial planet with a thin atmosphere, having surface features reminiscent both of the impact craters of the Moon and the valleys, deserts, and polar ice caps of Earth.

    Planetary Science Decadal Survey

    The Planetary Science Decadal Survey is a publication of the United States National Research Council produced for NASA and other United States Government Agencies such as the National Science Foundation. The document identifies key questions facing planetary science and outlines recommendations for space and ground-based exploration ten years into the future. Missions to gather data to answer these big questions are described and prioritized, where appropriate.

    Phobos Surveyor is a mission concept under preliminary study by Marco Pavone of Stanford University, the NASA Jet Propulsion Laboratory (JPL), and the Massachusetts Institute of Technology as a part of NASA's Innovative Advanced Concepts program.

    Asteroid Redirect Mission United States space mission to collect samples from an asteroid

    The Asteroid Redirect Mission (ARM), also known as the Asteroid Retrieval and Utilization (ARU) mission and the Asteroid Initiative, was a space mission proposed by NASA in 2013. The Asteroid Retrieval Robotic Mission (ARRM) spacecraft would rendezvous with a large near-Earth asteroid and use robotic arms with anchoring grippers to retrieve a 4-meter boulder from the asteroid.

    Phobos And Deimos & Mars Environment

    Phobos And Deimos & Mars Environment (PADME) is a low-cost NASA Mars orbiter mission concept that would address longstanding unknowns about Mars' two moons Phobos and Deimos and their environment.

    Mars Base Camp

    Mars Base Camp (MBC) is a crewed Mars laboratory orbiter concept under study that was commissioned by NASA from Lockheed Martin in US. It would use both future and proven concepts as well as the Orion MPCV, also built by Lockheed Martin.

    The following outline is provided as an overview of and topical guide to Mars:

    Martian Moons Exploration Japanese space mission for the exploration of Mars and its moons

    The Martian Moons Exploration (MMX) is a robotic space probe set for launch in 2024 to bring back the first samples from Mars' largest moon Phobos. Developed by the Japanese Aerospace Exploration Agency (JAXA) and announced in 9 June 2015, MMX will land and collect samples from Phobos once or twice, along with conducting Deimos flyby observations and monitoring Mars' climate.

    Deimos and Phobos Interior Explorer (DePhine) is a European mission concept to use a dedicated orbiter to explore the two Moons of Mars: Phobos and Deimos. The mission concept was proposed in 2016 to the European Space Agency's Cosmic Vision programme for launch in 2030, but it was not chosen as a finalist for the M5 mission class.


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