Changes

== Discovery ==

== Discovery ==

Phobos was discovered by the astronomer [[Asaph Hall]] on August 18, 1877, at the [[United States Naval Observatory]] in [[Washington, DC]].<ref>Authors unknown. "[http://adsabs.harvard.edu//full/seri/Obs../0001//0000181.000.html Notes: The Satellites of Mars]." ''The Observatory'', 1:181-185, 1877. Accessed February 11, 2008, from the SAO/[[NASA]] Astrophysics Data System, [[Harvard University]].</ref><ref>Hall, A. "[http://adsabs.harvard.edu//full/seri/AN.../0091//0000013.000.html Observations of the Satellites of Mars]." ''Astronomische Nachrichten'', 91(2161):11-14, 1877. Accessed February 11, 2008.</ref><ref>Morley, TA. ''[http://adsabs.harvard.edu//full/seri/A+AS./0077//0000220.000.html A catalogue of ground-based astrometric observations of the Martian satellites], 1877-1982'', ''Astronomy and Astrophysics Supplement Series'' (ISSN 0365-0138), 77(2):209-226, February 1989. Accessed February 11, 2008.</ref> Astronomers were especially excited by the very short sidereal month of Phobos—about one-third of a Martian sidereal day—a phenomenon without precedent at the time.

Phobos was discovered by the astronomer [[Asaph Hall]] on August 18, 1877, at the [[United States Naval Observatory]] in [[Washington, DC]].<ref>Authors unknown. "[http://adsabs.harvard.edu//full/seri/Obs../0001//0000181.000.html Notes: The Satellites of Mars]." ''The Observatory'', 1:181-185, 1877. Accessed February 11, 2008, from the SAO/NASA Astrophysics Data System, [[Harvard University]].</ref><ref>Hall, A. "[http://adsabs.harvard.edu//full/seri/AN.../0091//0000013.000.html Observations of the Satellites of Mars]." ''Astronomische Nachrichten'', 91(2161):11-14, 1877. Accessed February 11, 2008.</ref><ref>Morley, TA. ''[http://adsabs.harvard.edu//full/seri/A+AS./0077//0000220.000.html A catalogue of ground-based astrometric observations of the Martian satellites], 1877-1982'', ''Astronomy and Astrophysics Supplement Series'' (ISSN 0365-0138), 77(2):209-226, February 1989. Accessed February 11, 2008.</ref> Astronomers were especially excited by the very short sidereal month of Phobos—about one-third of a Martian sidereal day—a phenomenon without precedent at the time.

The astronomer V. Knorre named the satellite ''Phobos'' (and also provided the name ''Deimos'' for the other satellite that Hall had discovered six days earlier), per a suggestion by Henry G. Madan of Eton, based on the names given in [[The Iliad]] for the two servants of Ares, the Greek god of war, named Fear (Phobos) and Panic (Deimos).<ref>Knorre, V. "[http://adsabs.harvard.edu//full/seri/AN.../0092//0000031.000.html Entdeckung zweier Planeten]." ''Astronomische Nachrichten'', 92(2187):47-48, March 14, 1878. Accessed February 11, 2008.</ref>

The astronomer V. Knorre named the satellite ''Phobos'' (and also provided the name ''Deimos'' for the other satellite that Hall had discovered six days earlier), per a suggestion by Henry G. Madan of Eton, based on the names given in [[The Iliad]] for the two servants of Ares, the Greek god of war, named Fear (Phobos) and Panic (Deimos).<ref>Knorre, V. "[http://adsabs.harvard.edu//full/seri/AN.../0092//0000031.000.html Entdeckung zweier Planeten]." ''Astronomische Nachrichten'', 92(2187):47-48, March 14, 1878. Accessed February 11, 2008.</ref>

Phobos orbits Mars at a distance closer than the distance of a synchronous orbit. For that reason, Phobos actually rises in the ''west'' and sets in the ''east'' of the Martian sky.<ref name=planetsoc>"[http://www.planetary.org/explore/topics/our_solar_system/mars/phobos.html Entry for Phobos]." The Planetary Society. Accessed February 11, 2008.</ref> Yet because Phobos orbits Mars so closely, it is not visible above the horizon at latitudes higher than 70° north or south.

Phobos orbits Mars at a distance closer than the distance of a synchronous orbit. For that reason, Phobos actually rises in the ''west'' and sets in the ''east'' of the Martian sky.<ref name=planetsoc>"[http://www.planetary.org/explore/topics/our_solar_system/mars/phobos.html Entry for Phobos]." The Planetary Society. Accessed February 11, 2008.</ref> Yet because Phobos orbits Mars so closely, it is not visible above the horizon at latitudes higher than 70° north or south.

Phobos is not round, but is shaped like a [[potato]], with dimensions 27 x 21.6 x 18.8&nbsp;km. Its most remarkable surface feature is Crater Stickney, named for the family of Mrs. Asaph Hall. Stickney has a diameter of 10&nbsp;km, and its outer rim has pronounced surface striations suggesting a tremendous impact.<ref name=planetsoc/><ref name=nasa2>"[http://marsprogram.jpl.nasa.gov/facts/phobos.html Entry for Phobos]." Mars Exploration Program, [[NASA]]. Accessed February 11, 2008.</ref>

Phobos is not round, but is shaped like a [[potato]], with dimensions 27 x 21.6 x 18.8&nbsp;km. Its most remarkable surface feature is Crater Stickney, named for the family of Mrs. Asaph Hall. Stickney has a diameter of 10&nbsp;km, and its outer rim has pronounced surface striations suggesting a tremendous impact.<ref name=planetsoc/><ref name=nasa2>"[http://marsprogram.jpl.nasa.gov/facts/phobos.html Entry for Phobos]." Mars Exploration Program, NASA. Accessed February 11, 2008.</ref>

Phobos is spiraling in toward Mars at a rate of 18&nbsp;mm per year. At that rate, if it had 50 million years to wait, it would enter the Martian atmosphere and drop to the surface in a cataclysmic crash, or else disintegrate into a ring.<ref name=nasa1/>

Phobos is spiraling in toward Mars at a rate of 18&nbsp;mm per year. At that rate, if it had 50 million years to wait, it would enter the Martian atmosphere and drop to the surface in a cataclysmic crash, or else disintegrate into a ring.<ref name=nasa1/>

The [[Union of Soviet Socialist Republics|Soviet]] spacecraft ''Phobos 2'' discovered that Phobos was venting something into space, but all its systems failed before its controllers could determine what that substance was—most commentators believe that it was water.

The [[Union of Soviet Socialist Republics|Soviet]] spacecraft ''Phobos 2'' discovered that Phobos was venting something into space, but all its systems failed before its controllers could determine what that substance was—most commentators believe that it was water.

In the 1950's and 1960's, a number of American and Soviet astrophysicists actually speculated that Phobos was hollow, and even that it was an artificial construct.<ref>Öpik, E. J. "[http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1964IrAJ....6..281. Is Phobos Artificial]?" ''Irish Astronomical Journal'', 6:281, September 1964. Accessed February 11, 2008.</ref> This speculation had part of its basis on a lack of understanding of Phobos' density and a gross overestimation of the rate of Phobos' orbital decay—5&nbsp;cm per year instead of the 1.8&nbsp;cm per year that present observations have established.

In the 1950s and 1960s, a number of American and Soviet astrophysicists actually speculated that Phobos was hollow, and even that it was an artificial construct.<ref>Öpik, E. J. "[http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1964IrAJ....6..281. Is Phobos Artificial]?" ''Irish Astronomical Journal'', 6:281, September 1964. Accessed February 11, 2008.</ref> This speculation had part of its basis on a lack of understanding of Phobos' density and a gross overestimation of the rate of Phobos' orbital decay—5&nbsp;cm per year instead of the 1.8&nbsp;cm per year that present observations have established.

== Exploration ==

== Exploration ==