Calliope

The asteroid Calliope is officially listed as 22 Kalliope in the NASA database. It is is a fairly large main belt asteroid that is classified as an M-type. It was discovered by J.R Hind on November 16, 1852. It is named after the Greek Muse that inspired epic poetry. It has a mass of 8.09 x 10¹⁹ kg and a density of 3.35 g/cm³ . The asteroid is slightly elongated with an approximate diameter of 180 km. The asteroid’s perihelion is 2.610 AU and its aphelion is 3.208 AU. This is one of the few asteroids that have captured their own moons. Kalliope’s companion is Linus.

M-type asteroids generally have a partially known composition and are moderately bright with albedos ranging from 0.1 to 0.2. They are the third most common asteroid type. A portion of them are iron-nickel in composition in either a pure state or mixed with rock. This type of asteroid are thought to be pieces of the metallic core of differentiated asteroids that were fragmented by impacts. They are also believed to be the source of iron meteorites. There are many more M-type asteroids whose composition is uncertain. Calliope is one of them. Its known density is far too low for a solid metallic object or even a metal and rock rubble pile. A rubble pile of iron-nickel and rock would need about 70% porosity which is inconsistent with packing considerations. Kalliope has spectral properties which seem to indicate the presence of hydration minerals and silicates, an anomalously low radar albedo that is inconsistent with a metallic surface, and other characteristics more in keeping with C type asteroids.

Kalliope has one known satellite, 22 Kalliope I Linus, or simply Linus. It is a large object, between 30 and 40 km in diameter, and would be a noticeable asteroid by itself. It orbits about 1065 km from the asteroid. Linus was discovered on August 29, 2001 by Jean-Luc Margot and Micheal Brown. Scientists believe that Linus’ orbit precesses rapidly, making one cycle in several years. This is most likely due to the non-spherical shape of Kalliope. Linus’s brightness has varies noticeably between observations, indicating that it is elongated. Linus most likely formed from an impact event, possibly as ejecta or as a fragment of Kalliope itself.

Asteroids like Calliope can make scientists scratch their heads in wonder as they present contradictions to widely held theories. Scientists are still trying to determine why some asteroids have moons, but even larger ones do not. The Dawn space mission to the main belt may offer some clues.

Here is a link with information about the asteroid. On Universe Today we have a great article about mining asteroids like Calliope. Astronomy Cast offers a good episode about the chance of an asteroid striking Earth.

Sources:
http://starbrite.jpl.nasa.gov/pds/viewTargetProfile.jsp?TARGET_NAME=22+KALLIOPE
http://solarsystem.nasa.gov/planets/profile.cfm?Object=Asteroids&Display=Moons
http://www.nasa.gov/mission_pages/dawn/news/dawn20091113.html

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