Difference between revisions of "Borrelly"

Revision as of 17:11, 10 March 2010

Borrelly was discovered by Alphonse Louis Nicolas Borrelly during a routine search for comets at Marseilles, France on December 28, 1904. Deep Space 1 flew about 1350 miles from the nucleus on September 22, 2001. Borrelly is a typical short period comet; it has a period of 6.8 years (currently; it has approached Jupiter 3 times in the 20th century and had its orbit and period adjusted each time). At perihelion, it is 1.35AU from the sun (just inside the orbit of Mars) and at aphelion, it is 5.83 AU from the sun (just outside the orbit of Jupiter); it has an eccentricity of 0.624. The nucleus is shaped sort of like a bowling pin, 8x4 km. It has three "jets" firing material off into the coma; the longest (the one on the rotation axis) is ~60 km long. Each jet comes out of a particularly bright and smooth, but concave part of the comet, and it is believed that the comet is eroding fastest near them. The nucleus is about the colour of asphalt (3% albedo); the coma and tail produce much of the light viewed. This makes the nucleus the darkest known object in the solar system (although Halley is #2 at 4%, and the fact that the only two comets approached are extraordinarily dark suggests that most comets are this dark). The crust is believed to be iron or carbon. The fact that the relatively smooth erosion centres are much brighter has led to suggestions that the albedo comes partly from colour but also from texture, of which the comet has a great deal (hills, craters, general cragginess). The comet is not so much a "dirty iceball" as a "icy dirtball". Deep Impact indicated that comets (or at least Tempel-1) seem to be not very dense; Tempel-1 is maybe 25% by volume ice grains loosely packed together, covered in a layer a couple of meters deep of very fine silicate dust, trace hydrocarbons and organics (including clays and calcium carbonates).