The Rosetta Spacecraft moved into position alongside Comet 67P/Churyumov–Gerasimenko on Wednesday 6 August 2014, the first spacecraft to reach a cometary target, and ten years after the mission was launched. It will now spend six weeks making a series of close passes around the comet, before attempting to move into a stable orbit about 30 km above its surface. The Probe is equipped with a small lander, named Philae, which will attempt to reach the surface of the comet and take samples, probably in November. The Spacecraft will then remain with and monitor the comet as it moves towards and passes through its perihelion (the closest point on its orbit to the Sun) in August 2015.
The surface of Comet 67P/Churyumov–Gerasimenko. Rosetta/European Space Agency.
Images of the surface of 67P/Churyumov–Gerasimenko produced by Rosetta so far show an asteroid-like body. This is not altogether a surprise, as while asteroids and comets have been thought of as different categories of objects, recent studies have shown the division between them to be much less clear. During the formation of the Solar System the Sun would have been surrounded by a protoplanetary disk, within which the planets and smaller bodies orbiting the Sun would have formed. For a small body the main determining factor in what particles could accumulate in its composition would be temperature. In the inner parts of the Solar System only silica and metal based minerals could crystalize, due to the heat of the Sun, but further out within the protoplanetary disk a variety of ices could also form, with higher temperature ices such as water and carbon dioxide forming closer to the Sun than cooler ices such as nitrogen or cyanide.
Such bodies probably had roughly circular orbits when they formed but many have subsequently been disturbed and adopted more eccentric, elliptical orbits, which bring them closer to and further away from the Sun. When bodies come closer to the Sun they are warmed by the Sun's heat, and any ices in their composition will begin to sublimate (turn from solids directly to gasses - liquids cannot form in a vacuum), releasing clouds of snowflakes and dust in the process to form a visible cometary halo. Exactly how far from the Sun a comet begins to form such a halo depends on what ices are present, with lower temperature ices beginning to sublimate further from the Sun. Bodies have been found within the Main Asteroid Belt which behave as asteroids for most of the time, but which still produce a small halo around their perihelion (closest point on their orbit to the Sun), suggesting that they originated further out in the Solar System and at some point adopted a cometary trajectory, falling deeper into the Solar System then back out, from which they were then nudged into a more asteroid line path; these are thought to be fairly old bodies, still venting the last of their icy material.
67P/Churyumov–Gerasimenko is a short period, Jupiter Family comet (a comet with a period of less than 20 years with an orbit angled at less than 30° to the plane of the Solar System), with a 6.44 year orbital period and an elliptical orbit tilted at 7° to the plane of the Solar System, which takes it from 5.68 AU from the Sun (i.e 5.68 times as far from the Sun as the Earth, slightly outside the orbit of Jupiter) to 1.24 AU from the Sun (i.e. 1.24 times the average distance between the Earth and the Sun, roughly halfway between the orbits of Earth and Mars).
The orbit of 67P/Churyumov–Gerasimenko. JPL Small Body Database Browser.
67P/Churyumov–Gerasimenko was discovered in 1969 by Klim Ivanovych Churyumov and Svetlana Ivanovna Gerasimenko, working at the Kiev Astronomical Observatory. The name 67P/Churyumov–Gerasimenko implies that it was the 67th periodic comet discovered (technically all comets are likely to be periodic, but some have such long periods that they if they are seen again then the two events are unlikely to be connected; for practical purposes comets with periods of less than 200 years are regarded as periodic and designated with the letter P, while longer period comets are regarded as non-periodic and designated with the letter C). Many more comets are discovered today than was the case in the 1960s, due to the advent of automated sky surveys, and more recent comets tend to be named for the year in which they are discovered. To make matters worse, some comets are initially identified as asteroids, and given asteroid designations, then reclassified as comets by appending the letters P or C.
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