So what exactly is a comet, and what does Rosetta hope to find out?
Comets can be truly spectacular objects with tails stretching across a huge expanse of the sky. For example, nearly everyone has heard of Halley's Comet, the iconic object that reappears in our skies regularly every 75 to 76 years, and that was even portrayed in the Bayeux Tapestry nearly 1000 years ago. Its most recent apparition in 1986 was quite modest because of its distance from the earth but there have been several spectacular comets since, including Comet Hyakutake in 1996, Comet Hale–Bopp in 1997, Comet McNaught in 2007 and Comet Lovejoy in 2011.
However, more often comets are faint objects only visible with powerful telescopes.
A comet is basically a solid nucleus of ice (mostly water ice but also frozen ammonia and carbon dioxide) and perhaps some rocky elements, partially coated in dark organic matter. (Less than 20 km across. ) This is surrounded by a coma – a cloud of gas around the nucleus formed by vaporised ice as the comet gets closer to the sun. (Can be up over a million km across)
The most most striking feature of a comet is of course its tail. In fact there are always two separate tails, as you can see in the image of Comet Hale-Bopp above – the dust tail being the most visible, as well as an ion tail, both formed by the effects of the sun's radiation, and thus always pointing away from the sun. (Can be over 100,000 km long.)
This is why Rosetta's mission is so exciting. In August the spacecraft will approach within 2 km of Comet 67p/Churyomov-Gerasmenko to map its surface, gravity, shape and rotation, and in November it will also attempt to land a probe, named Philae, on the surface of the nucleus. Due to the low gravity there, it will have to use harpoons and ice-screws to stay put! The lander is equiped with instruments to sample and measure the composition of the ice and to drill below the surface to detect any amino acids, the building blocks of life.
The aim of the mission it to catalogue the the chemical composition of the various parts of the comment and ultimately to see if there is a match with similar matter found on Earth. The results are sure to be fascinating!
1. E. Kolmhofer, H. Raab; Johannes-Kepler-Observatory, Linz, Austria (http://www.sternwarte.at), via Creative Commons
2. Creative Commons http://commons.wikimedia.org/wiki/File:Io_diagram.svg#mediaviewer/File:Comet.svg
3. NASA, ESA and Philippe Lamy (Laboratoire d'Astronomie Spatiale) - http://hubblesite.org/newscenter/newsdesk/archive/releases/2003/26/
More information about comets and the Rosetta mission
http://sci.esa.int/rosetta/
http://www.newsweek.com/rosetta-probe-closing-comet-landing-253803
http://www.space.com/53-comets-formation-discovery-and-exploration.html