Origin: Kuiper belt
Orbital Period: 6.45 years
Size: ~4.3 x 4.1 kilometers
Kuiper belt comet approaching perihelion on 13 August 2015 while being orbited and observed by the European Space Agency’s Rosetta space probe along with its Philae lander module. Comets are theorized to be “dirty snowballs” of ice and rocky debris but Rosetta’s ALICE ultraviolet (UV) spectrograph revealed the comet’s surface to be extremely dark and detected no water ice.
“We’re a bit surprised at just how unreflective the comet’s surface is and how little evidence of exposed water-ice it shows,” says Dr. Alan Stern, Alice principal investigator and an associate vice president of the SwRI Space Science and Engineering Division.
The tails of comets are thought to form by the heating of a comet’s nucleus by sunlight and solar wind, resulting in the vaporization of volatile materials including water ice. These gases along with dust erupt from the comet’s surface forming streams of particles sometimes millions of kilometers long. However, no traces of any fissures, vents, holes or other points of outgassing have so far been detected by Rosetta though its OSIRIS instrument did observe a jet of material being ejected from the night side of the comet, which is shielded from sunlight and the solar wind.
Comet 67P/C-G contains very little, if any, water ice. Water vapor detected in the streams of particles flowing from the comet is formed by the electrical dissolution of silicates on the comet’s surface combined with hydrogen in nearby space. This dissolution is the result of plasma discharges to the comet’s surface following an electric charge imbalance between the comet and the Sun and the weak but vast current flow between them. Both Rosetta and Philae have detected a magnetic field around the comet even though observations from both show the nucleus is not magnetized. Malfunctions during Philae’s landing and close flybys by Rosetta further indicate the electrical nature of the comet’s dynamics.