The Hubble space telescope has observed the furthest active inbound comet ever known, which is currently 2.4 billion kilometers (1.5 billion miles) from Earth. The object, known as C/2017 K2 (PANSTARRS) or "K2", was discovered last May by the Pan-STARRS observatory in Hawaii.
K2 is already active. The observations reveal that although it’s still far away from the Sun, it sports an outgassing that stretches to 130,000 kilometers (80,000 miles). The outgassing is called a coma and is made of gas and dust liberated from the nucleus due to evaporation.
"K2 is so far from the Sun and so cold, we know for sure that the activity – all the fuzzy stuff making it look like a comet – is not produced, as in other comets, by the evaporation of water ice," lead researcher David Jewitt of the University of California, Los Angeles, said in a statement. "Instead, we think the activity is due to the sublimation [a solid changing directly into a gas] of super-volatiles as K2 makes its maiden entry into the solar system's planetary zone. That's why it's special. This comet is so far away and so incredibly cold that water ice there is frozen like a rock."
The comet comes from the depth of space – the region beyond the orbit of Pluto, called the Oort cloud, where the leftover debris of the early Solar System is located. K2 has been on a million-year journey to reach us. It has a 19-kilometer (12-mile) nucleus and the researchers believe it is coated in volatile substances like oxygen, nitrogen, carbon dioxide, and carbon monoxide. This composition isn't too unusual but it’s intriguing to see the gases being expelled.
"I think these volatiles are spread all through K2, and in the beginning billions of years ago, they were probably all through every comet presently in the Oort Cloud," Jewitt added. "But the volatiles on the surface are the ones that absorb the heat from the Sun, so, in a sense, the comet is shedding its outer skin. Most comets are discovered much closer to the Sun, near Jupiter's orbit, so by the time we see them, these surface volatiles have already been baked off. That's why I think K2 is the most primitive comet we've seen."
As reported in The Astrophysical Journal Letters, the team looked for past detections of the comet in archival data and discovered that the Canada-France-Hawaii Telescope spotted K2 in 2013. It was further than Uranus but so dim that no one noticed it.
The comet will come closer than the orbit of Mars by 2022 and once it gets this close it might even develop a tail.