NASA’s Mercury-orbiting MESSENGER spacecraft has captured images of two comets — 2P/Encke and C/2012 S1 (ISON) — setting the stage for observations later this month when both comets will be substantially brighter and much closer to Mercury and the Sun.
ISON was discovered in September 2012 by amateur Russian astronomers, who observed with a 16-inch telescope that is part of the International Scientific Optical Network (ISON), after which the comet was named. On November 28, ISON will fly within 700,000 miles (1.2 million kilometers) of the Sun’s photosphere, at which time it is expected either to flare brilliantly or disintegrate.
As part of an ISON observation campaign involving ground- and space-based NASA observatories, as well as many other observatories around the world, MESSENGER has been poised for several weeks to collect observations of ISON. From November 9 through November 11, the probe’s Mercury Dual Instrument System (MDIS) captured its first images of the comet.
“We are thrilled to see that we’ve detected ISON,” said Ron Vervack, of the Johns Hopkins University Applied Physics Laboratory, who is leading MESSENGER’s role in the ISON observation campaign. “The comet hasn’t brightened as quickly as originally predicted, so we wondered how well we would do. Seeing it this early bodes well for our later observations.”
A few days earlier, from November 6 through November 8, MESSENGER’s imagers picked up its first snapshots of Encke. Unlike ISON, Encke has been known for quite a while. It was discovered in 1786 and recognized as a periodic comet in 1819. Its orbital period is 3.3 years — the shortest period of any known comet — and November 21 will mark its 62nd recorded perihelion.