Yet another relatively bright comet has been discovered. Jiangao Ruan of China found the 10th magnitude comet on images taken by the SECCHI HI-1B instrument onboard one of the STEREO spacecraft. The comet was first visible on images taken on April 3 UT. Similar to SOHO (a spacecraft that was used to co-discover Comet C/2009 F6 (Yi-Swan)), the two STEREO spacecraft study the Sun and its immediate environment. Since the comet was found in spacecraft data it is named after the spacecraft, in this case, STEREO. This spacecraft based naming scheme is also used for comets found with the SOHO coronographs (named Comet SOHO) and the SOHO SWAN lyman-alpha imager (named Comet SWAN). The spacecraft data being used for comet discovery is in the public domain and many of the discoverers are amateur astronomers analysing the data on their computers.
This is Mr. Ruan’s 19th comet discovery with all of the his comets being found in data from the SOHO and STEREO spacecraft. All 19 comets have been discovered in the past 11 months. As impressive as that number is, the SOHO spacecraft has been used to find ~1600 comets since 1996. Most of these comets are very faint and are only bright enough to be observed when extremely close to the Sun. Unfortunately, the only place one can observe objects that faint and that close to the Sun is above the Earth’s atmosphere. Some more info on the discovery and movies of the discovery images can be found at the Naval Research Lab’s Sungrazing Comet page.
C/2009 G1 (STEREO) is different from the usual SOHO and STEREO comet. Instead of a small faint comet that only gets bright enough to see because it comes within a few solar radii of the Sun, Comet STEREO is a nice sized comet orbiting the Sun at nearly the same distance as the Earth. With perihelion on April 16 at 1.13 AU from the Sun, the comet is as close to the Sun as it will get. Luckily it will get closer to the Earth over the next 2 months, though it will get no closer than 1.06 AU from Earth. With a current estimated magnitude of ~10.5, the comet may brighten a bit as it gets closer.
The comet is located low in the southeastern sky right before the start of dawn. Moving through Aquarius the comet is much easier to see from the southern hemisphere. The comet’s orbit with an inclination of 108 degrees is carrying it further to the south. It will never be an easy object for northern observers. In fact, within a month it will only be observable from the southern latitudes.