Comet Siding Spring, also known as C/2007 Q3, was first discovered in 2007 by Australian astronomers when it was near its closest approach to Earth and the Sun at 1.2 and 2.2 AU, respectively. The comet is on a hyperbolic orbit, so will not return again to the neighborhood of the Sun. The Wide-field Infrared Survey Explorer observed it on January 10, 2010, by which time it was far north of the ecliptic plane. WISE views the sky in infrared wavelengths. The comet is 10 times colder than the background stars, so its emission is mostly in longer wavelengths, giving it its red color in this false-color view. (In this image, 3.4-micron light is colored blue; 4.6-micron light is green; 12-micron light is orange; and 22-micron light is red.) Visit the JPL Near Earth Object Program website to see an interactive diagram of C/2007 Q3 (Siding Spring)'s orbit.
The image made me curious about the expectations for WISE to observe, and discover, comets, so I fired off an email to WISE's principal investigator, Ned Wright. He told me "WISE has already seen lots of comets, mostly already known. Siding Spring is the biggest we've seen so far. We could with luck discover a dozen or more comets, with most of them being quite small and faint." I wrote before about the first comet they discovered. A quick check of the Near-Earth Object Confirmation Page at the Minor Planet Center shows five new objects recently observed by WISE (they are the ones with codes beginning in "W"), waiting for followup by ground-based observers so that orbits can be established. On that page they make no notation as to whether the objects are asteroids (more common) or comets (less common), so as not to prejudice observers; followup observers are expected to study their images and note if the object appears to look cometary, that is, if it has a coma.
As WISE's scans run from ecliptic pole to ecliptic pole, you will see WISE objects spanning a wider range of declinations than is normally covered by ground-based surveys. We realize this may present a challenge to follow up observers, but this sky coverage may reveal new knowledge of the distribution of orbits of solar system objects.
You will note that there are now a number of WISE targets on the confirmation page. We encourage you to follow these up as WISE targets have high priority scientifically. The combination of visible light brightness measurements, improved orbit information and the WISE infrared fluxes will enable the determination of accurate diameters, albedos, surface properties, thermal forces, etc. This all requires ground-based follow-up. While most of our targets will be observed via established collaborations, the participation of the broader astronomical community plays a crucial role that has already been demonstrated for a number of WISE discoveries.The Siding Spring image is just one of several stunners released today; I am particularly enamored of the huge Andromeda Galaxy image, which I think I may have to print out and hang on my wall!