3 May 2011

Hale-Bopp photographed at 30.7 AU!

Who doesn't remember that magnificent comet that lit up the skies during the first months of 1997, becoming one of the most spectacular of the last century? It was comet Hale-Bopp (C/1995 O1), and only those that didn't see it will not remember it.
As always happens, once the comet was no longer visible to the naked eye it slipped out of the attention of the media, and the general public heard no more of it. However, some researchers continued to follow it in its huge orbit (it takes 2,500 years to complete), among them, a team lead by Gyula M. Szabó, whose results have been published in Astronomy & Astrophysics.
They imaged the comet at red wavelengths using the ESO 2.2 metre telescope at La Silla, Chile. The most impressive thing about the Hale-Bopp images (above and to the right, taken in December 2010) is the huge distance at which the comet was photographed, 30.7 astronomical units, well beyond the orbit of Neptune, the greatest distance at which a comet has ever been observed.
The comet appeared star-like and had a magnitude of 23.3. Its angular diameter was less than 1.9 arcseconds, without any clear evidence of a coma or tail (with a surface brightness higher than 26.5 magnitude per square arcsecond), beyond 2.5 arcseconds (equivalent to 55 thousand km at that distance).
It looks as though there is no longer any cometary activity, as might have been expected for an object so far from the Sun. However, some doubt remains, because although the reflecting surface area has been estimated to be 485 km² if the nucleus has an albedo of 4% (typical of an inactive nucleus), which is 9 times less than measured in 2007, also by Szabó and colleagues, it would imply a diameter of 60-65 km for the nucleus.
This is much larger than the previously accepted value of 35 km. So, either the nucleus has a much higher albedo of 13%, which would be difficult to explain, or there is a residual, low level activity. This might be limited to just a single "hot spot", although "hot" would be a bit of an exaggeration given the current temperature of the comet is actually 50-53 K, about -220°C.
This temperature interval may in fact be just that which signals the transition between an inactive and an active nucleus, and is of great interest because the jets that emerge from cometary nuclei when they become active can modify their trajectory around the Sun.