I was at the OSIRIS Full Team meeting held at the Max Planck Institut für Sonnensystemforschung in Göttingen, Germany, last week. We had a great meeting, and the good news are piling up – the spacecraft Rosetta performs well, our imaging camera system OSIRIS is fully operational (as are all the other instruments), orbit manoeuvres are successfully executed to enable Rosetta to rendezvous with the comet in early August, and we have already started to do science.
First of all, we have detected the nucleus of Comet 67P/Churyumov-Gerasimenko and are tracking its motion. Secondly, the lightcurve is being monitored regularly, which has allowed us to measure a 12.4 hour rotation period of the nucleus. The lightcurve is a periodic variation in the observed brightness of the nucleus. The variations arise since the nucleus is not spherical but irregular, so that the amount of solar light that is reflected by the nucleus towards the spacecraft is changing with time as the nucleus rotates. The third discovery is that the comet nucleus – which was dormant and quiet at our first observations in late March – now has become active.
Comet activity means that the ice in the nucleus surface layers has become heated sufficiently by sunlight to sublimate, i.e., turn directly to vapor without first becoming liquid. At these distances, at the time of writing 4.03 AU from the Sun, the temperature is too low to allow water ice to sublimate. Instead, more volatile substances like carbon monoxide and carbon dioxide are responsible for the activity. OSIRIS do not see these gases directly. However, the sublimation also liberates a large amount of micrometer-sized dust grains that are entrained in the gas as it rushed into space. OSIRIS detects the solar light that is reflected by this dusty coma, that currently measures about 2600 kilometers across.