Observations of Comet 9P/Tempel 1 around the Deep Impact event by the OSIRIS cameras onboard Rosetta

Abstract

The OSIRIS cameras on the Rosetta spacecraft observed comet 9P/Tempel 1 from 5 days before to 10 days after it was hit by the Deep Impact projectile. The Narrow Angle Camera (NAC) monitored the cometary dust in 5 different filters. The Wide Angle Camera (WAC) observed emissions from OH, CN, Na, and OI together with the associated continuum. Before and after the impact the comet showed regular variations in intensity. The period of the brightness changes is consistent with the rotation period of Tempel 1. The overall brightness of Tempel 1 decreased by about 10 % during the OSIRIS observations. The analysis of the impact ejecta shows no new coma structures created by the impact. Instead, material from the impact cloud may have enhanced existing structures. Most of the material moves with ~200 m/s and is preferably pushed into existing coma structures of Tempel 1. The light curve of the comet after the impact and the amount of material leaving the comet (4.5-9 x 106 kg of water ice and a presumably larger amount of dust) suggest that the impact ejecta were quickly accelerated by collisions with gas molecules. Much of the material left the comet in the form of icy grains which sublimated and fragmented within the first hour after the impact. Na D-line emission was detected in the impact ejecta. If the source of the sodium is evaporation of hot dust grains, approximately 3800 kg of hot dust have been created in the impact, consistent with measurements from the Deep Impact spacecraft. The impact ejecta are quickly accelerated by gas in the near-nucleus region. The motion of the ejecta cannot be described by ballistic trajectories. Therefore, density and tensile strength of the nucleus of Tempel 1 cannot be determined with models using ballistic ejection of particles.