Ground-based visible and near-IR observations of Comet 9P/Tempel 1 during the Deep Impact encounter

Abstract

We present the results of our visible and near-IR observations of Comet 9P/Tempel 1 during the Deep Impact encounter. The comet was observed before, during, and after impact from Kitt Peak National Observatory (J, H, K) and Observatorio Astronómico Nacional-San Pedro Mártir, Mexico (B, V, R, I). High time-resolution images in R, J, H, and K the night of impact with a 3 . ″ 5 radius aperture revealed a rapid brightening which had multiple slopes and lasted for approximately 25 min before leveling off. The brightness decreased on subsequent nights and returned to near pre-impact levels by July 8 UT. The R–J, R–H, R–K, J–H, J–K, and H–K colors became bluer the night of impact. The R–J, R–H, and R–K colors remained blue on the night after impact while the J–H, J–K, and H–K colors returned to baseline levels. The observed color changes suggest the bluening was due to an increase in small grains relative to the ambient coma, an increase in ice relative to refractory dust in the coma, or a combination of the two. The ejecta were initially directed towards the southwest but had been driven southeast by solar radiation pressure by the second night after impact. The mean projected ejecta velocity was estimated at 0.20–0.23 km s −1 over the first 24 h after impact.