Determination of the light curve of the Rosetta target asteroid (2867) Steins by the OSIRIS cameras onboard Rosetta

7 pp.-- Article published by EDP Sciences and available at http://www.aanda.org or http://dx.doi.org/10.1051/0004-6361:20066694.-- Table 2 is only available in electronic form at http://www.aanda.org.[Context] In 2004 asteroid (2867) Steins has been selected as a flyby target for the Rosetta mission. Determination of its spin period and the orientation of its rotation axis are essential for optimization of the flyby planning.[Aims] Measurement of the rotation period and light curve of asteroid (2867) Steins at a phase angle larger than achievable from ground based observations, providing a high quality data set to contribute to the determination of the orientation of the spin axis and of the pole direction.[Methods] On March 11, 2006, asteroid (2867) Steins was observed continuously for 24 h with the scientific camera system OSIRIS onboard Rosetta. The phase angle was 41.7 degrees, larger than the maximum phase angle of 30 degrees when Steins is observed from Earth. A total of 238 images, covering four rotation periods without interruption, were acquired.[Results] The light curve of (2867) Steins is double peaked with an amplitude of ≈0.23 mag. The rotation period is 6.052 ± 0.007 h. The continuous observations over four rotation periods exclude the possibility of period ambiguities. There is no indication of deviation from a principal axis rotation state. Assuming a slope parameter of G = 0.15, the absolute visual magnitude of Steins is 13.05 ± 0.03.The OSIRIS imaging system on board Rosetta is managed by the Max-Planck-Intitute for Solar System Research in Katlenburg-Lindau (Germany), thanks to an International collaboration between Germany, France, Italy, Spain, and Sweden. The support of the national funding agencies DLR, CNES, ASI, MEC, and SNSB is gratefully acknowledged. We acknowledge the work of the Rosetta Science Operations Centre at ESA/ESTEC and of the Rosetta Mission Operations Centre at ESA/ESOC who made these observations possible on short notation and operated the spacecraft. S.C.L. acknowledges support from the Leverhulme Trust. This research made use of JPL’s online ephemeris generator (HORIZONS).Peer reviewe

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

17 pp.-- Final full-text version of the paper available at: http://dx.doi.org/10.1016/j.icarus.2006.09.023.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 through filters sensitive to 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 that no new permanent coma structures were created by the impact. Most of the material moved with ~ 200 m/s. Much of it left the comet in the form of icy grains which sublimated and fragmented within the first hour after the impact. The light curve of the comet after the impact and the amount of material leaving the comet (4.5 - 9 x 10e6 of water ice and a presumably larger amount of dust) suggest that the impact ejecta were quickly accelerated by collisions with gas molecules. Therefore, the motion of the bulk of the ejecta cannot be described by ballistic trajectories, and the validity of determinations of the density and tensile strength of the nucleus of Tempel 1 with models using ballistic ejection of particles is uncertain.The OSIRIS imaging system on board Rosetta is managed by the Max-Planck-Institute for Solar System Research in Katlenburg-Lindau (Germany), thanks to an International collaboration between Germany, France, Italy, Spain, and Sweden. We acknowledge the funding of the national space agencies ASI, CNES, DLR, the Spanish Space Program (Ministerio de Educacion y Ciencia), SNSB and ESA. IRAF is distributed by the National Optical Astronomy Observatories, which is operated by the Association of Universities for Research in Astronomy, Inc. (AURA) under cooperative agreement with the National Science Foundation. We acknowledge JPL's Horizons online ephemeris generator for providing the comet's position and rate of motion during the observations. This research has made use of NASA's Astrophysics Data System.Peer reviewe