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

Possible physical and thermodynamical evidence for liquid water at the Phoenix landing site

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95444/1/jgre2665.pd

The primordial nucleus of comet 67P/Churyumov-Gerasimenko

Context. We investigate the formation and evolution of comet nuclei and other trans-Neptunian objects (TNOs) in the solar nebula and primordial disk prior to the giant planet orbit instability foreseen by the Nice model. Aims: Our goal is to determine whether most observed comet nuclei are primordial rubble-pile survivors that formed in the solar nebula and young primordial disk or collisional rubble piles formed later in the aftermath of catastrophic disruptions of larger parent bodies. We also propose a concurrent comet and TNO formation scenario that is consistent with observations. Methods: We used observations of comet 67P/Churyumov-Gerasimenko by the ESA Rosetta spacecraft, particularly by the OSIRIS camera system, combined with data from the NASA Stardust sample-return mission to comet 81P/Wild 2 and from meteoritics; we also used existing observations from ground or from spacecraft of irregular satellites of the giant planets, Centaurs, and TNOs. We performed modeling of thermophysics, hydrostatics, orbit evolution, and collision physics. Results: We find that thermal processing due to short-lived radionuclides, combined with collisional processing during accretion in the primordial disk, creates a population of medium-sized bodies that are comparably dense, compacted, strong, heavily depleted in supervolatiles like CO and CO2; they contain little to no amorphous water ice, and have experienced extensive metasomatism and aqueous alteration due to liquid water. Irregular satellites Phoebe and Himalia are potential representatives of this population. Collisional rubble piles inherit these properties from their parents. Contrarily, comet nuclei have low density, high porosity, weak strength, are rich in supervolatiles, may contain amorphous water ice, and do not display convincing evidence of in situ metasomatism or aqueous alteration. We outline a comet formation scenario that starts in the solar nebula and ends in the primordial disk, that reproduces these observed properties, and additionally explains the presence of extensive layering on 67P/Churyumov-Gerasimenko (and on 9P/Tempel 1 observed by Deep Impact), its bi-lobed shape, the extremely slow growth of comet nuclei as evidenced by recent radiometric dating, and the low collision probability that allows primordial nuclei to survive the age of the solar system. Conclusions: We conclude that observed comet nuclei are primordial rubble piles, and not collisional rubble piles. We argue that TNOs formed as a result of streaming instabilities at sizes below ~400 km and that ~350 of these grew slowly in a low-mass primordial disk to the size of Triton, Pluto, and Eris, causing little viscous stirring during growth. We thus propose a dynamically cold primordial disk, which prevented medium-sized TNOs from breaking into collisional rubble piles and allowed the survival of primordial rubble-pile comets. We argue that comets formed by hierarchical agglomeration out of material that remained after TNO formation, and that this slow growth was a necessity to avoid thermal processing by short-lived radionuclides that would lead to loss of supervolatiles, and that allowed comet nuclei to incorporate ~3 Myr old material from the inner solar system

The Light Curve Of The Dust Cloud Ejected By The Collision Between The Deep Impact Projectile And The Nucleus Of Comet 9P/Tempel 1

Contribution presented at the 39th Annual Meeting of the Division for Planetary Sciences of the American Astronomical Association (DPS 2007), October 7-12, 2007 Orlando, Florida.When Deep Impact fired its projectile into the nucleus of comet 9P/Tempel 1, a cloud made of dust and icy grains was ejected from the impact crater. The dust was subsequently accelerated by gas drag. About a week after the impact event, the dust cloud has dispersed due to its expansion and the force exerted by solar radiation pressure. The light curve of the dust cloud contains information about its formation and evolution: the time scale of the production of impact created material can be derived from the time scale of the brightness increase. The velocity distribution of the cloud is indicative of acceleration processes in the inner coma of the comet. Finally, the abundance of large dust particles created by the impact can be estimated from the brightness of the cloud several days after the impact when small particles have been pushed away by radiation pressure.Here we analyze data obtained by the Narrow Angle Camera (NAC) of OSIRIS onboard the ESA spacecraft Rosetta to derive the velocity distribution of the dust cloud from an inversion of its light curve. OSIRIS observed comet Tempel 1 near-continuously for more than two weeks around the impact. A model of the expansion of the ejecta is compared to the light curve seen by the NAC. We derive a broad velocity distribution of the dust particles, which peaks at around 225 m/s, in good agreement with published estimates. The velocity suggest that the impact ejecta were quickly accelerated by gas in the cometary coma. We will discuss implications of our results for the evolution of the dust cloud during the first hours after the impact and provide estimates of the released dust mass.We acknowledge the funding of the agencies ASI, CNES, DLR, the Spanish Space Program (Ministerio de Educación y Ciencia), SNSB and ESA.Peer reviewe

Interest of Stereophotoclinometry Models for Geological Studies

International audienceNew techniques have been developed during the last decade to reconstruct the global and/or local topography of solar system objects from visible images. These techniques called "stereophotoclinometry" combine the stereo information extracted from image patches with pixel-to-pixel variations of the intensity on a set of images of the same area. These techniques can complement the classical stereophotogrammetry approach. We will discuss how stereophotoclinometry techniques can contribute to geomorphology studies by revealing the 3D structure of fine and narrow features observed on visible images but not always captured in 3D terrain models. Our discussion will be illustrated by examples taken from sets of images acquired by the OSIRIS imaging instrument onboard the ROSETTA spacecraft, for which both stereophotogrammetry and stereophotoclinometry models are available

Determination Of The Light Curve Of Rosetta Target Asteroid 2867 Steins With The Osiris Narrow Angle Camera Onboard Rosetta

International audienc