The Rosetta Fly-by at Asteroid (2867) Steins
The International Rosetta Mission is one of ESA's Planetary Cornerstone Missions on its way to rendezvous with comet 67P/Churyumov-Gerasimenko. On route to the comet Rosetta has encountered its first asteroid target, main belt asteroid (2867) Steins. Closest approach occurred on 5 September 2008, 18:38:20 UT at a distance of 802.6 km. The spacecraft passed on the sunlit side of the asteroid with a relative velocity of 8.6 km/s in the plane defined by the relative velocity vector and the Sun direction. This fly-by strategy allowed continuous pointing on the asteroid before, during and after closest approach as well as passing through a phase angle close to zero. The minimum phase angle (0.27°) was reached at 18:36:23, about 2 minutes before closest approach. Optical navigation on the asteroid started already on 4 August 2008 as the spacecraft had to target the small asteroid (effective radius ~ 5 km) within accuracy better than 2 km to keep it in the field of view of the science instruments during closest approach. In addition an attitude flip manoeuvre of 20 minutes duration was performed before autonomous tracking on asteroid (2867) Steins started. Altogether 14 instruments were switched on during the fly-by, providing spatially resolved multi-wavelength observations of the asteroid and in-situ measurements of its dust, plasma, magnetic, and radiation environment. Its detailed characterization will add to the understanding the different types of asteroids and to solving the puzzle of how the solar system formed and evolved. (2867) Steins was Rosetta's first nominal scientific target in its 11.5-year mission. The next scientific encounter is however already in view. In July 2010 Rosetta will fly-by at asteroid (21) Lutetia, a 100 km-sized object (95.5 ± 4.1 km) before moving out to Jupiter's orbit to meet and explore the nucleus of 67P/Churyumov-Gerasimenko.
Imaging Asteroid (2867) Steins with OSIRIS onboard Rosetta
The Optical, Spectroscopic, and Infrared Remote Imaging System OSIRIS observed the E-type asteroid (2867) Steins during the fly-by of ESA's Rosetta spacecraft. Observations over a large phase angle range (from near 0 to 140) by the scientific camera system OSIRIS revealed the illuminated hemisphere of the asteroid's diamond-like shaped body with a mean radius of 2.7 km and a projected surface at zero phase angle of 5.3 x 3.9 km2. A large crater (diameter 2 km) is evidence of an almost disastrous impact and implies that Steins is not a solid rock. More than 30 craters or crater-like features with diameters > 150 m are identified. 7 round concavities are arranged along a line pointing radially away from the big impact crater. The shape and volume of asteroid Steins is derived from models based on the images of both OSIRIS cameras and earlier observations of the photometric light curves. Its resemblance to a spinning top suggests that it was influenced by the YORP effect making it the first optical observation of such a body. Analysis of the images provide the disk integrated albedo, reveal a strong opposition effect, and photometric properties of the surface showing very little variegation. Its very uniform, bright surface suggests that this asteroid is homogeneously formed out of the igneous (magmatic) minerals found in enstatite achondrite meteorites that are produced in melts requiring temperatures of more than 1000 C. Consequently (2867) Steins is a fragment of the interior of a large parent body.
VIRTIS OBSERVATIONS DURING ROSETTA FLYBY OF 2867 STEINS
At present, several asteroids have been remotely observed by space probes; yet they present new surprises. On 5 September 2008, the Rosetta spacecraft encountered the asteroid 2867 Steins on its way to the comet 67P/Churyumov-Gerasimenko. This was the first of two planned asteroid flybys performed by the probe, the second being the much larger 21 Lutetia in July 2010. From ground-based observations, Steins was classified as an E-type asteroid. The designation is linked with the mineral enstatite, common in aubrite meteorites. Therefore, we are in presence of a differentiated object whose study can give information on the differentiation processes that took place in its parent body. We will discuss here this hypothesis in the light of the data acquired by the VIRTIS imaging spectrometer on board Rosetta, that collected resolved observation of Steins, performing spectroscopic measurements in both the visible and near-infrared ranges. In particular, we will describe the results of the observations performed in the 2 channels of VIRTIS, VIRTIS-M and VIRTIS-H. The VIRTIS -M channel is an imaging spectrometer covering the 0.25-5 micrometers spectral range with medium spectral resolution. Each pixel of the VIRTIS-M spectrometer consists of 864 spectral bands, allowing to measure surface reflectance with a spectral resolution of about 2 nm in the VIS and 10 nm in the IR. VIRTIS-H is a point spectrometer that reaches a high spectral resolution (about 1500) in a single aperture from 2 to 5 micrometers. Despite the difficulties for an imaging spectrometer to operate during a fast (8.616 km/s) and close (800 km) flyby, VIRTIS-M was able to acquire high signal/noise hyperspectral data of different regions of Steins. The spatial resolution on the asteroid's surface is of the order of 200 m/pix. VIRTIS-H acquired several unresolved spectra. Both channels were perfectly working: using this data set we have investigated the spectral behavior of the asteroid's surface, as well as its superficial temperature. Moreover, the asteroid was observed in high resolution by VIRTIS -H. Also from the data of VIRTIS H the temperature of the asteroid was measured. The analysis of the data of both channels revealed a substantial agreement, the observed value ranging between 200 and 230 K. The analysis of spectroscopic data is in substantial agreement with an average enstatitic-achondrite composition.
Rosetta Alice Far Ultraviolet Observations of (2867) Steins
During Rosetta's flyby of the main-belt, E-type asteroid (2867) Steins on 5 Sept. 2008, the U.S. Alice UV imaging spectrometer was used to obtain the first far-ultraviolet (FUV) spectrum of an asteroid. A ten minute integration, averaging over a variety of geometries at closest approach, shows very good signal from 850 Å to 2000 Å representing the first spectrum of an E-type asteroid below the atmospheric cutoff. We find that the far ultraviolet albedo of Steins is very low, ∼5%, compared to its visible albedo, 41% (Keller et al. 2009; Weissman et al. 2008; Jorda et al. 2008), as is expected from the UV behavior of many refractory materials. We also find that the albedo does not show a dramatic color variation over the FUV spectral range; however, there is a pronounced dip near 1600 Å. In addition, Alice obtained the total FUV count rate integrated with 1 second resolution during the encounter to determine the average variation of reflected UV flux with phase angle. In comparison to the OSIRIS WAC data, Alice data show clear wavelength dependent phase reddening and that the opposition effect is greater in the FUV than in the visible. In addition to observing Steins at closest approach, a ∼22 hour exosphere search was conducted prior to closest approach with Steins in the Alice slit. As expected from the only existing model (Schläppi et al. 2008), a deep search for any exosphere (e.g., hydrogen, oxygen) yielded no obvious detections in our initial analysis. We have placed upper limits on an atomic hydrogen and oxygen exosphere at Steins. Jorda, L., et al. 2008. A. and Ap. 487, 1171. Keller, H. U., et al. 2009. Rosetta Steins Fly-by Scientific Workshop, Tegernsee, Germany. Schläppi, B., K. Altwegg, and P. Wurz 2008. Icarus 195, 674. Weissman, P.R., et al., 2008. Meteoritics and Planetary Science, 43, 1-10.
Continuum Observations of Asteroid (2867) Steins With the MIRO Millimeter and Submillimeter Instrument on Rosetta
The European Space Agency Rosetta Spacecraft passed within 802 km of the asteroid (2867) Steins on September 5, 2008, on its long journey to Comet 67P/Churyumov-Gerasimenko which will be encountered in 2014. The Rosetta Spacecraft carries a number of scientific instruments including a relatively small and lightweight millimeter and submillimeter instrument, the first of its kind launched into deep space. The instrument, named MIRO (Microwave Instrument for the Rosetta Orbiter), consists of a 30-cm diameter, offset parabolic reflector telescope followed by two heterodyne receivers. Center-band operating frequencies of the receivers are near 190 GHz (1.6 mm) and 562 GHz (0.5 mm). Broadband continuum channels are implemented in both frequency bands for the measurement of near surface temperatures and temperature gradients in two asteroids, (2867) Steins and (21) Lutetia, and Comet 67P/Churyumov-Gerasimenko. A 4096 channel CTS (Chirp Transform Spectrometer) spectrometer having 180 MHz total bandwidth and 44 kHz resolution is, in addition to the continuum channel, connected to the submillimeter receiver. We present the continuum observations of Asteroid (2867) Steins obtained during the flyby with the MIRO instrument. Thermal models and radiative transfer calculations will be presented to deduce near surface (regolith) physical properties of (2867) Steins. The work at the Jet Propulsion Laboratory, California Institute of Technology was supported by NASA.
Magnetic Field Investigations During ROSETTA's Steins Flyby
During the recent Steins flyby of the ROSETTA spacecraft magnetic field measurements have been made with both, the RPC orbiter magnetometer and the ROMAP lander magnetometer. These combined magnetic field measurements allow a detailed examination of any magnetic signatures caused either directly by the asteroid or indirectly by Steins different modes of interaction with the solar wind. Comparing our measurements with simulation results show that Steins does not possess a significant remanent magnetization. The magnetization is estimated at less than 1 mAm2/kg. This is significantly different from results at Braille and Gaspra.
Steins Properties in Comparison with Those of the Other Asteroids Visited by Space Missions
Rosetta spacecraft flown-by the asteroid Steins of September 5th, 2008 exploring for the first time an E type asteroid. The onboard camera OSIRIS, the visible and infrared spectroimager VIRTIS, the ultraviolet spectrometer ALICE, and the microwave spectrometer MIRO collected an important set of data which allows us to characterize the asteroid. As indicated by its meteorite analogues (the aubrites), the material believed to compose Steins is highly differentiated: the asteroid is a remnant of a large body destroyed by a chatastrophic collision. The surface morphology, dominated by several "fresh" craters, implies that the disruption of the Steins' parent body is relatively recent. The Steins' diamond shape is the consequence of the YORP effect. A comparative analysis of the properties of Gaspra, Ida, Eros, Mathilde, Itokawa and Steins provides some important hints for the understanding of the evolution of the solar system through the physical, chemical, and dynamical history of the asteroid population.
Coordinated International Laboratory Studies of Meteorites Supporting Rosetta Mission's Asteroid Flybys
The Rosetta spacecraft flew by asteroid 2867 Steins in September 2008, collecting images, UV, visible and IR spectra and radar reflections. A flyby of the ∼95 km diameter asteroid 21 Lutetia is scheduled for July, 2010. Laboratory studies using meteoritic and terrestrial samples are designed to support interpretation of the observations of the asteroid targets. The goal is to study likely meteoritic analogues of Steins and Lutetia in several laboratories using the same samples in multiple experiments simulating the conditions and types of measurements made during the flybys. The first sample, 5g of an aubrite, ALHA78113,82, is an achondrite consisting of very low iron, high magnesium silicates, with small amounts of metallic and sulfide grains that may be a fragment from Steins. Chips and powdered samples have been measured in reflectance at Brown University, RELAB. Spectral imaging in visible, at INAF, Roma, was conducted on a chip before it was powdered. The spectrum has high albedo (20-40%) depending on grain size and abundance of opaque minerals. It also has an ultraviolet absorption band with two slopes and no 1- nor 2-μm bands. There is no absorption at 0.5 μm as there is in telescopic spectra of Steins. There are two separate questions related to aubrites and Steins. First, what is the nature of the absorption band first measured in the sample of ALH78113,101 at 0.42 μm? And quite separately, what is the spectroscopically active feature in the ground-based spectrum and OSIRIS photometry of Steins at 0.50 μm? Polarization and phase functions have been measured by PROGRA2-vis and -surf covering 6-150° at two wavelengths. Very small phase angle measurements are planned at Kharkiv, and ultraviolet spectra will be measured at Southwest Research Institute, Boulder along with olivine, enstatite, troilite (FeS) and Fe0. Plans are developing to measure the dielectric constant and magnetic susceptibility of the sample. Irradiated samples simulating space exposure will be measured last as it will damage the sample.