Rosetta flyby of Asteroid Steins, 5th September 2008 Options

[peter59]

Only 78 days to Stein encounter ! I can't believe I forgot about this event.

[Paolo]

Some details about Steins that I collected for the third volume of my book "robotic exploration of the solar system"

Steins is a small body less than 10 km across, discovered on 4 November 1969 by Soviet astronomer N. Chernykh at the Nauchnyj Observatory in Crimea and named after Karlis Steins, former director of the Latvian University Astronomical Observatory. While the properties of Lutetia, the second asteroid target are relatively well known, being a largish and bright object discovered more than a century ago, almost nothing was known about Steins, and observational campaigns were started in 2004 to characterize it. Steins was observed by the largest astronomical observatory on Earth, including the European Very Large Telescope (VLT) in Chile, JPL's Table Mountain Observatory, the Spitzer infrared astronomy satellite and Rosetta itself.
The Rosetta orbiter observed continuously Steins (1.06 AU away) for 24 hours with its science camera on 11 March 2006 in order to collect a “light curve” at phase angles larger than ever achievable from Earth, due to observational geometry constrains and for a longer time span, uninterrupted by day and night cycles. A total of 238 images were taken, covering four rotations.
Although researchers initially catalogued Steins in the S class (like most previously-visited bodies), the observational campaigns found that its spectral and polarimetric properties placed it in the E taxonomic class of reddish bodies with high albedo believed to be thermally evolved and of igneous origin, which underwent at least a partial melting and differentiation early in their history. E asteroid spectra probably makes them related to some rare enstatite chondrite or aubrite meteorites and therefore these bodies are believed to have a surface consisting of iron-free or iron-poor silicates. Observations pinpointed Steins' rotation period at about 6.05 hours, while the measured diameter could vary between 2 and 5 km, depending on its albedo. Asymmetries in the light curve confirmed that the small body has an irregular shape, with a ratio between the main axes of about 1.3. Other researchers pointed out that some of the characteristics of Steins could hint at a young and very rough surface at most a few million years old.
While fewer than 30 members of the E class of asteroids were known, including (44) Nysa, the largest, and two near-Earth objects, (3103) Eger and (4660) Nereus (a recurrent space mission target), little is known about the evolutionary history of the type. Steins was initially placed in the same family as (64) Angelina, but other studies showed that it shared most of the same spectral characteristics as Eger, both bodies being believed to be members of an old eroded family which formed in the inner asteroid belt close to the present position of Steins, which appears to be the largest member of the family. The presence of Eger in an Earth-intersecting orbit of course provides a path for enstatite and aubrite meteorites to hit our planet.
The encounter on 5 September 2008 will be at a distance of 1745 km and a relative speed of 8.6 km/s