Rosetta wakes up for encounter with asteroid (2867) Steins / La sonda Rosetta pasará asteroide Steins el 5 de septiembre

Wow, just this morning I mentioned that Rosetta would be passing by the asteroid 2867 Šteins, and less then a day later there's an article on the craft at spaceref.com about how it has been dragged out of its slumber for the encounter:

Spacecraft controllers have just awoken Rosetta from hibernation to prepare for its encounter with asteroid (2867) Steins on 5 September. ESA's comet chaser will study the relatively rare asteroid as it flies by on its way to comet 67/P Churyumov-Gerasimenko.

This is the first of two:

Rosetta has swung by Earth twice and Mars once, performing gravity-assist manoeuvres, that gave it the necessary boost to continue on its journey. The third and last Earth swing-by is scheduled for November 2009. The spacecraft will also fly by two asteroids and study them on the way: (2867) Steins in September this year and (21) Lutetia in June 2010. As it closes in on (2867) Steins in September, Rosetta will have travelled about 3700 million km and will be 2.1 AU from the Sun.

Also, here are the objectives of the flyby:

The science objectives of the fly-by observations are as follows:

• To characterise the asteroid by studying its physical and chemical properties
• To study its kinematic properties (its rotation, for example)
• To study the asteroid's surface and to perform comparative studies with surfaces of other asteroids in order to understand differences between asteroid types
• To study the interaction between the solar wind and the asteroid
• To study the asteroid's environment, including the presence of natural satellites, the magnetic and electrical properties of the immediate environment, and gas or dust orbiting the asteroid

So what do we know about this asteroid so far? Well, it has a diameter of some 2 to 5 km, and rotates once every six hours. It's much smaller than the second asteroid it will by flying by, but the flyby speed is about half that of the second and the distance is also closer, at 800 km out compared to 2000 km for 21 Lutetia.

So what can we expect the flyby images to look like? One particularly notable flyby of an asteroid was that of 243 Ida in 1994 by Galileo as it was on its way to Jupiter. Here are some details to help imagine: Ida is 53.6×24.0×15.2 km in diameter, and has a moon named Dactyl that was discovered during the flyby that has a diameter of 1.4 km, and from a distance of 10,500 km out it looked like this:



So imagine the tiny moon on the right but two to three times as big, and taken from a distance some twelve times closer. I have no idea what sort of image resolution capabilities Rosetta has compared to Galileo though, so that's the one factor that I can't take into account.

Also, on the space.com forums my favourite poster has also commented on what the encounter should be like. Here's what he says:

It promises to be interesting, as we know virtually sod all about Asteroid 2867 Šteins.

Regarding sizes, a lot of it is assumption, based on brightness & spectra verses distance, other than those we have either seen up close (951 Gaspra, 243 Ida, 253 Mathilde, 433 Eros, 5535 Annefrank, etc) or those we have actually seen a shape for like 1 Ceres, 3 Juno, 4 Vesta, 511 Davida, etc.

So do not be surprised if 2867 Šteins turns out to be a completely different size to that expected. Because this willbe a relatively slow encounter, Rosetta should see much of 2867 Šteins during approach (earlier Rosetta obs suggests a rotational period of about 6 hours), as Galileo did with 243 Ida & 951 Gaspra.

AFAIK, the OSIRIS camera, also has a much higher resolution than did Stardust, so together with this & the much closer pass, the imagery & other data of 2867 Šteins, should be pretty spectacular. It is something to look forward to.

Also later on in July 2010 (IIRC), Rosetta encounters the much larger type M asteroid 21 Lutetia.

Best Stardust view of Asteroid 5535 Annefrank.

This Asteroid 5535 Annefrank photograph was taken under these conditions (quoted from the poster just above the last one):

For comparison, we have this picture taken of asteroid 5535 Annefrank taken from a distance of 3,300 km. The asteroid has a of length 8 km (it was predicted to be of half of that).

So, conclusion? This should be a very good flyby. Expect nice pictures.