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ESA Releases Lutetia Flyby Images
The European Space Agency has released images from yesterday's close approach of asteroid 21 Lutetia by the Rosetta probe. At its closest, the probe was a mere 3,162 km from the asteroid, passing at 15 km/s and snapping photos sharp enough to make out features as small as 60 meters.
"Rosetta operated a full suite of sensors at the encounter, including remote sensing and in-situ measurements. Some of the payload of its Philae lander were also switched on. Together they looked for evidence of a highly tenuous atmosphere, magnetic effects, and studied the surface composition as well as the asteroid’s density. ... The flyby marks the attainment of one of Rosetta's main scientific objectives. The spacecraft will now continue to a 2014 rendezvous with its primary target, comet Churyumov-Gerasimenko. It will then accompany the comet for months, from near the orbit of Jupiter down to its closest approach to the Sun. In November 2014, Rosetta will release Philae to land on the comet nucleus."
There is also a replay of the media event webcast on the ESA's website.
I've read the replies to this question, and would like to give my (insider's) input. In simple terms, the flyby distance was determined to be no closer than 3160km because the whole object needed to fit inside the tracking camera's field of view. Otherwise, we would get unpredictable results in the spacecraft attitude at closest approach. Given that the NAVCAMs have a 5deg x 5deg FOV and we could use about 45% of that (for processing speed/exposure time reasons), and the asteroid was estimated to be 130km on its longest axis, add some safety margin, and you get something of the order of the given distance. Incidentally, we were 3160km from the center of the object, so a bit closer to the surface. There was some speculation about the fuel impact a closer flyby would have had. There would have been none, since we would have aimed closer at a very early time already. As it was, we actually had to move out from the asteroid with a correction maneuver.