Rosetta's most important job over the last few months has been to observe how the position of asteroid (21) Lutetia shifts against the background of fixed (fixed, that is, as far as Rosetta can see) stars. In a technique whose heritage can be traced to the skywatching of Earth's most ancient mariners, Rosetta (and, of course, its human navigators) employs the background stars to help it determine its course.
That optical navigation campaign is now complete; Rosetta has done its best to chart the course of Lutetia, and ESA's navigators will examine its photos to determine whether Rosetta's course is okay as currently set, or whether one final tiny puff of its thrusters is needed to steer the spacecraft past the asteroid at the desired distance. They've already said, though, when they made their decision to cancel the possible encounter-minus-40-hour thruster firing, that they expected the encounter-minus-12-hour firing would not be needed.
Here is one of the last photos taken as part of the optical navigation campaign, by Rosetta's sharpest camera, OSIRIS. You can't really say much about the asteroid from this photo -- its exposure is set to pick up faint background stars, so the view of the asteroid itself is overexposed -- but clearly Lutetia is in Rosetta's sights.
ESA 2010 MPS for OSIRIS Team MPS / UPD / LAM / IAA / RSSD / INTA / UPM / DASP / IDA
Rosetta approaches Lutetia (OSIRIS image)
Rosetta captured this image of asteroid (21) Lutetia on July 9, 2010, at 01:00 UTC, when the spacecraft was still about two million kilometers (and 36 hours) from the asteroid.
Closest approach happens at 15:44:56 UTC July 10, or about 21 hours from now. Let's all hope all the instruments and electronics operate perfectly for the upcoming encounter with the biggest asteroid yet seen up close!