Europe's Rosetta spacecraft flew within 800 kilometers of asteroid 2867 Steins on September 5, 2008. It performed observations related to the flyby during a two-month period from August 4 to October 6, 2008.
Mission literature states that closest approach occurred at 18:32:22 UTC, but an investigation of the spacecraft TARGET_CENTER_DISTANCE given in image headers shows that it occurred closer to 18:38:14. The latter time corresponds well to the growing and diminution of Steins in the images.
Steins is a small asteroid, 6.67 by 5.81 by 4.47 kilometers, equivalent in volume to a sphere with a radius of 2.65 kilometers. Steins' south pole is dominated by an impact crater 2.1 kilometers in diameter. The encounter was the first time a European mission had performed autonomous asteroid tracking with its onboard camera.
The NAC data set is available here. The WAC data set is available here. Information about the OSIRIS cameras is available here. Information in this page came from the MISSION.CAT file for the WAC data set.
Stares at Vega and 16 Cygni for instrument calibration
Steins lightcurve observations two weeks before closest approach
Steins lightcurve observations 1 day before closest approach
Close encounter imaging
Random encounter notes
The geometry of the flyby required a flip of the spacecraft near closest approach. The spacecraft took the time from 40 minutes to 20 minutes before closest approach to accomplish the flip.
The Narrow-Angle Camera went into safe mode about 10 minutes before closest approach.
Following is a list of the images acquired during the encounter using both NAC and WAC images. The observation titles in bold text came from the INDEX.TAB files for NAC and WAC data sets. The notes in italics are Emily Lakdawalla's so are totally unofficial!