It is part of a series of images taken for optical navigation purposes as the spacecraft approached:
ESA / Rosetta / MPS for OSIRIS Team MPS / UPD / LAM / IAA / SSO / INTA / UPM / DASP / IDA
Comet Churyumov-Gerasimenko develops a coma, March 27 to May 4, 2014
A sequence of images showing comet 67P/Churyumov–Gerasimenko moving against a background star field in the constellation Ophiuchus between 27 March and 4 May 2014, as the distance between the spacecraft and comet closed from around 5 million to 2 million kilometers. The comet (and Rosetta) were between 640 million km and 610 million km from the Sun during the sequence. The comet is seen to develop a dust coma as the sequence progresses, with clear activity already visible in late April. Exposure times are 720 seconds for each image, taken with the OSIRIS Narrow Angle Camera. The globular cluster M107 is also clearly visible in the field of view.
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Original image data dated on or about April 30, 2014