Which spacecraft first entered orbit around a comet?

Rosetta was the first spacecraft to enter orbit around a comet.

The European Space Agency spacecraft reached comet 67P/Churyumov–Gerasimenko in August 2014 and began orbiting it after a decade-long journey. Rosetta used gravity assists and extended hibernation to complete the trip.

Its companion lander, Philae, made the first landing on a comet in November 2014. Although Philae’s anchoring system did not work as planned and the lander bounced before stopping, it returned valuable measurements from the comet’s surface.

Giotto had previously flown close to Halley’s Comet, but it did not enter orbit. Stardust collected comet dust during a flyby, while Deep Impact released an impactor. Rosetta’s defining achievement was sustained orbital study of an active comet.