The 1978 fix-up In the Ocean of Night is the first volume in Gregory Benford’s Galactic Centre series1.
In the far-off year of 1999, British-American astronaut Nigel Walmsley is part of a two-man team sent by NASA to the asteroid Icarus. Unexplained out-gassing has transformed a body remarkable only for its eccentric orbit into an impending Earth-impacter. Nigel and Len’s mission is to determine how much, if any, of Icarus remains. If enough material is left to present a significant risk to the Earth, they are to destroy or divert Icarus with the Egg, a fifty-megaton fusion bomb.
The hope was that nothing would remain after the Egg had been used. The expectation was that a chunk of rock and iron might head for Bengal. The reality was a surprise: the large mass that had survived the out-gassing was an alien spaceship.