Frederik Pohl & Jack Williamson - [Cuckoo 01] Farthest Star

CUCKOO WAS COMING

No one knew what it was--but everyone knew it was trouble.

Designated Object Lambda when it first appeared on the fringes of the galaxy, 20,000 light-years away, it was traveling at one sixth the speed of light.

The astrophysicists said that it was vast... lights . . and had the potential for utter destruction. So an existing space probe was reoriented to intercept, and staffed with replicates of both humans and aliens.

Then the space probe began to leak radiation...