The Mote in God's Eye
From SolarSailWiki
By Larry Niven and Jerry Pournelle.
Simon and Schuster, New York, 1974. ISBN 0-671-21833-6
This is a science fiction story about humanity's first contact with alien intelligence about 1000 years after attaining faster-than-light travel. The aliens contact humanity by sending a piloted laser sail to an inhabited human world.