Original: The Giotto probe found that Comet Halley's nucleus reflects approximately 4% of the light that falls on it, and Deep Space 1 discovered that Comet Borrelly's surface reflects 2.4 - 3.0% of the light that falls on it.
Simplified: When light shone on Comet Halley's nucleus, the comet reflected only 4% of the light back to us.