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 .