Why slam a copper impactor into Comet Tempel 1? Wouldn’t copper vapor contaminate the spray? Why not a high-temperature ceramic?

P.M. deLaubenfels
Corvallis, Ore

According to Casey Lisse of the Deep Impact team, copper was chosen because its density put a lot of mass into a small package, its relative softness reduced bounce at impact, and its large atomic number and density contrast with those of elements that were expected to spew from the comet at impact (see “Deep Impact,” in this week’s issue).—R. Cowen