Re: Why copper?
From the NASA Deep Impact mission's web page at: http://deepimpact.umd.edu/faq2.html#q2
Spectroscopy, yes, but also because "engineering". My assumption is that these reasons apply to Hayabusa2 too.
<quote>Copper was chosen because it will cause the least interference with the measurements that will be made during the impact, will not leave a residue that would confuse potential future measurements, and can be made into a structurally strong impactor. In particular, all the inner shells of electrons for copper are completely filled. This means that it reacts very slowly with other elements, such as with the oxygen in cometary water, and it will end up producing relatively few bright emission lines in the spectrum of the vaporized materials. Other materials such as aluminum would produce far more and stronger emission lines (mostly due to aluminum oxides). There are only a few materials that satisfy this criterion and copper is the least expensive of them that is structurally sound. The material used to make the impactor is actually a copper alloy with about 3% beryllium to make the copper more stiff.</quote>