How Big of a Problem Is Space Contamination?

This question originally appeared on Quora.

Answer by Robert Walker, writer of articles on Mars and space:

For me, the No. 1 issue that is rarely discussed is contamination of the solar system by Earth microbes and of Earth by samples returned from space. It is not so much that the issues are not discussed at all. There are many papers and workshop reports about them. The problem is that they are rarely discussed by the public and are ignored in news reports. So there is hardly any public awa...