Yeah, but can it be fired from orbit? I'll have to look it up, but for a time the US was knocking around the idea of essentially dropping a guided tungsten rod from orbit to knock out deep bunkers and other high-value targets. (A tank would actually NOT be considered a high-value target in this scenario.)
Here's a link:
http://www.sfgate.com/cgi-bin/articl...NGS6HID5A1.DTL
Here's the Google Search I used:
http://www.google.com/search?q=rods+...utf-8&aq=t&rls
They're called Rods from God.