[MetalMan]

I think they are concerned about it falling out of orbit, and the heavy materials being spread out far in the atmosphere. But as long as the craft left Earth's gravity to explore deep space, this really isn't an issue.

Unless they're concerned about us pissing off some aliens or something. Who knows how those environment freak's brains work anyhow?

[NormsRevenge]

Here's some info from NASA on Prometheus (formerly the Nuclear Systems Initiative) . Pretty neat stuff.