[api]

Aliens from different biospheres will never be able to have physical contact.

If they are close in biochemistry the dangers are extreme to catastrophic. If we actually found a crashed UFO with bodies we should drop a thermonuclear bomb on it immediately. Just ask the Native Americans.

If they are not biochemically close chances are we and our environment would be horribly toxic, freezing, or boiling hot to them and vice versa.