Perhaps this is not such a great scenario for a campaign, but here goes:
Orion and a few back up cruisers/whatever jump into Sol. The guvnor orders the appropriate hails to be sent out on all channels. They wait for half an hour, but no response from anybody. Even hours of long range sensor sweeps pick up nothing moving.
A course is laid in for Earth and the jump is made. When the Orion finally settles into orbit, the crew can't get a reply from the planet or the Moon. Sensors detect no orbital platforms or satellites at all, and no functional electrical systems down below.
After a few hours, just as a mission to the surface is being prepared, a weak signal is picked up from the planet. After being filtered and boosted in every way conceivable, the crew of the Orion can just make it out...
"If you're hearing this, you've got to get out! NOW! They're comming!*static*.........." or words to that effect.
And then either:
1> 'They' appear
2> The expedition attempts to find out what happened
3> The expedition is never heard of again. Command sends another scouting party, which also disappears, followed by an entire GTVA fleet. Not one ship makes it back. Command closes the node until further notice.
Or whatever else seems appropriate... hmm, I think I feel a piece of fanfic comming on... better go lie down a while...
