Captain: Morning all.
Cadet: Good morning sir!!
Cadet: Sir. All ship systems normal. No intruders in the no fly volume. Nothing ... to report.
Captain: You sound disappointed, mister.
Cadet: It was my first time taking the conn and ... I did nothing sir.
Captain: Excellent. If something came up ... what'd you do do?
Cadet: Call you and the X.O. immediately.
Captain: Capital. Good man. Now get out of my chair Shavetail.
Cadet: Aye sir. Thank you sir.
Captain: Beautiful planet isn't it? The blue oceans, the white clouds. Makes you a little homesick.
Cadet: Yes sir. Only home is two A.U.s away ... what happened here sir?
Captain: Now you're asking question above your pay grade, Cadet. For that matter, you are asking questions above my pay grade. No it's all right to ask questions. Just learn when to stop asking them out loud. The short simple answer is ... no one knows.
Cadet: But they are working on it, sir?
Captain: I should hope so. One minute all is well the next we have another Earth orbiting in opposition to our Earth. Most theories involve some kind of temporal incident. That's only because Contra-Terra is at the point in history we labeled 1957, old calendar.
Cadet: Why does this weird shit always happen in the Solar System, sir? Sorry sir.
Captain: I think we've pissed off too many of them big glowing heads myself. In any case we've had a merry dance these last six months, putting up deflector satellites to screen in system chatter, establishing an observation post on Contra-Luna, rescheduling Fleet assignments to mount a blockade here. Good thing Mars was on the far side of the sun with Earth. That shielded the doppelgangers from most of our comm signals. It was still dicey.
Cadet: Sir, Mr. Tivk was saying the beanpoles are going over Klordhop's theories to account for this being that the Cluster Shunt Effect was proven true.
Captain: Right. Klordhop is suddenly a celebrity and under heavy guard. Everyone though he was nuts before this. You'd think that you need someone a little crazy to figure out time travel.
Cadet: The last I heard ... he was saying that Contra-Terra belonged here and the rest of the galaxy got shunted back in time.
Captain: Makes as much sense as the big glowing head theory.
Contra-Terrene just is. It's an example of a major temporal shake up in a campaign that while certainly disconcerting is not going to pull the plug on the game (hopefully). What exactly caused it is up to the referee of course. Answers include calling an elder god a smug bastard, time travel hijinx, playing with an alien artifact or a planet slipping from an alternate reality for any of the previous reasons.
The era of Contra-Terra is also up to the referee. In the example above 1957 is as good a year as any. The Cold War and paranoia is in full bloom. The UFO craze is just getting off the ground and in a few years manned spacecraft will be in orbit. In a few months Sputnik is going to be launched and require further cover ups. Will a big reveal change history? Is this the past of the campaign's timeline?
Why pick Earth? They always pick Earth. It can be used with any planet with interesting results. An evolved peaceful race could suddenly be confronted by their warlike ancestors. A primitive race could be exploited by their descendants who squandered their resources.
Down the road characters might be recruited for covert missions to Contra-Terra to find out what the bejeebers is happening. They might be humans or recruited from humanoid aliens to avoid any paradoxes. The Fleet has already foiled twelve attempts to engineer a grandfather paradox and taken the criminals into custody.
What happens when the Fleet discovers the flying saucer sightings of the era are real (at least on Contra-Terra).
Cadet: Port engine is out sir. Torpedoes coming back online!
Captain: They never told me I'd have to turn back a damned invasion from Dimension X to preserve what might be our history ... fire torpedoes 1, 2 and, 3 on my mark! Target the big saucer!
Cadet: this explains what those Nazis were doing on the moon at least ... incoming!! Brace for collision!!