Andromeda

Have you ever played spin the bottle, except instead of a bottle, it's a gun, and instead of a gun, it's a two-sided gun that can shoot up or down, in any direction? You have? Wow. OK, then, you already know about the kind of danger facing Earth from gamma ray bursts. But, listen to this description of "megatars" (Yes, you have to read it aloud in order to "listen" to it. But read it aloud, in a dramatic voice, with thrilling theme music):

Thought to be a mighty cataclysm in a super-dense, highly magnetized star called a magnetar, it emitted as much energy in two-tenths of a second as the sun gives off in 250,000 years. Its intrinsic power was a thousand times greater than the power of all other stars in the galaxy put together, and ten thousand times brighter than the brightest supernova.

Isn't that just like the opening of "Superman" or something? Not "Gilligan's Island," though. A "megatar" is a gamma ray flare. In this artist's conception, the megatar looks a lot like a "Daddy Longlegs" spider on fire. Are you scared? Well, then, you don't even want to know about gamma ray bursts. But GRBs -- Earth-directed and close enough to endanger life on Earth -- are expected to occur one or two times every billion years (pdf).

So why aren't we terraforming Mars yet? We'll never survive until the Sun becomes a Red Giant, vaporizing Earth, and Andromeda Galaxy crashes into our galaxy, vaporizing Earth, if we don't start colonizing now! If we just shipped fifty percent of our Republicans to Mars, they and their factories, SUVs, and cigars would have that planet warm and toasty in no time. But then they'd outlaw immigration.