Given the incredible amounts of energy in a supernova explosion – as much as the sun creates during its entire lifetime – another erroneous doomsday theory is that such an explosion could happen in 2012 and harm life on Earth. However, given the vastness of space and the long times between supernovae, astronomers can say with certainty that there is no threatening star close enough to hurt Earth.
Astronomers estimate that, on average, about one or two supernovae explode each century in our galaxy. But for Earth's ozone layer to experience damage from a supernova, the blast must occur less than 50 light-years away. All of the nearby stars capable of going supernova are much farther than this.
So if a nearby star goes supernova we'll know it is due to an alien attack aimed at wiping out our species. You never can tell what Vogons might rationalize as sensible.
Why we need more space telescopes watching near and distant stars: Look for signs that stars or solar systems are being engineered. Even if a nearby star isn't being tampered with and we do not face a need to burrow underground to survive a period of missing ozone layer we still should find out whether alien species are operating at such scales of engineering that astronomers can detect it.
But we are still free to worry about gamma ray bursts. So we've got that going for us.
A gamma-ray burst could affect Earth in much the same way as a supernova -- and at much greater distance -- but only if its jet is directly pointed our way. Astronomers estimate that a gamma-ray burst could affect Earth from up to 10,000 light-years away with each separated by about 15 million years, on average. So far, the closest burst on record, known as GRB 031203, was 1.3 billion light-years away.
So did the Mayans get a visit from aliens who told them a gamma ray burst was headed our way?
As with impacts, our planet likely has already experienced such events over its long history, but there's no reason to expect a gamma-ray burst in our galaxy to occur in the near future, much less in December 2012.
So if a gamma ray burst hits us in December 2012 that is strong evidence that some aliens have faster-than-light spaceships, rushed ahead of a gamma ray burst from 10,000 light years away, and told the Mayans about it. So in December 2012 we could discover that there's intelligent life out there. How cool is that?