This One Didn't Have Our Number After All...While ridiculous ancient prophecies don't pose much of a threat to our planet, there are real risks out there. Many of them are caused by humans, such as releasing toxins in the air, water, and soil of our planet, messing with the composition of the atmosphere, etc. But some are natural in origin, and while we might never be able to do much about super-volcanoes (which are super-scary), we could probably do something about space rocks on a collision course with Earth if we had enough time. That's why it's so important to monitor the skies, keep track of what's out there, and calculate orbits far into the future to make sure that there's no danger. Impacts from large near Earth objects (NEOs) might be rare, but their potential for destruction is high enough that it's worth spending the resources on a few telescopes and astronomers; it's like vaccinating against a rare but deadly disease -- better safe than sorry.
One example of why it's important to watch the skies is 2011 AG5. It's an asteroid about 500 feet in diameter that astronomers thought had a less than a 1% chance of hitting Earth in February 2040. Now that might seem like a small probability, but when the stakes are high, you don't want to rely on luck. Would you take a bit of a sandwich that I tell you has a 1% chance of being full of cyanide and scorpion venom (or whatever...)?
I said that the astronomers "thought" it had a less than 1% chance because after more observations and refining orbits, they now believe that we're safe and that it'll be a clear miss.
To narrow down the asteroid's future course, NASA put out a call for more observation. Astronomers from the University of Hawaii at Manoa took up the task and managed to observe the asteroid over several days in October.
"An analysis of the new data conducted by NASA's Near-Earth Object Program Office at the Jet Propulsion Laboratory in Pasadena, California, shows that the risk of collision in 2040 has been eliminated," NASA declared Friday.
The new observations, made with the Gemini 8-meter telescope in Mauna Kea, Hawaii, reduce the orbit uncertainties by more than a factor of 60. That means the Earth's position in February 2040 is not in range of the asteroid's possible future paths. (source)
2011 AG5 is now believed to come no closer to Earth than 890,000 kilometers (553,000 miles), or more than twice the distance to the moon. Now that's good news for xmas!