What does a one-in-ten-million chance of apocalypse look like? Well, it used to look like this:
That is asteroid 2005 YU55, a near-Earth object (or NEO) that also happens to be a PHA, or potentially hazardous asteroid. It has an orbit which intersects the Earth, which means that someday it could possibly hit us.
Now before you panic -- and I'll make this clear: DON'T PANIC -- that doesn't mean you'll wake up tomorrow to see flaming death streaking across the sky. Think of it this way: when you walk to the local convenience store to get a squishy, you have to cross the street. The path you take intersects the street, but as long as you don't try to occupy the same spot as a moving car, you won't get hit. Same with PHAs: their orbits cross the Earth's orbit, but space is big. As long as the Earth and the asteroid aren't at the same place at the same time, we're OK. Since we don't know the orbits of these objects perfectly, we assign a probability they will hit us over some period of time. Up until recently, YU55's chance of hitting us over the next century was calculated to be about 1 in 10,000,000, which is reasonably close enough to 0 for me. However, it's always good to get better data. In this case, very good: new observations have eliminated the chance that YU55 will ruin our day for at least a century to come.
YU55 was observed with the monster 300 meter (1000 foot) Arecibo radio telescope in Puerto Rico. Arecibo can send little radar pings into space, aimed at an asteroid. The pings reflect off the rock, come back to Earth, and the timing of each one can be logged. This tells us how far away the asteroid is, how big it is, and even (by carefully measuring the different arrival times of the pings back on Earth) the shape of the asteroid.
If this sounds familiar, that's because this is how dolphins and bats sense their environment. They use sound, not light, but the principle is the same. So what did Arecibo tell us when it dolphinated YU55?
The good news is that the orbit of the asteroid was nailed down better, and that 1 in 10,000,000 chance of an impact in the next century dropped to 0. Nada. Nil. And astronomers are so confident of that they removed YU55 from their Risk Page.
So we're safe from YU55 ruining our day for quite some time at least.
And that's good, because, as it turns out, YU55 is bigger than expected: about 400 meters (a quarter mile) across, twice as large as previous estimates showed! Something that big hitting us at orbital speeds would explode with the force of a lot of nuclear weapons -- a few thousand megatons, or a hundred times the yield of the largest bomb ever detonated.
So yeah, yay! It won't hit us, and that's by any definition good.
But the middlin' bad news is that this also means is that it's tough to get good size estimates for asteroids without this technique. Usually, the size of a rock is determined by measuring how bright it is. A bigger asteroid reflects more light, and by measuring how well it reflects sunlight we can estimate the size. But that doesn't always work so well, as YU55 is telling us. Clearly, we need to use multiple methods to get the sizes of these guys.
Arecibo's funding is constantly under attack, yet it's the best machine we have to get the sizes of and, more importantly, accurate orbits for these potentially life-threatening objects. YU55 is off the list now, but there's a long line of rocks ready and waiting to take its place there.