The sky is big. Searching it for potentially hazard objects like asteroids and comets is hard. The best way to do it? A big 'scope, equipped with a BIG camera, and a wide, wide field of view. That's just what the Panoramic Survey Telescope & Rapid Response System -- PanSTARRS -- brings to the table. It's just a prototype, but it has a 1.8 meter 'scope on -- wait for it, wait for it -- Mount Haleakala, and it sports a 1.4 gigapixel camera. You read that right: 1.4 billion pixels. It scans the skies looking for threatening objects, and astronomers just announced they have found their first one: 2010 ST3, an asteroid 50 meters (150 feet or so) across. It was found September 16, when it was still 30+ million kilometers (20 million miles) from Earth. Here's the object in question:
How big a threat is this object? Well, not very: there's "a very slight chance" it will hit Earth in 2098, so I'm not terribly concerned. When astronomers map an orbit of an object, there's some uncertainty in the measurements. It's hard to get the exact position of the object, and its motion over a day or two isn't enough to get a good idea of its trajectory. The farther you try to project where it'll be in the future, the fuzzier the prediction gets. For something like 2010 ST3, there's a huge volume of potential space it might occupy come 2098, and it so happens that the Earth is in that same volume of space at that time. But the Earth is near the edge of the projected position, and as time goes on, and the orbit is better determined, the volume of space the asteroid might be in will shrink. Eventually, what almost always happens is that the Earth winds up outside that volume as our data get better. That's why the odds of it hitting us are so low.
Now, if it did hit us, it would be bad. An object a bit smaller than that carved out Meteor Crater in Arizona, a hole over 1.5 kilometers across (that's me posing in front of it; click it to get an idea of how big this scar is, and bear in mind the far rim is almost a mile away). An impact by something like that is about the same as exploding a 20 megaton bomb. So yeah, bad. The good news here... let me correct myself: the great news here is that Pan-STARRS found this thing at all! From that distance, an object this small is really hard to see, and no other asteroid survey could've found it. That means that as time goes on, Pan-STARRS will find lots and lots of threatening objects. And they're out there whether we look for them or not! So it's best to find the beasties before they find us. And when we do find them, we need to keep a really good eye on them. We need accurate orbits, and good statistics, so that we can figure out just how big a threat these guys are. 2010 ST3 is almost certainly benign, at least for the next century. But there are thousands more like it roaming the sky. We don't get hit very often, but we do get hit. For those of you who fret about such things, I like to say that this is something to be concerned about, but not something to worry about. Worry accomplishes nothing, but concern means we're turning our brains to the problem. And that's the very best way to solve problems. Image credit: PS1SC