I've been thinking about the satellite collision that happened last week and just how amazing it is that it actually happened. I mean, I know there's a lot of stuff up in space orbiting our planet. But there's also a lot of space up there.
If you think about it, two objects traveling along the same line (essentially one dimension) have a pretty good chance of running into each other. You get one going in the wrong direction or at a different speed and, well, it's gonna happen (think about two trains running on the same track). Adding an additional dimension, like with cars driving on the road, there's still a reasonable chance that two will collide, but the odds go way down. Think about how many cars are on the roads each day and how few of those actually hit each other. There's just more room in which to move and more ways to move out of the way of each other. Then, adding the third dimension, the odds of a collision go down even further. Think about airplanes. It's rare to hear about a collision of two aircraft that are in the air. The chances of two airplanes being over the same bit of ground, at the same altitude at the same time aren't very high. Airplanes are just too small compared to how much air is up there to fly around in. And airplanes only have a couple of miles of altitude in which to fly. Once you get into space, there's not only a whole lot more miles of altitude to inhabit, but the farther you go up, the more space there is. (Think about it like this: Draw a small circle. Now draw a bigger circle around it. Do that a couple of times. Now, start at the center of the circle and draw a line to the edge of the biggest circle. Do that again. Notice how the point where the lines intersect the circle get farther away as the circles get bigger? Same idea with altitudes. The farther up you go, the more room you have on that circle...or in that altitude.)
So, in order for these two satellites to collide, they had to be over the same bit of ground, at the same altitude, at the same time. Looking at the satellites' orbits (top inset picture here) , these two satellites were rarely over the same bit of ground at the same time. Add in that they needed to be at the same altitude (objects in orbit are constantly losing altitude. It's not a lot, but that's why you sometimes hear of satellites needing to be "boosted" in orbit, or the space station needing to fire it's engines in order to increase the altitude. So, satellites are always changing altitude. It's slow, but over time it makes a difference.), and the odds have to be astoundingly low. It just boggles my mind thinking about all the things that had to come together to get these two little specks (compared to the gianourmous size of space) to collide.
Also, the Astronomy Picture of the Day is a very cool web site and should be in everyone's RSS feed. (What! You don't have an rss reader? Start here. Best thing since the iPod...which is even better than sliced bread.)