I've been scratching my head for a long time, trying to figure out why NASA hasn't been taking the idea of preventing asteroid impacts more seriously. This idea has everything you'd want in a project: it's cool (I mean, c'mon, we're talking asteroid impacts!), it's doable, it's not terribly expensive, it's already on the public's mind thanks to Hollywood, and there's always the eensy-weensy possibility that you might save all of humanity.
Yet, despite this, it's been an uphill battle to get NASA to pay attention. While the space agency has been very good about supporting early detection programs, the support for a space mission to prevent an impact has been lacking. Of course, given their relatively small budget (<1% of the federal spending) I imagine taking on anything like this would be difficult.
So I'm pretty chuffed that the European Space Agency is looking into saving our collective skins. They've being studying the feasibility of a mission to test methods of asteroid impact mitigation, including a very very cool space mission they've dubbed Don Quijote (first proposed in 2002, and may launch sometime after 2020). It's actually two separate spacecraft: one to impact a small near-Earth asteroid, and another to monitor the event carefully to see what happens, including how much the orbit of the asteroid was changed.
The idea here isn't complicated: if we see an asteroid on an impact trajectory with Earth, we want to change the orbit so it doesn't hit us. We could try blowing it up, but that's actually a bad idea: at best it creates a lot of debris that can still smack into us, some of which may still be big enough to do us serious harm. So a better idea is to make sure it doesn't hit us at all.
Impacting an incoming asteroid is a good way to do this. It probably won't shatter the rock, and if done with enough advance time the orbit can be changed enough to prevent an impact with Earth. Think of it this way: imagine you're crossing the street. If you see a car really close, you have to move quickly, but if it's far away you can take your time. If you have enough lead time, even a small change in the car's velocity can make the difference between a hit and miss. Slamming a space probe into an asteroid changes its velocity, and if it changes enough we'd prevent an impact.
The problem is we've never tried this, so it's not clear how big an impact you need. Don Quijote would be a big step toward that goal. By making extremely precise measurements of the impact and the change in the asteroid's velocity (which may be impossible to do from the ground due to the tiny orbital changes made) we can get a handle on how well we can do this.
One recent paper published looked into what sort of instruments would be needed to achieve the goals of the mission. They looked at deflecting two different asteroids; they have different sizes (320 vs. 680 meters in diameter), masses, and orbits as well: one is on a more elliptical orbit making it harder to get to but easier to deflect, and the other on a more circular orbit that's easier to get to and harder to deflect. This study helps clarify what equipment will need to be built to do all this, and is a great step toward understanding what must be done.
Mind you, NASA did something like this back in 2005 with the Deep Impact mission, but there the primary goal wasn't seeing how much the orbit of a comet was changed after an impactor whacked it, but instead to try to dig up surface material and see what lay beneath. Also, comets vent material which changes their orbit all the time, making it difficult to know just how much of a difference Deep Impact made. Don Quijote would avoid that by hitting an inert rock.
I certainly hope the ESA continues to fund this project, and that it eventually gets the green light. I would love to see something like this happening soon. It's pretty unlikely that an asteroid big enough to really hurt us will be spotted in the next few years, but the thing is, one will eventually turn up. While it may not be tonight, over the next few decades the odds go up considerably. Since it generally takes many years for a mission to get from an idea to launch -- and many more to get to its target and do what needs to be done -- the sooner we get started on this the better.
Image credit: ESA - AOES Medialab. Tip o' the Whipple Shield to Smithsonian Magazine.