Last week was pretty much exciting. We had 2012 DA14 passing above our heads and many scientists took the chance to explore it better - it was very nice opportunity to do it. However, headlines were stolen by meteoroid which showed some resistance entering our atmosphere and did a bit of damage in Russia. Needless to say, media exploded in hysteria writing down all sorts of stupid things - I support idea of re-introducing electric chairs as some "journalists" deserve to take some rest there.
Before I go to our little friend which is now known as the Chelyabinsk meteor, let's see what did we learn about 2012 DA14. We know now for sure that its dimensions are 20x40 meters and yes, it is a rock. If this kind of rock would land upon us, we would be in trouble. An impact by asteroid like DA14 would be like detonating 20 million tons of TNT on the Earth. It is not like dinosaur-killer, but it’s not something you want either. But we were never in danger as all estimates kept saying it would pass some 27000 km away from us and so it did. If it didn't pass so near, probably no one would care about it.
The rock that came in over Russia was probably some 17 meters across and had a mass between 7000 to 10000 tons. It was moving many times faster than a rifle bullet, and thus had a lot of kinetic energy (the energy of motion). Slowed by the Earth’s atmosphere, it released that energy as a series of explosions that totaled something like 30000 to 500000 tons of TNT, roughly the yield of a small nuclear weapon. It was so huge and bright it was seen for hundreds of kilometers. Russians commonly have video cameras on their dashboards, so videos of the event popped up on the internet within minutes. In first moments, media was all over the place connecting these two events, but that was just not possible for anyone educated. For example, Russian meteor happened 16 hours before the passing of DA14. In that time, the Earth moved 1.7 million kilometers. And even if you will suspect debris cloud, these two came from different orbits. So just forget it, there is no connection between these too. After 24 hours, most of media came to their senses, but not all.
Soon media started to write down how we should have detected this, how we should have monitoring for small rocks as this and all related fantasies. Let's be clear on this and I will use words by Phil Plait here - another big impact like Chelyabinsk may not occur again for 500 years, or another may come in tonight. There are billions of such rocks in space, and they are simply too small and faint to track. We can only talk about them statistically. The same is true, more or less, with DA14-sized rocks as well, but most likely there are a million or so of them, with 10000 having been spotted. And that's 1% only. So yeah, we need monitoring, but monitor what? When people think asteroid impacts, they think of dinosaur killers. But we have most of the 10km asteroids mapped out, and we know we’re pretty safe from them. Going back to Russia example, does it make sense to monitor those? I'm pretty much on the same page as Lubos Motl who doesn't see this as good approach. As Lubos states, one should be reasonable about the costs-and-benefits analysis. If we were hypothetically able to prevent the damage caused by meteorites similar to the Russian one (by size as well as population density in the target), we could save millions or billions of dollars per century. Not much. A meteorite similar to the Russian one is as harmful as an earthquake of a certain moderate magnitude. It has a similar local impact, too. Earthquakes of this equivalent magnitude are taking place much more frequently than the collisions with the Russian-size meteoroids. So the overall impact of the meteorites of this size is a small fraction of the impact of the earthquakes - and we don't really fight against the earthquakes too efficiently, either (even the arresting of innocent Italian seismologists fails to be an efficient way to fight earthquakes).
In first sequel of Hit Me Baby One More Time, I wrote of some methods at our disposal to protect ourselves from this manace coming towards us. Of course, I don't feel we have choice in fight against small rocks like Russian one - but I do not see those much of the threat. Of course, they might hit (very unlikely) some populated city and kill millions of people, but still that's not big price to pay. Bigger rocks are the problem and as you might have guessed. detecting it is just one part of the problem. Deflecting it is second part. If you imagine that we would have kind of anti-missile-like batteries deployed around the globe (or specific selfish country), you still need to deploy in fashion that you know what you are doing and that calculations and timing is right (bare in mind these things move fast and you must move even faster to leave Earth). However, getting some good method to deflect big boys which we would detect ahead of the time does make sense. Right now we have nothing tested in real life. But interesting ideas pop up every now and then.
Space Generation Advisory Council made so called Move An Asteroid contest and 2012 winner is MIT grad student Sung Wook Paek. MIT guys are clever folks and sometimes they really come up with interesting ideas. For example, Sung suggested we fight asteroid with paintballs. And it is not that we should shoot asteroid with paintballs, but rather help Sun to do its thing. How? What? Simple. Moving asteroid will have hige momentum (mv). Those paintballs will have low mass (m) so no big deal for our rushing asteroid. The sneaky part of the attack will be to increase the reflectance of the asteroid so that instead of absorbing solar photons, it reflects them. Why you may wonder? While photon is seen as massless particle, it still has momentum (E/c) so every time asteroid absorbs a photon, conservation of momentum requires that the late photon's momentum transfers to the asteroid. Astronomers are aware of this radiation pressure, and they include it in predictions for asteroid trajectories. However, if instead of absorbing the photon, the newly anemic asteroid reflects it, that means the photon not only loses its original momentum, but it now has the same amount of momentum in the opposite direction. So to cancel that out, the asteroid has to gain an equal amount of momentum in the photon's original direction.
As Robert Cooper draw parallel, imagine playing catch on ice skates. When you catch the ball, it transfers momentum to you and pushes you backward. Now, if like a paintballed asteroid you throw the ball back whence it came, you get pushed backward once again. This increase of momentum transfer is what pushes the asteroid off course. The momentum from each photon is tiny, but the photons are unlimited and free.
Credits: MIT, Phil Plait, Lubos Motl, Robert Cooper