# MIT student defends the Earth with paintball

On Friday, Sung Wook Paek, a PhD student in Aeronautics and Astronautics at the Massachusetts Institute of Technology (MIT), took 1st Prize in the Move an Asteroid 2012 Competition with his novel idea for protecting the Earth from potentially dangerous asteroids — paintballs.

Paek suggests that a small spacecraft could be used to launch clouds of paintballs, filled with white paint powder, at an asteroid. Two clouds would be arranged to match the size and shape of the asteroid, and timed so that one cloud would cover one half of the asteroid, and the second cloud would cover the opposite side. The timing of the impacts from the paintballs themselves would be carefully managed so that they would all be from the same general direction. These impacts would cause a small shift in the path of the asteroid — an idea that was part of last year's winning proposal — but it is the addition of the white paint that gave Paek's idea the winning spot this year.

Covering the surface of the asteroid with white paint would increase the asteroid's albedo, thus reflecting more sunlight off of its surface. This would cause an increase in 'solar radiation pressure' acting on the asteroid, which would act to push the asteroid further away from the Sun.

Photons — particles of light — have zero mass, but they do have both energy and momentum. It is the momentum that is important to this process. Momentum, like energy, cannot be created or destroyed; It can only be transferred to other things. When photons are completely absorbed by the surface of an object, they transfer their momentum to that object. The force exerted on the object by this is determined by how much its momentum changes.

This is what happens with most asteroids, because they are composed of very dark materials, so they almost completely absorb any sunlight that shines on them.

However, if photons are completely reflected from the object — which is what Paek's plan tries to achieve — they arrive at the object and depart from the object with exactly the same momentum. This means that the total change in momentum is actually doubled, therefore the force exerted on the object is doubled.

Paek tested his hypothesis using simulations using Apophis, a 320 metre-diameter asteroid that is estimated at around 30 million metric tons, which is set to swing very close by Earth in both 2029 and 2036. According to Paek, it would take around five metric tons of paint to coat the surface of Apophis, and it would likely take around 20 years for solar radiation to push the asteroid off course.

However, this method could be used to coat an asteroid with other materials. Since part of the threat of these objects is the speed at which they're traveling, you could use this idea to slow an asteroid down by painting it with particles that would add atmospheric drag and slow it down. Or, you could just paint it with specific colours to make it easier to track and identify.

As we become more aware of the number of near-earth asteroids that are out there, more and more attention is being given to ideas such as Paek's.

"It is very important that we develop and test a few deflection techniques sufficiently so that we know we have a viable 'toolbox' of deflection capabilities to implement when we inevitably discover an asteroid on an impact trajectory," said Lindley Johnson, program officer for NASA's Near Earth Objects (NEO) Program.

(Photo courtesy of The Canadian Press)

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