Swarm of pebbles could safely deflect asteroids
http://www.newscientist.com/article/mg21428635.700-swarm-of-pebbles-could-safely-deflect-asteroids.htmlFLINGING pebbles at an asteroid sounds like a fruitless task, but a new calculation shows that this could deflect an Earthbound rock.
It takes surprisingly little force to deflect an asteroid, provided it is done several years before the projected impact. Previous ideas have included landing an engine on the asteroid to push it away from a collision, and using mirrors or lasers to vaporise its surface and provide thrust to shift its course.
Alison Gibbings and Massimiliano Vasile, aerospace engineers at the University of Strathclyde, UK, have another solution. A 500-kilogram swarm of fingernail-sized spacecraft would, they calculate, deflect a fast-moving, 250-metre asteroid by nearly 35,000 kilometres - easily enough to avoid a collision, provided the swarm hits eight years, or about three orbits, before the expected Earth impact. A swarm could be launched from Earth in a single rocket. After release, pebbles could harness the thrust provided by reflected sunlight to steer themselves into a tight cloud directed at the asteroid.
Best of all, each pebble would be too small to crack the asteroid into still-dangerous pieces, the pair reported on 17 April at the Astrobiology Science Conference in Atlanta, Georgia.
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The original article doesn't give much away on their model, so I don't know, if they took these into account:
1. The pebbles wouldn't impact at the same time. For the sake of simplicity, let's say they impact in two waves. The first wave will impact and sputter the surface. This will create a cloud of dust, pebbles and rocky material around the impact area. The second wave will hit this cloud first, greatly reducing the efficiency of momentum transfer.
2. Quote from article: "After release, pebbles could harness the thrust provided by reflected sunlight to steer themselves into a tight cloud directed at the asteroid." This is total bullshit. That's like focusing the shrapnel of a shotgun-shell in-flight with a fan.
Actually, the cloud of pebbles would get ripped apart by gravitational tidal forces over the course of those many years until they hit the asteroid. The cloud would increase in diameter over time. There sure would be gravitational attraction within the cloud, but it wouldn't be strong enough to keep it together.
Let this sink in:
Earth has 1 AU distance from the Sun, the Asteroid Belt 2,3-3,3 AU.
Their plan is to hit a moving 250m target over a distance of 1,3 AU. That's a ratio of roughly 1
78.000.000 (If you can hit a grain of salt from half a mile away with a sniper-rifle, your accuracy still wouldn't be good enough by far.)
the mother ship until within effective distance. The ship could even fire them off in waves, letting the dust cloud disperse between waves.
The secret to hitting something with a gun is to get close to the target. 
As for using solar thrust ... this is like steering a SAIL with a (very big !) fan.
