Uhh, physics troll? h = gt^2/2; t = sqrt(2h/g); v = gt = g(sqrt(2h/g)) = sqrt(2gh) = 47 feet per sec = 32 mph. Not sure yet how you get more energy out of an inclined plane, but work it. Anyway, http://www.nasa.gov/topics/aeronautics/features/helo-droptest.html says: "We crash-tested the helicopter by suspending it about 35 feet (10.7 m) into the air using cables. Then, as it swung to the ground, we used pyrotechnics to remove the cables just before the helicopter hit so that it reacted like it would in a real accident," she explained. The test conditions imitated what would be a relatively severe helicopter crash. The flight path angle was about 33 degrees and the combined forward and vertical speeds were about 48 feet per second or 33 miles per hour (14.6 meters per second, 53.1 kph).

Hmm. I guess we should deorbit all those expensive weather satellites too. We can't really change the course of approaching hurricanes, so there's no reason to know about them.

Even without Bruce Willis, a month's notice of a km-class asteroid collision might save a billion lives, if the time were used wisely:

o Distribute and cache potable water, water purification, food, and antibiotics.

o Move people away from the coasts

o Prepare to move people from projected ground zero.

What would _you_ do if you had a month to get ready?