Colin Stuart aka. Sky Ponderer asked:
How many LCROSS NASA missions would it take to change the orbit of the moon by 1%?
Dave - Interesting question. What they were doing was firing the top stage of a centaur rocket and crashing it into the moon. Theyíve been trying to watch the plume of stuff that comes up from that to see if there is water in that plume.
Now the centaur rocket weights about 2.3 tons and itís going at about 10,000 kilometres per hour, thatís 2,800 metres per second, which means itís got 6.4 million (6.4 x 106) kilogram metres per second of momentum. Thatís an awful lot of momentum. For anything on Earth, thatís a scary amount of momentum. However, the moon has got awful lot more momentum than that. Itís moving at a kilometre per second and it weighs 7.3 x1022 kilograms.
That means the moon has got 7.3 x 1025 kilogram metres per second of momentum.
So, how many LCROSSís crashing into it would change itís momentum by 1%?
7.3x1025 minus 6.4x106 is roughly 1x1019
So about 1019 collisions. So thatís 1 with 19 zeros after it (10,000,000,000,000,000,000!).
And actually, an LCROSSís momentum compared to the moon is about the same as 1 millilitre of water compared to all the water in all the earthís oceans.