Science Questions

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.  They’ve 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 it’s going at about 10,000 kilometres per hour, that’s 2,800 metres per second, which means it’s got 6.4 million (6.4 x 10⁶) kilogram metres per second of momentum.  That’s an awful lot of momentum. For anything on Earth, that’s a scary amount of momentum.  However, the moon has got awful lot more momentum than that.  It’s 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 LCROSS’s crashing into it would change it’s momentum by 1%?

7.3x10²⁵ minus 6.4x10⁶ is roughly 1x10¹⁹

So about 10¹⁹ collisions.  So that’s 1 with 19 zeros after it (10,000,000,000,000,000,000!).

And actually, an LCROSS’s momentum compared to the moon is about the same as 1 millilitre of water compared to all the water in all the earth’s oceans.

October 2009