Spacecraft Deep Impact - Comet's mass

[treverd]

Homework Statement

On July 4, 2005, the NASA spacecraft Deep Impact fired a projectile onto the surface of Comet Tempel 1. This comet is about 9.0 {\rm km} across. Observations of surface debris released by the impact showed that dust with a speed as low as 1.0 {\rm m/s} was able to escape the comet.

1)Assuming a spherical shape, what is the mass of this comet? ( The escape speed for an object at the surface of Earth is 11.2 {\rm km/s} ).

2)How far from the comet's center will this debris be when it has lost 70 {\rm \%} of its initial kinetic energy at the surface?

I have absolutely no idea what to do. any help would be appreciated.

Homework Equations

I know that for part a it is V=sqrt((2GM)/r)

but then how I calculate the lost kinetic energy.

The Attempt at a Solution

 

[Hootenanny]

but then how I calculate the lost kinetic energy.

You know that it is 70% of it's intial value...