1. The problem statement, all variables and given/known data 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 km across. Observations of surface debris released by the impact showed that dust with a speed as low as 1.0 m/s was able to escape the comet. How far from the comet's center will this debris be when it has lost 90 % of its initial kinetic energy at the surface? 2. Relevant equations 3. The attempt at a solution The equation for escape velocity can help determine the mass: 1.0 m/s = sqrt((2*G*M)/4500 m), M = 3.4 x 10^13. KE at the surface = (1/2)(3.4 x 10^13)(1.0)^2 = 1.7 x 10^13 -- 90% of that is 1.53 x 10^13 J. So 90% of the kinetic energy has been converted into potential energy, right? Potential energy is (GMm)/r... but then I have two unknowns (m & r). How do I solve this? Is there another equation that can make the m cancel?