the force between Venus and the object is:
F=GM1M2/r²
to lift the object to 900km space from the surface needs to overcome the force F to do work,this work is equal to the energy.
W=∫Fdr=∫(GM1M2/r²)dr=GM1M2(1/r1-1/r2)
这里要用积分.就跟将地球上的物体提高到一定高度需要克服重力做功一样.r1就是表面的半径,r2就是r1+900km,M1=4.87*10^24kg,M2=1kg