Assume an asteroid's moon is 1.6 km wide, and 148 km from the center of the asteroid, which is 55 km long. The moon's orbit is circular with a period of 21 h. Whatis the mass of the asteroid? Ok This has to do with Kepler's Laws and I don't really understand Kepler's Law. I tried using the Law Of Periods so I did (75600 s)^2=((4pi^2)/(GM))*(148 km)^3. I get the wrong answer so I don't know what I did wrong. Any ideas?