How to solve for the asteroid's m when given information about spacecraft's orbit?

After the NEAR spacecraft ped Mathilde, on several occasions rocket propellant was expelled to adjust the spacecraft's momentum in order to follow a path that would approach the asteroid Eros, the final destination for the mission. After getting close to Eros, further small adjustments made the momentum just right to give a circular orbit of radius 41 km around the asteroid. So much propellant had been used that the final m of the spacecraft while in circular orbit around Eros was only 500 kg. The spacecraft took 1.03 days to make one complete circular orbit around Eros. Calculate what the m of Eros must be.