Answer: 10000 kg
Step-by-step explanation:
The momentum 
 is given by the following equation:
 is given by the following equation: 
 (1)
 (1) 
Where: 
 is the mass of the object
 is the mass of the object 
 is the velocity of the object
 is the velocity of the object
Now, in this case and according the conservation of momentum:
 (2)
  (2) 
Where:
 is the mass of the spacecraft
 is the mass of the spacecraft
 is the mass of the asteroid
 is the mass of the asteroid
 is the initial velocity of the spacecraft
 is the initial velocity of the spacecraft
 is the initial velocity of the asteroid (because we are told the asteroid is stationary, as the spacracft is on the sateroid it remains stationary as well)
 is the initial velocity of the asteroid (because we are told the asteroid is stationary, as the spacracft is on the sateroid it remains stationary as well)
 is the final velocity of the spacecraft
 is the final velocity of the spacecraft
 is the final velocity of the asteroid
 is the final velocity of the asteroid
Rewritting (2):
 (3)
  (3) 
 (4)
  (4) 
Finding 
 :
:
 This is the mass of the asteroid
 This is the mass of the asteroid