Given :
Two astronauts, one of mass 64 kg and the other 86 kg , are initially at rest together in outer space.
To Find :
How far apart are they when the lighter astronaut has moved 10 mm.
Solution :
Since, there is no force in space.
So, applying conservation of momentum :

We know, distance travelled by smaller astronaut is :

For bigger astronaut distance travelled :

Therefore, they are ( 10 + 7.44 ) = 17.44 mm apart from each others.
Hence, this is the required solution.