The Alien Spacecraft

This is the time that the inner solar system is being utilized from Earth to the asteroid belt. The asteroid belt is being explored and mapped.
Identifiable asteroids are suddenly exiting the belt and when the astronomers check they find them still in their original orbits. Upon closer examination they spot what appears to be a spacecraft drifting. How is this mystery spacecraft causing asteroids to be in two places at once and what danger does it pose to the inner planets.
The ship sent to examine this mystery spacecraft finds much more than a derelict. Can they find their way back and at what cost to civilization as they know it.