Entropy's Compass

Is time linear? Can events in spacetime only travel in one direction? Does 'direction' even have meaning? Is there a past, present and future and must they always occur in that order, if they occur at all? Is it possible to prevent something which has already taken place?
In 2209 C.E.., a routine mission to the Kuiper Belt is launched, its purpose to calibrate a newly placed addition to the Spaceguard Early Detection Net, whose purpose is to detect potential asteroidal threats to the Inner System worlds and remove any possibility of an extinction level impact before it can occur. En route, the new equipment begins to transmit data which is bizarre, even impossible. A mission which had begun as boringly routine quickly becomes of critical importance as it becomes evident that, if true, the data threatens far more than just the Earth and her colonies but the entire galaxy