In a Mirror, Darkly

When an interstellar object known as the Mirror lands on Mars, a team is sent to investigate. The lead scientist, Kai Stellan, at first thinks the Mirror is an inert object, despite being made of theoretical materials–until it seems to sample his bioenergy. Two of the marines accompanying him have the same experience. 

While being taken back to base under quarantine, they are attacked by an unknown alien ship: the Coldwight, who have pursued the Mirror across the galaxy. But the Mirror didn't just draw energy off the human team–it made them immortal. Despite being shot down, they manage to hijack the alien ship. Knowing that Command has questionable intent for their future, they flee Mars, only to wind up as prisoners on the alien mothership.

Now they must escape the Coldwight, steal a ship, and get back to Earth, all while avoiding recapture by their own side–assuming they don't start a war between Earth and the Coldwight along the way.