Fifth Man, (Oxygen, book 2) by John B. Olson and Randall Ingermanson

Fifth Man
(The second book in the Oxygen series)

A novel by
John B. Olson and Randall Ingermanson

Will Their Journey Prove We Are Not Alone? Valkerie Jansen has made the discovery of a lifetime-evidence of past life on Mars. But her personal life is coming unglued. Fellow astronaut Bob Kaganovski is so...infuriating. With NASA scheduling her every minute, she hasn't got time for relationships. Especially while she's under the media microscope.

Bob Kaganovski is looking over his shoulder. Is the mission commander cracking up-or is Bob? When Valkerie insists that she's seen "something" out there, Bob is forced to ask why nobody else can see it. Who-or what-is Valkerie's "fifth man"? And why is it trying to keep them from going home?