The Doomsday Conspiracy by Sidney Sheldon

When a mysterious weather balloon crashes to earth in the Swiss Alps, the head of the NSA handpicks Robert Bellamy to track down and identify the ten known witnesses to the event. A man whose obsession with his covert assignments has cost him the only woman he can ever love, Bellamy now faces the impossible. But as he searches for clues from Rome to Budapest to Texas, this mission blows up in his face -- and rips the lid off an incredible conspiracy that stretches around the globe and even into space...

Alone and betrayed on every side, Bellamy must run for his life -- holding an astonishing secret and the key to the planet's very survival.