Heaven's Shadow — David S. Goyer; Michael Cassutt

Three years ago, an object one hundred miles across was spotted on a trajectory for Earth's sun. Now, its journey is almost over. As it approaches, two competing manned vehicles race through almost half a million kilometers of space to reach it first. But when they both arrive on the entity, they learn that it has been sent toward Earth for a reason. An intelligent race is desperately attempting to communicate with our primitive species. And the message is: Help us.