Fenrir

When astronomer Stephanie Bronson uncovers a massive alien vessel named Fenrir racing toward the Sun, she must lead a desperate mission to rescue its crew before a catastrophic failure dooms them both to fiery destruction.
It was just a dot of light . . .
. . . a dot of light that appeared out of nowhere. Stephanie Bronson thought she might have found a distant supernova. Instead, what she’d found was something more than a thousand miles across, moving towards the Sun at almost a third the speed of light from the constellation of Lupus, the Wolf. Even more frightening, the object—called Fenrir, after the Norse Wolf of the World’s End—was slowing down. It was not a comet or even a rogue planet.
It was an alien vessel.
And despite the increasingly detailed and insistent messages of greeting from Earth, it showed no sign of responding. As the world braced itself for the arrival of the immense, frighteningly silent alien visitor, Stephanie held hope that they still might communicate with Fenrir—until something happened that no one had imagined: Fenrir flared and its drive died, leaving the huge ship careening out of control to a fiery death inside the orbit of Venus.
There was—just possibly—a chance for Earth to act—to rescue the now-shipwrecked “Fens.” But even Stephanie—or those trying to stop her and the rescue ship Carpathia—could not imagine that not merely the fate of the aliens, but that of Earth itself, would be decided by Fenrir.