More pocked with craters than any other object in our solar system, Jupiter's outermost and second-biggest Galilean moon, Callisto, appears geologically unremarkable. In the 1990s, however, NASA's ...

The giant-impact hypothesis posits that billions of years ago a Mars-sized body named Theia collided with the early Earth.

But the mission will also bring something that’s never been installed on the moon or anywhere else in space before—a fully ...