This artist rendering provided by NASA shows Voyager 1 at the edge of the solar system. AP hide caption

itoggle caption AP

Will we need to tread lightly, in deference to the locals, when finally do make it to Mars? Pat Rawlings/SAIC/NASA hide caption

itoggle caption Pat Rawlings/SAIC/NASA

On its way to Jupiter, the Galileo spacecraft looked back and captured this remarkable view of Earth and the moon. The image was taken from a distance of about 3.9 million miles. NASA hide caption

itoggle caption NASA

An artist's impression of a gamma-ray burst, a powerful jet of energy lasting from less than a second to several minutes. The most powerful events in the universe, they are thought to be mostly associated with the explosion of stars that collapse into black holes. A. Roquette/ESO hide caption

itoggle caption A. Roquette/ESO