One of the twin Voyager spacecraft heads into interstellar space in this artist's concept. The two probes are still working more than 40 years after launch, and they transmit data on charged particles, magnetic fields, and other properties of the space outside the Sun's protective magnetic "bubble." Voyager 1 is more than 14 billion miles from Earth, so the round-trip communications time with the craft is more than 42 hours. [NASA/JPL/Caltech]