Spacecraft launched decades ago still return faint signals from far beyond the planets.

The longer version

Old probes keep transmitting because their radioisotope power sources decay slowly and their transmitters were built to run for decades with no maintenance. They coast outward on trajectories set long ago, gradually slowing under the Sun's pull but never returning. Several craft launched decades ago are still tracked by deep space networks.

Signals arriving from such distances are extraordinarily weak by the time they reach Earth, so huge antennas and sensitive receivers are needed to pull them out of the background noise. Round trip communication takes many hours, and eventually the power output falls below what the instruments require.

Where this entry sits

The night sky looks calm, yet almost everything up there is moving, burning, or slowly changing. These facts cover planets, stars, the Moon, gravity, black holes, comets, and the story of how people began exploring beyond Earth. The full list sits on the Space and Sky page, where every entry is listed in order.