On August 12, 1978, the International Earth-Sun Explorer-3 (ISEE-3), the third satellite in the ISEE program, launched from Cape Canaveral on a mission to study how the sun affects the Earth. Heat and light aren't the only important resources that the sun sends our way. Phenomena like solar wind, coronal mass ejections and solar flares—particles and electromagnetic fields that wallop the Earth—affect communications, satellites, the stability of the electrical grid and other crucial systems.
For five years ISEE-3 watched the sun, hovering out in space part way between the Earth at the star. Then, in 1983, NASA used the pull of the Moon's gravity to launch the satellite on the next phase of its life, a trip around the sun, For just over 30 years, ISEE-3 has been orbiting our star, traveling just a little bit faster than the Earth. But ISEE-3 has nearly caught up, says Emily Lakdawalla for the Planetary Society, and this coming August will lap us, passing the closest its been to Earth in at least 31 years.
ISEE-3 is still alive and kicking: it's still got fuel, it's still sending out signals, it's ready to get to work on a new mission. There's just one problem: NASA has lost the ability to speak ISEE-3's language.
The transmitters of the Deep Space Network, the hardware to send signals out to the fleet of NASA spacecraft in deep space, no longer includes the equipment needed to talk to ISEE-3. These old-fashioned transmitters were removed in 1999. Could new transmitters be built? Yes, but it would be at a price no one is willing to spend. And we need to use the DSN because no other network of antennas in the US has the sensitivity to detect and transmit signals to the spacecraft at such a distance.
NASA can see ISEE-3 and they can hear its signal, they just can't talk back to it to tell it what to do next. So off it will go, continuing on its path around the Sun, watching and recording and yelling into space.
According to Lakdawalla, recovering and repurposing ISEE-3 was never really part of the plan: "If they had planned for it to still be functioning at this point, they would have maintained the capability to communicate with it." But technological obsolescence is a problem that plagues much of science. As Joseph Stromberg wrote for Surprising Science recently, the vast majority of raw science data is lost forever, locked away in archives and formats that we've stopped using long ago.