Launched as the International Sun-Earth Explorer-3 in 1978, a joint NASA-ESA mission, the ISEE-3 probe studied the solar wind and Earth’s magnetic field from the L1 Lagrange point. When its mission was complete in 1981, NASA proposed repurposing the craft by using it to intercept a comet; such reuse would save the money and time of building a new spacecraft. Renamed the International Cometary Explorer, ICE was redirected through a series of lunar flybys, setting it up to intercept Comet 21P/Giacobini-Zinner. On Sept. 11, 1985, the probe passed through the tail of the comet, approximately 4,885 miles (7860 kilometers) from its nucleus. The spacecraft returned data on the make-up of the comet’s tail, supporting the theory that they are “dirty snowballs.” It was the first-ever encounter of a human-made spacecraft with a comet. NASA continued to reuse ICE afterwards, flying it past Comet Halley in 1986 to measure solar wind. Though the agency ceased contact with the probe in 1997, an independent team of researchers called the ISEE-3 Reboot Project successfully reestablished communication with the craft in 2014, firing its thrusters for the first time in 27 years.
