A Historic Milestone in Spaceflight
On September 11, 1985, the International Cometary Explorer (ICE) performed the first ever flyby of a comet, passing comet Giacobini‑Zinner. The mission was a joint effort between NASA and the European Space Agency ESA and represented a major advance in the study of small solar system bodies.
Originally launched in 1978 as the International Sun‑Earth Explorer 3 (ISEE‑3), the probe initially investigated the solar wind and Earth’s magnetosphere. After completing its primary objectives, the still‑healthy spacecraft was retargeted toward a comet. A series of thruster maneuvers shifted its trajectory from a Lagrange point to an intercept course with the inner solar system.
During the encounter ICE approached the comet at a distance of roughly 7 800 kilometers while traveling at about 21 kilometers per second. Its instruments measured plasma, magnetic fields and dust particles in the comet’s coma and tail, delivering the first direct data on the composition and dynamics of gas and dust emitted by an active comet.
The success demonstrated that existing spacecraft could be repurposed for new scientific questions and paved the way for later comet missions such as Giotto, Rosetta and Deep Impact. ICE data continue to be used in models of comet physics and solar wind–comet interactions.