On September 11, 1985, a spacecraft designed to watch the solar wind entered the invisible environment of a comet. The International Cometary Explorer carried no camera and would return no portrait of Giacobini–Zinner. Instead, its instruments measured the magnetic fields, plasma and charged particles flowing through the comet’s tail. For the first time, humanity had sent a working spacecraft into direct contact with a comet.
The achievement was remarkable not only because it was a first. The probe had not been built for comets at all. It reached Giacobini–Zinner because engineers and scientists found a second mission hidden inside an aging spacecraft, then used lunar gravity and careful navigation to make it possible.
A spacecraft stationed ahead of Earth
NASA launched International Sun–Earth Explorer 3, usually called ISEE-3, on August 12, 1978. It was part of a three-spacecraft program conducted with the European Space Agency to study the relationship between the Sun and Earth’s magnetic environment.
ISEE-3 occupied a halo orbit around the Sun–Earth L1 Lagrange point, roughly 1.5 million kilometers sunward of Earth. It was the first spacecraft to operate in such an orbit. From there, it could sample the solar wind before the same stream of charged particles reached Earth and compare its observations with measurements made closer to the planet.
The machine was a spin-stabilized, sixteen-sided cylinder covered by solar cells, with long booms extending instruments away from the body. Its payload measured magnetic fields, plasma waves, energetic particles, solar-wind ions, cosmic rays, X-rays and gamma-ray bursts. There was no imaging camera. ISEE-3 was built to encounter an environment through data.
The second mission inside the first
After its original objectives were met, mission planners led by Robert Farquhar proposed sending ISEE-3 away from L1. A comet encounter demanded far more energy than the spacecraft could provide with its small propulsion system. The solution was orbital mechanics: a complex series of passes through the Earth–Moon system, including multiple lunar flybys.
The most important passage came in December 1983, when the spacecraft flew close behind the Moon. Lunar gravity altered its speed and direction, placing it on a path toward Comet 21P/Giacobini–Zinner. The old name no longer described the mission. ISEE-3 became ICE, the International Cometary Explorer.
This was not a straightforward journey to a bright object visible against the stars. Giacobini–Zinner is a small periodic comet that returns to the inner solar system about every 6.6 years. Navigators had to aim the spacecraft toward where the comet and its plasma tail would be on the encounter date, then preserve communication with a vehicle launched seven years earlier.
September 11: through the comet’s tail
On September 11, 1985, ICE passed roughly 7,800 kilometers from the comet’s nucleus and crossed the plasma tail. The Deep Space Network coordinated with large antennas including Arecibo in Puerto Rico and Usuda in Japan to maintain the weak telemetry link during the encounter.
Because ICE carried no camera, there was no dramatic image arriving from close range. The discovery was written in instrument traces: changes in magnetic field direction, plasma density and energetic particles. Measurements identified structures associated with the interaction between the solar wind and the comet’s atmosphere, including a bow-wave region and the boundary where the magnetic field changed across the tail.
A comet produces its extended atmosphere when sunlight heats ices in the nucleus and releases gas and dust. The solar wind then interacts with ionized gas, shaping a plasma tail that points generally away from the Sun. ICE sampled this process in place. Ground-based telescopes could observe the large-scale comet; the spacecraft measured the local physics.
First, but part of an international comet era
ICE became the first spacecraft to encounter a comet. It was also an advance scout for a larger moment in planetary science. Comet Halley returned in 1986, and spacecraft from the Soviet Union, Europe and Japan formed what became known as the Halley Armada. ESA’s Giotto passed close to Halley’s nucleus and returned the first close-up images of a cometary nucleus.
ICE did not approach Halley closely. From millions of kilometers away it measured the solar wind upstream of the comet, providing context for observations by the other missions. The reused spacecraft had become part of a coordinated international campaign.
A very long afterlife
NASA continued using ICE for heliospheric observations and ended routine operations in 1997, though the transmitter remained detectable. In 2014, as the spacecraft again approached the Earth–Moon system, a privately organized team re-established two-way communication. They obtained data and attempted to alter its trajectory. A longer engine burn failed, likely because the propulsion system lacked sufficient nitrogen pressurant, and contact was later lost.
The 2014 effort did not return ICE to its old orbit, but it underlined the durability of both the spacecraft and the knowledge required to communicate with it. A machine launched with 1970s electronics had remained a scientific object across multiple generations of spaceflight.
Why the first comet encounter matters
ICE demonstrated three ideas that became increasingly important: Lagrange-point operations, gravity-assisted mission design and the productive reuse of spacecraft after their primary assignments. Most of all, it changed comets from distant telescopic targets into places where instruments could directly sample matter and fields.
Continue through the Science & Space archive with Voyager 2 at Neptune, the first view of Earth from lunar orbit, and Luna 2 reaching the Moon. Return to the complete Today in History edition for September 11.
Sources and method. Mission dates, orbit, instruments and encounter status were checked against the NASA/GSFC HEASARC mission record, the NASA Technical Reports Server mission paper, and a contemporaneous Deep Space Network encounter report. The article distinguishes visualizations and artist concepts from photographs and does not imply that ICE carried a camera.
Written and fact-checked by the Discover Today in History editorial team. Last reviewed September 10, 2026.
