Halley Armada

• 1. Introduction

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  The Halley Armada was a fleet of space probes dispatched from three different space agencies to intercept Halley's Comet during its 1986 passage through the inner Solar System. Five probes succeeded. One agency, conspicuously, was absent: NASA sent nothing. The questions this raises go beyond simple budget politics. How did a comet that appears roughly once every 75 years bring together the Soviet Union, France, Japan, and Europe in a shared scientific mission? What did the probes actually find when they got close enough to see? And what was lost when two missions never made it off the ground?

• 2. The Closest Look

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  On the 14th of March 1986, the European Space Agency's probe Giotto closed to within 596 kilometers of Halley's nucleus. That made it the first spacecraft in history to return close-up color images of a comet's core. The distance sounds comfortable enough, but it almost did not happen. Without data gathered by the other armada probes first, Giotto's engineers could only have aimed to within 4,000 kilometers. The measurements from the wider fleet allowed mission controllers to refine the trajectory and bring Giotto four times closer than would otherwise have been possible. The images it returned changed what scientists understood about the structure of cometary nuclei.

• 3. Soviet-French Partners

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  Vega 1 and Vega 2 were joint projects between the Soviet Union and France, operated through the Intercosmos program. Neither probe flew directly to Halley. Both stopped at Venus first, dropping a balloon probe and a lander onto that planet before continuing on to their cometary target. Vega 1 reached Halley on the 6th of March 1986, passing at a distance of 8,889 kilometers. Vega 2 followed three days later, on the 9th of March, at 8,030 kilometers. The Venus detour was not a detour in the wasteful sense. The gravitational assistance from Venus helped redirect both spacecraft toward the comet, and the balloon experiments produced their own stream of atmospheric data entirely separate from the cometary mission.

• 4. Japan's Debut in Deep Space

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  Sakigake, launched by Japan's Institute of Space and Astronautical Science, made its closest approach to Halley on the 11th of March 1986, at a distance of roughly 6.99 million kilometers. That enormous gap was not a failure. Sakigake was explicitly a test of interplanetary mission technology, and it earned the distinction of being Japan's first probe to leave the Earth system entirely. The data it returned informed the design of its companion mission. Suisei, also from the Institute of Space and Astronautical Science and known alternatively as PLANET-A, flew in on the 8th of March and passed at 151,000 kilometers. Its mission was dedicated specifically to the scientific study of Halley, and the engineering lessons learned from Sakigake fed directly into its preparation.

• 5. Supporting Eyes

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  Pioneer 7, launched on the 17th of August 1966, had been circling the Sun in heliocentric orbit at a mean distance of 1.1 AU for two decades before Halley arrived. On the 20th of March 1986, it passed within 12.3 million kilometers of the comet and monitored how the cometary hydrogen tail interacted with the solar wind. Pioneer Venus Orbiter, already stationed in orbit around Venus, was positioned to observe Halley during its perihelion on the 9th of February 1986. Its UV spectrometer tracked the comet's water loss at a time when ground-based observers on Earth could barely see the object at all. The International Cometary Explorer had been repurposed as a cometary probe in 1982, visited Comet Giacobini-Zinner in 1985, and then transited between the Sun and Halley in late March 1986 to take its own measurements.

• 6. Missions That Never Arrived

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  Space Shuttle Challenger lifted off on the 28th of January 1986 carrying SPARTAN-203, an instrument package designed specifically to observe Halley's Comet. The vehicle broke apart during ascent on mission STS-51L, killing the crew and destroying the spacecraft. The consequences cascaded far beyond that single flight. Dozens of subsequent shuttle missions were cancelled in the aftermath. Among them was STS-61-E, scheduled for the 6th of March 1986, which had been carrying the ASTRO-1 observatory with Halley observations among its planned objectives. An earlier cancellation had happened years before. The International Comet Mission, conceived as a carrier NASA probe paired with a smaller European spacecraft based on the ISEE-2 design, was intended to release the European probe for a close Halley flyby before the NASA vehicle continued to Comet 10P/Tempel. That American probe was cancelled in November 1979, which is a large part of why NASA had no representative in the armada at all.