Giotto Probe to Halley's Comet

Halley's Comet is the first comet ever recorded in history, going back more than 2000 years and witnessed by different civilizations. It is the only comet which can be observed without a telescope or binoculars in modern times. Halley's Comet is observable by humans during its pass thru the inner solar system once every 75 years.

In 1980, the European Space Agency (ESA) proposed a spacecraft mission to Halley's Comet. The Giotto mission was actually a group of 5 spacecraft, called "the Halley Armada", with Giotto built by the European Space Agency (ESA), with launches coordinated with Vega 1 and Vega 2 from the Soviet Union, and the Japanese probes Sakigake and Suisei, the latter two being Japan's first spacecraft to leave Earth orbit into interplanetary space, Japan becoming the third country to do so. The United States initially intended to be a partner but NASA budget cuts put an end to that.

In 1985, Giotto launched from an ESA Ariane, the Japanese probes were launched from Japan, and the Russian probes launched from the Soviet Union. The Vega probes went past Venus to drop off landers and balloon explorers before diverting to Halley's Comet with a Venus gravity assist.

All probes encountered Halley's Comet, the Giotto and Vega probes making closer approaches than the Japanese probes. Giotto came within 600 km, the Vega spacecraft within 8000 to 9000 km, and the Japanese craft at 150,000 and 7 million km.

Images showed at least 3 outgassing jets on the side of the comet facing the sun. The jets consisted of approximately 80% water, 10% carbon dioxide, 2.5% methane and ammonia (CH4 and NH3), and the rest various hydrocarbons and other stuff. The comet itself was darker than coal, and the instruments determined that the surface was carbon rich. The dust ejected was thought to be finer than cigarette smoke, and consisted of two types. One was carbon, hydrogen, oxygen, and nitrogen. The other was calcium, magnesium, iron, silicon, and sodium. The comet was found to be low density but there was a wide variation of estimates.

Several particles from the comet struck Giotto. One knocked out its multicolor camera, though it had already taken good photos which should have been its best. Another particle knocked off the orientation of the spacecraft, which took 30 minutes to re-establish so that it could communicate back to Earth.

After completing the encounter with Halley's Comet, Giotto went on an extended mission to fly by comet 26P/Grigg–Skjellerup in 1992, though with its camera knocked out, there was limited scientific gain. After that, Giotto was put back into hibernation mode and has not been reactivated since.

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