Giotto mission

Giotto was a European unmanned space mission from the European Space Agency, intended to fly by and study Halley's Comet.

Mission

On March 13, 1986, Giotto approached at a 596 kilometre distance from Halley's nucleus.

The spaceprobe "Giotto" from the European Space Agency was designed to study Halley's Comet. The spacecraft was named after the medieval Italian painter Giotto di Bondone. He had observed Halley's Comet in 1301 and was inspired to depict it as the star of Bethlehem in his painting The Christmas Story.

Originally a United States partner probe was planned that would accompany Giotto, but this fell through due to budget cuts at NASA. There were plans to have observation equipment on-board a Space Shuttle in low-Earth orbit around the time of Giotto's fly-by, but they fell through with the Challenger disaster.

The plan then became a cooperative armada of five spaceprobes including Giotto, two from the Soviet Union's Vega program and two from Japan: the Sakigake and Suisei probes. The idea was for Japanese probes to make long distance measurements, followed by the Russian Vegas which would locate the nucleus, and the resulting information sent back would allow Giotto to make precisely target very close to the nucleus. Because Giotto would pass so very close to the nucleus ESA was mostly convinced it would not survive the encounter due to bombardment from the many high speed cometary particles.

The craft

The spacecraft is derived from the GEOS research satellite built by British Aerospace, and modified with the addition of a dust shield as proposed by Fred Whipple and comprising a thin aluminium sheet separated by a space and a thicker Kevlar sheet. Later the Stardust (spacecraft) would use a similar Whipple shield.

Timeline

The mission was given the go-ahead by ESA in 1980, and launched on an Ariane 1 rocket (flight V14) on 1985 July 2 from Kourou.

The Russian Vega 1 starts returning images of Halley on 1986 March 4, and the first ever of its nucleus, and made its flyby on March 6, followed by Vega 2 making its flyby on March 9.

Giotto passed Halley successfully on 1986 March 14 at 600 km distance, and surprisingly survived despite being hit by some small particles. One impact sent it spinning off its stablised spin axis so that its antenna no longer always pointed at the Earth, and importantly, its dust shield no longer protected its instruments. After 32 minutes Giotto re-stabilised itself and continued gathering science data.

Another impact destroyed the Halley Multicolor Camera, but not before it took spectacular pictures of the nucleus at closest approach.

Giotto's trajectory was adjusted for a return to Earth (flyby) and its science instruments were turned off on 1986 March 15 02:00 UT.

In 1990 Giotto was commanded to wake up and on July 2 flew by the Earth in order to sling shot to its next cometary encounter.

Giotto also flew by the Comet Grigg-Skjellerup in 1992 July 10, which it approached to a distance of ca. 200 kilometres. Afterwards, on 1992 July 23 Giotto was again switched off.

In 1999 Giotto made another Earth flyby but was not reactivated.

Results

Images showed Halley's nucleus to be a dark peanut-shaped body, 15 km long, 7 to 10 km wide. Only 10% of the surface was active, with at least three outgassing jets seen on the sunlit side. Analysis showed the comet formed 4.5 billion years ago from volatiles (mainly ice) that had condensed onto interstellar dust particles. It had remained practically unaltered since its formation.

Of the volume of material ejected by Halley: 80% was water, 10% carbon monoxide, and 2.5% a mix of methane and ammonia. Other hydrocarbons, iron, and sodium were detected in trace amounts.

Halley's nucleus was blacker than soot, which suggests there is proportionally more dust than ice.

The nucleus's surface was rough and of a porous quality, with the density of whole nucleus only 0.3 kg/m³.

The quantity of material ejected was found to be 3 tonnes per second for seven jets, and these caused the comet to wobble over long time periods.

The dust ejected was mostly only the size of cigarette smoke particles, the largest being 40 milligram. Although the one that sent Giotto spinning was not measured, from its effects its mass has been estimated to lie between 0.1 and 1 gram.

Two kinds of dust were seen: one with carbon, hydrogen, nitrogen and oxygen; the other with calcium, iron, magnesium, silicon and sodium.

The ratio of abundances of the comet's light elements excluding nitrogen (ie. hydrogen, carbon, oxygen) were the same as the Sun's. The implication is that the constituents of Halley are among the most primitive in the solar system.

The plasma and ion mass spectrometer instruments showed Halley has a carbon-rich surface.

External link

• Official site (http://sci.esa.int/home/giotto/)

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