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'Giotto' was a European robotic spacecraft mission from the European Space Agency, intended to fly by and study Halley's Comet. On March 13, 1986, the mission succeeded in approaching Halley's nucleus at a distance of 596 kilometers.
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 Adoration of the Magi.

Contents

Mission

The craft

Timeline

Launch

Halley encounter

First Earth flyby

Grigg-Skjellerup encounter

Second Earth flyby

Results

Scientific Results

Spacecraft achievements

External links

Mission

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 and the pre-existing American probe International Cometary Explorer 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 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 coordinated group of probes became known as the Halley Armada.

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. The later Stardust spacecraft would use a similar Whipple shield.

Timeline

Launch

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 craft was controlled from the European Space Agency ESOC facilities in Darmstadt (then West Germany) initially in Geostationary Transfer Orbit (GTO) then in the Near Earth Phase (NEP) before the longer Cruise Phase through to the encounter. While in GTO a number of slew and spin-up manoeuvres (to 90 RPM) were carried out in preparation for the firing of the Apogee Boost Motor (ABM), although unlike orbit circularisations for Geostationary Orbit, the ABM for Giotto was fired at perigee. Attitude determination and control used sun pulse and IR earth sensor data in the telemetry to determine the spacecraft orientation.

Halley encounter

The Soviet 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 stabilized 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-stabilized 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.

First Earth flyby

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

Grigg-Skjellerup encounter

Giotto was commanded to wake up and on July 2, 1990 when it flew by Earth in order to sling shot to its next cometary encounter.
The probe then flew by the Comet Grigg-Skjellerup in July 10, 1992 which it approached to a distance of about 200 kilometres. Afterwards, Giotto was again switched off on July 23, 1992.

Second Earth flyby

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

Results

Scientific 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.
Measured volume of material ejected by Halley:

★ 80% water,

★ 10% carbon monoxide

★ 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.

Spacecraft achievements

★ Giotto made the closest approach to Halley comet and provided the best data for this comet.

★ Giotto was the first spacecraft to provide pictures of a cometary nucleus.

★ Giotto was the first spacecraft do a close flyby of two comets. Young and active comet Halley could be compared to old Grigg-Skjellerup.

★ Giotto was the first spacecraft to return from interplanetary space and perform a Earth swing by.

★ Giotto was the first spacecraft to be re-activated from hibernation mode.

External links

★ Official site

★ Interview with the mission's Deputy Project Scientist by NASA's Solar System Exploration

★ Nature 1986 about Giotto mission

★ Giotto di Bondone's 'Adoration of the Magi' painting that includes his rendition of Halley's Comet