Giotto (spacecraft)

Did You Know?

• A proposal was made to use spare parts to build a backup craft.
• A second proposal was made to use a Giotto-based probe to return a sample of a comet coma with a re-entry capsule in place of the Mage motor.

Mission

The ESA's Solar System Working Group began studying a mission to Halley's comet in 1977, but rejected it in August 1978 in favor of a lunar orbiter. The Science Advisory Committee reversed this decision, and the ESA started examining a joint mission with NASA called the International Comet Mission, which would have used a NASA carrier probe and a smaller European probe based on ISEE-2. The American probe was to release the European probe toward Halley for a close flyby before proceeding to Comet 10P/Tempel, but the NASA probe was cancelled in November 1979. Proposals then shifted to an Ariane 1 launched mission. One option, named HAPPEN and suggested by Giuseppe Colombo, would have used parts from a planned Geos-3 satellite to examine Earth's magnetotail before flying through Halley's tail in March 1986; the Solar System working group rejected it on 24 January 1980 for not returning enough information on Halley. In February 1980, a proposal was made to launch two Geos-based probes on Ariane 1—one for the magnetotail and one targeting Halley—and the name Giotto began to be used. The Science Advisory Committee approved it, but the Science program committee initially rejected it in favor of the Hipparcos satellite, while allowing a second submission without the magnetotail mission but with more extensive planning and a budget of 80 million accounting units. In July 1980, the committee approved the second proposal. British Aerospace submitted a Geos-based design in March 1981, which was rejected due to power and temperature control issues; the Geos design had become outdated and parts were no longer obtainable, so a new, larger craft was designed with British Aerospace remaining the primary contractor. Originally planned for launch on an Ariane 2 with a commercial satellite, then shifted to an Ariane 3, difficulties finding a customer for the launch window led to Giotto being assigned solo to an Ariane 1 available after EXOSAT was launched by a Thor-Delta.

Design

The Giotto spacecraft was a cylinder measuring 1.87 meters across and 2.85 meters long, with three internal platforms. British Aerospace in Filton, Bristol, handled its construction. It featured a Whipple shield, a dust protection system conceived by Fred Whipple, made from a thin 1 mm aluminum sheet separated from a thicker 12 mm Kevlar sheet. A similar shield later appeared on the Stardust spacecraft. Giotto communicated with Earth via a 1.51-meter antenna. Its white paint was an electrically conductive coating developed by Centre national d'études spatiales. A mock-up of the spacecraft is displayed at the Bristol Aero Collection hangar in Filton, Bristol. The craft used a Mage motor from Societe Europeenne de Propulsion. Giotto carried ten scientific instruments: a magnetometer (MAG), the Halley Multicolour Camera (HMC), a Dust Impactor Detector System (DID), the Rème Plasma Analyser (RPA), the Johnstone Plasma Analyser (JPA), a Particulate Impact Analyser (PIA), an Optical Probe Experiment (OPE), an Energetic Particle Analyser (EPA), a Neutral Mass Spectrometer (NMS), an Ion Mass Spectrometer (IMS), and the Giotto Radio Experiment (GRE).

Timeline

The ESA took possession of the probe on 22 April 1985, and later that month it was flown to French Guiana minus its camera, which made the transatlantic trip on 17 May. A 22-day launch window began at 11:13 UTC on 2 July. In the final minutes before launch, the probe's temperature dropped below minus 20 centigrade due to cooling by the rocket's liquid helium, but this did not cause problems. The mission launched on an Ariane 1 rocket (flight V14) on 2 July 1985 from Kourou, French Guiana. The craft was controlled from the ESA's 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 to the encounter. In GTO, slew and spin-up maneuvers (to 90 RPM) were carried out for the firing of the Apogee Boost Motor (ABM), which was fired at perigee. Attitude determination used sun pulse and IR Earth sensor data. On 13 September 1985, Giotto's camera was tested by photographing Vega; Earth was imaged on 18 and 23 October. In January 1986, a delay in sending instructions caused Giotto to enter safe mode, but it was returned to normal operations with help from the NASA deep space network. The probe was hit by a solar flare on 8 February but suffered no damage. The Soviet Vega 1 returned the first images of Halley's nucleus on 4 March 1986 and made its flyby on 6 March, followed by Vega 2 on 9 March. On 4 March, Giotto took its first distant photo of the comet from 59 million kilometers. A final course correction on 12 March aimed to put the probe 540 km from the nucleus, chosen because 500 km was the minimum distance for camera tracking, with 40 km added for position uncertainty. Three hours before closest approach, the camera began its encounter program taking a photo every 4 seconds; it was found to be tracking at the wrong speed, and a last-minute correction was made.

Scientific results

When Giotto flew by Halley’s Comet, it got the closest look and returned the best data of any spacecraft. Halley’s nucleus turned out to be a dark, peanut-shaped object 15 kilometers long and 7 to 10 kilometers wide. Only a tenth of its surface was active, with at least three jets spewing gas on the sunlit side. The comet formed 4.5 billion years ago when volatiles, mostly ice, condensed onto interstellar dust and stayed nearly unchanged ever since. The material it ejected was 80% water, 10% carbon monoxide, and 2.5% a mix of methane and ammonia, plus traces of other hydrocarbons, iron, and sodium. The nucleus was dark, hinting at a thick dust layer, and its surface was rough and porous. Density estimates ranged from 0.3 to 0.6 grams per cubic centimeter, but one researcher warned the error bars were too big to be useful. Seven jets together ejected three tonnes of material per second, making the comet wobble over time. Most dust particles were as small as cigarette smoke, with masses from 10 attograms to 0.4 grams. One particle that hit Giotto and spun it—likely breaking off a piece of the spacecraft—probably weighed between 0.1 and 1 gram. Two dust types appeared: one rich in carbon, hydrogen, nitrogen, and oxygen; the other in calcium, iron, magnesium, silicon, and sodium. The comet’s light elements, except nitrogen, matched the Sun’s ratios, meaning Halley’s ingredients are among the most primitive in the solar system. Instruments also showed a carbon-rich surface. Giotto was the first spacecraft to photograph a cometary nucleus, fly by two comets (young Halley and old Grigg–Skjellerup), return from interplanetary space via an Earth swing-by, be reactivated from hibernation, and use Earth for a gravity assist.