Giotto and Halley’s Comet in line for close encounter in space

Press, 7 February 1986, Page 17

Giotto and Halley’s Comet in line for close encounter in space

By

DERRICK ROONEY

Astronomers last had an opportunity to study and photograph Halley’s Comet in 1910. This year, when the comet reappears in the skies, an astronomically more sophisticated array of instruments will be trained on it from observatories in many parts of the earth and in space, but only one observer will be close enough to look into the comet’s core.

This is Giotto, the European Space Agency’s space vehicle which is winging its way towards an encounter with the comet — and possible destruction by it.

Launched from French Guiana in July, Giotto carries a "payload” of 10 experiments which will photograph the comet’s nucleus, examine the chemistry of its coma, analyse its contents and study the dust particles in its vicinity. There will be only a small "window” through which the space vehicle’s instruments will be able to complete this task. Because the comet and Giotto are travelling in different directions around the sun they will pass at very high relative speed, the space vehicle’s instruments will have only a few hours in which to make their observations and relay them to earth.

The “close encounter” will occur about midnight, Greenwich mean time, on March 14, at which time Giotto will be a mere 100 km from the comet’s nucleus. There will be no second chance. At "encounter” Giotto will be about eight light minutes from earth, which means its radio signals will take the time to reach the ground stations. To Minimise the risk tof data loss

should some of the vehicle’s equipment fail during this critical fly-by period, the designers of Giotto have included two onboard computers designed to reconfigure the electronic subsystems in the event of failure. As Giotto approaches the comet its front will be bombarded with particles of comet dust at a velocity of almost 70km per second, an onslaught which has led some writers to describe it as a kamikaze mission.

To protect its instruments Giotto has a very thin “bumper shield” of Imm aluminum sheet, positioned, 23cm in front of a second shield of foam and kevlar, 1.35 cm thick. The idea is that when a dust particle hits the bumper shield it will be vaporised, and the debris will spread out in the space between the shields, thus delivering only a mild impact to the rear shield.

The designers reckon that this should give the vehicle time to complete its scientific observations before the threat of destruction becomes a reality. This dust-protection system is something new, developed especially for Giotto. Also new in its design are the “design” mechanism for its main antenna — which must be rotated against the spacecraft’s spin to maintain a constant position relative to earth — and a star mapper to determine the craft’s position. From the public’s point of view, the most spectacular of the 10 experiments in the comet’s scientific payload is the German camera which will relay images of the comet’s coma and the surface of its icy nucleus.

At take-off the camera’s 13kg weight accounted for less than

1.5 per cent of the space vehicle’s total weight, but it will provide almost half the data relayed to earth. The camera does not point at the comet, but photographs its reflection in a mirror, mounted in a rotating baffle on the craft’s side. The camera itself is inside the craft, where it is protected from direct impact with dust particles. The focusing element is not a lens, but a reflecting telescope, chosen because its long focal length, about one metre, can obtain detailed images from a lorsg distance.

The mirror baffle will rotate 180 degrees to track the comet as Giotto flies past; this movement will be controlled by the camera, which will also process the images in four colours, and make corrections, to eliminate the effects of the spacecraft’s motion.

Other equipment on board Giotto will study the dust and ionised gas (plasma) in the comet’s coma. Sensors installed by the University of Kent will analyse the impact, distribution, and properties of comet, dust; a Freneh-built optical will

study the density and distribution of dust particles and gas molecules, in the coma and tail, and West German equipment will study and analyse dust particles in the inner coma.

Four plasma experiments — designed in England, West Germany, France, and Ireland — will study the coma and tail. An English experiment designed by the Mullard Space Science Laboratory will study the composition and distribution of positive ions and interactions with the solar wind. Higher-energy particles will bejy

measured by a detector provided by St Patrick’s College, Ireland, and the French and German experiments will be concerned with the magnetic configuration of the coma, and the composition

and distribution of electrons and positive ions. Giotto has already had two “dress rehearsals” for its encounter with the comet and began a third rehearsal on February 5. On March 4, preparations for the encounter will begin with a final rehearsal. A final orbit correction, based on data to

be collected by Russian and

Japanese space probes, will be made if necessary on March 11, and at midnight GMT, on March 14 the encounter phase will begin.

By 0400 GMT on March 15 Giotto will be passing beyond range of the comet, but by then it should have taken the first close look at the comet’s heart, thus completing what Sir Bernard Lovell describes as one of the few visionary and exciting space projects of the decade. The craft takes its name, from the Florentine painter, Giotto di Bondome (1266-

1337) who is perhaps most renowned for his series of frescoes in the Cappella dell’ Arena in Padua. Halley’s Comet is depicted as the Star of Bethlehem in the most famous of the 38 frescoes, “The Adoration of the Magi,” which was painted in 1303, two years after an appearance. of the comet

Although it is now known that the Star of Bethlehem could not have been Halley’s Comet the European Space Agency thought that the choice of Giotto as the spacecraft’s name was entirely appropriate. .* «