GIOTTO’S FINAL MISSION

THE ESA’s (European Space Agency) spacecraft, Giotto, is set to complete its final mission in July 1992. The battered space probe is currently on route to rendezvous with the comet Grigg-Skjellerup approximately 630 million km (390 million miles) from earth.

In 1986 Giotto made a spectacular journey through the tail of Halley’s comet, approaching within 605 km (375 miles) of the comet’s nucleus, transmitting the first ever close-up data on a big comet. The journey was not without cost. Two seconds before reaching its nearest point to the nucleus the spacecraft went dead – the ESA believes – as a result of a massive flash of radiation. Power returned twenty one seconds later but the spacecraft had suffered the effects of dust and rocks smashing through its Kevlar/aluminium shield at 240,000 kph (150,000 mph). The colour camera, main platform battery and several other instruments were destroyed or damaged.

The spacecraft was then put into hibernation in an orbit circling the sun at a distance of 100 million km (62 million miles) from earth.

Four years later (1990) scientists working at NASA’s Deep Space Network in Madrid used a 100,000-watt transmitter and 70 metre (230 foot) diameter antenna to send a wake-up call to Giotto. Two hours and twenty-one minutes later they received a weak response from the spacecraft.

Scientists at the ESA’s research centre at Noordwijk in the Netherlands and the agency’s Space Operations Centre in Darmstadt, Germany have now used most of Giotto’s remaining fuel to turn the spacecraft around so that its high-gain antenna is facing Earth and to correct a wobble. This means that future signals from Giotto should be strong enough to send back meaningful data. They have also manoeuvered the craft on a slingshot course around the Earth, using terrestrial gravity to increase its speed to catch up with Grigg-Skjellerup.

 Although the spacecraft is free, until last month the mission was in doubt due to financial restrictions on ESA projects. In August the ESA finally approved US$5 million for technical support on the ground.

Comets are classified into two main types; short-period comets such as Grigg-Skjellerup which orbits the sun every 5.09 years, and long-period comets such as Halley’s comet which has a mean-period of 76 years.

Comets are composed of a nucleus of ice particles, dust and rock only a few kilometres across. As a comet approches the sun, the nucleus gives rise to a coma (or head), a dust tail and a plasma tail. The three components of the comet are between one and 10 million times larger than the nucleus and are visible only by their interaction with solar radiation.

Comets are considered to be as old as the solar system itself and may reveal much about its history.

Sources: Jane’s Spaceflight directory, New Scientist, Economist, AP, The Cambridge Atlas of Astronomy (Cambridge University Press)