When Giotto kept its rendezvous with Halley’s comet in March 1986, no one really expected it to survive the encounter. The spacecraft closed on the comet at a relative velocity of 68.4 kilometres a second, so fast that a collision with even a small grain of dust could have completely destroyed it. Accordingly, the official mission ended just 15 minutes after the closest approach to the icy nucleus at the heart of Halley’s comet; and the European Space Agency, which planned the encounter, had no detailed plans for an extended mission.
Surprisingly Giotto emerged battered but unbowed. Although it was hit 13 seconds before its closest approach to the comet, a blow that knocked the spinning craft so hard it began to wobble, breaking its radio link with Earth for about half an hour, Giotto was not fatally damaged. When full contact was re-established, ESA scientists took the first of the steps that would eventually send Giotto to meet a little-known comet called Grigg-Skjellerup. All being well, Giotto was expected to meet its second comet on Friday, 10 July, and send back exciting new data on the plasma and dust in and around the comet.
As soon as Giotto emerged from Halley, controllers at ESA’s Space Operations Centre (ESOC) at Darmstadt in southwestern Germany, decided to carry on for another 24 hours to record extra data and to evaluate the condition of the craft. They found that several of the scientific instruments were still working and that they could still control Giotto. Secure in this knowledge, they switched off the remaining experiments and fired Giotto’s thrusters to place it on a trajectory that would bring it back towards Earth in July 1990. This would enable the project team to use Earth’s gravity to swing Giotto towards another comet.
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Next, to ensure that Giotto’s systems could survive unattended for several years, mission controllers had to position the spacecraft so that enough sunlight fell on the solar panels to provide the power it needed without the craft’s interior becoming too hot and without other parts falling into the shade and becoming too cold. This meant that Giotto could no longer point its radio antenna towards Earth, so on 2 April 1986, after it received the command to go into hibernation, Giotto fell silent.
Technical checkup
Although the idea of sending Giotto to a second comet had been briefly considered before the Halley encounter, plans for any extension of the mission depended on what the spacecraft could still do. In February 1990, ESA engineers embarked on a complex series of operations to contact Giotto, switch on various systems and point its radio antenna towards the Earth again. Their detailed investigation of Giotto’s condition showed that the spacecraft had survived extremely well, although the thermal protection system had been damaged, making the interior of the spacecraft warmer than expected.
The big disappointment was the camera. Although most of its internal systems still worked, Giotto was unable to see even bright stars. The scientists worked out that the camera was blocked by a piece of a cylindrical tube known as the outer straylight baffle, which had been almost totally destroyed during the Halley encounter. All the remaining scientific instruments, apart from the Neutral Mass Spectrometer, were still working, although not as well as they had been.
Another mission
The next stage was to put Giotto on course for comet Grigg-Skjellerup. As Giotto sped past the Earth on 2 July 1990, five years to the day after its launch, it was manoeuvred onto the right path with the aid of the Earth’s gravity. It passed within 22 731 kilometres of the Earth, making observations of the magnetosphere before returning to hibernation as it cruised towards its new objective.
Over the next few months ESA pondered Giotto’s fate. First, a panel of ESA scientists confirmed that Giotto could make useful observations of another comet, even without its camera. Then the agency had to find the money to pay for an extended mission. For a while it looked as if no funds would be available, but in December 1990, ESA’s science budget was increased and by treating the extended mission as part of ESA’s mandatory scientific programme, the agency avoided having to ask individual countries to provide the extra cash. With the project approved and the money available, the mission hinged on the ability of the engineering team to reactivate the spacecraft after its second hibernation and navigate it to its target.
Comet Grigg-Skjellerup, chosen from five possible targets for Giotto’s second rendezvous, had several factors in its favour. First, its orbit is fairly well known. Secondly, Grigg-Skjellerup is an old and not particularly dusty comet, with a nucleus which is probably only about 2 kilometres in diameter. This makes it an interesting contrast to Halley, which is relatively new with an irregular nucleus varying between 8 and 16 kilometres across. Halley produces gas about 100 times as fast as Grigg-Skjellerup. Finally, Giotto could reach this comet within two years of swinging past Earth, reducing both the risk of instruments failing and the cost of the mission.
The comet was discovered on 23 July 1902 by John Grigg, an amateur astronomer in New Zealand, but because he tracked it for just 10 days, its orbit could not be calculated accurately. It was rediscovered in 1922 by John F. Skjellerup, an Australian who had moved to South Africa, where, with others, he founded the Cape Astronomical Society. Skjellerup’s discovery was soon linked with Grigg’s observations, confirming that they had seen the same body. Subsequent calculations have shown that the comet may also have been observed in 1808 by the prolific comet hunter Jean-Louis Pons.
Comet Grigg-Skjellerup does not take long to orbit the Sun, and its period has altered three times this century, as a result of close approaches to the giant planet Jupiter. In 1902, the comet circled the Sun in 4.82 years and its orbit was tilted at an angle of just 8.3 degrees to that of the Earth’s: today the orbit is inclined at 21.1 degrees and the period has increased to 5.17 years.
Regular observations
Although Grigg-Skjellerup has been observed at every return since 1922, it is a relatively faint comet and professional astronomers have not observed it closely. The 1992 apparition is not a very good one for ground-based observations. In December last year, the comet was on the opposite side of the Earth from the Sun, in a good position for observations but then it was extremely faint. It began to brighten during this spring, but as it did so it moved to a part of the sky closer to the Sun and was only visible near the horizon in the twilight sky. In this position it became difficult to study from large observatories, which generally point their telescopes clear of the horizon because at low angles, where they must observe through a great thickness of atmosphere, the data they produce are often of much poorer quality.
Ground-based observations of comet Grigg-Skjellerup’s orbit were particularly important to the ESA teams who had to navigate Giotto towards its tiny target. Unlike the Halley encounter, where data from the Soviet and Japanese spacecraft that preceded Giotto could be used to calculate final adjustments in the course, navigation for this mission depends entirely on observations from Earth. Fortunately, the possibility that Giotto might be sent to the comet encouraged astronomers to observe Grigg-Skjellerup when it appeared in 1987. Special efforts were made to follow the comet as it receded from the Sun and, in December 1989 astronomers observed it with a 2.2-metre telescope in Hawaii. At this time the comet was almost at its farthest point from the Sun, and it appeared only as a point of light a million times fainter than the faintest star visible to the naked eye. Observations began again in September 1991 using the 3.5-metre telescope at Calo-Alto in Spain as the comet started to approach the Sun. Earlier this year, attention switched to the southern hemisphere. The comet was observed from the Schmidt Telescope in New South Wales, the Australian National University’s 40-inch telescope in Coonabarabran and the European Southern Observatory at La Silla in Chile.
Observations from Earth were scheduled to continue throughout yesterday’s encounter, when the comet was expected to be close to its peak brightness. Unfortunately, it is invisible from the northern hemisphere, and observing conditions in the southern hemisphere are poor. At the moment of encounter, the comet was visible from just one piece of dry land, the Indian Ocean island of Reunion, which has no professional observatory. The comet was briefly visible from Perth, Australia a few hours before the encounter and from the South African Astronomical Observatory two hours after Giotto’s closest approach.
Early in May this year, Giotto was successfully reactivated. Preparation for the encounter started at the beginning of last month, with teams checking the surviving instruments and preparing them for their brief surge of activity on 10 July. The engineers have had to take great care because Giotto’s batteries no longer work and all its power has to come directly from the solar cells, which, after seven years in space, no longer work as well as they did. The plan was to settle Giotto into a final alignment that provided maximum light for the solar panels about 48 hours before the encounter. This gave just enough power to operate the surviving instruments during the hectic hours of the flyby.
Fleeting visit
Giotto met Grigg-Skjellerup yesterday afternoon at about 1525 hours GMT, some 210 000 kilometres from Earth directly above the Ivory Coast. Controllers at Darmstadt communicated with the spacecraft via a NASA radio station in Spain. Giotto closed in on the comet at a much slower relative speed than it did on Halley, with Grigg-Skjellerup catching up with Giotto rather than smashing into it almost head-on. The difference in their velocities around the Sun is about 8 kilometres per second but because Giotto was travelling close to the plane of the Earth’s orbit while the comet was moving northward, their relative velocity was close to 14 kilometres per second. When Giotto approached Halley, the risk from dust collisions was high, but this time the risk was nowhere near as great. Grigg-Skjellerup is less dusty than Halley, and Giotto’s relative speed was much lower.
The spacecraft was aimed straight at where ESA scientists thought the nucleus of the comet would be, but a direct hit was unlikely because they knew the comet’s position to an accuracy of only 1000 kilometres. Scientists hope that Giotto’s star mapper detected the nucleus during the flyby. Although unlikely, with only about a 10 per cent chance of success, any information on the position of the nucleus would enable researchers to reconstruct the exact time and precise geometry of the encounter and help them to interpret data sent back to Earth.
Enough of Giotto’s instruments survived the encounter with Halley’s comet for worthwhile observations to be made. In particular, Giotto could still search for boundaries where the comet’s tenuous outer atmosphere interacts with the solar wind. The behaviour of charged particles, or ‘plasma’, that exists under these conditions is still uncertain. Data from the Giotto encounter could fill gaps in the models of plasma physics. NASA’s International Cometary Explorer, an old Earth satellite diverted to comet Giacobini-Zinner in 1985, did similar studies but Giotto’s instrumentation, even in its debilitated state, is far superior. Plasma observations at Grigg-Skjellerup had two advantages over those at Halley. As Giotto’s approach was slower, the craft could record the shape and position of any plasma features in more detail. And because the comet is smaller, the structure of its plasma will be less affected by fluctuations in the solar wind, which should simplify analysis of the results.
Using the best available theoretical models, plasma scientists predicted that about 26 minutes before Giotto reached the comet it met a ‘bow shock’ where the solar wind is deflected away from the comet. After the bow shock they expected to find a chemical transition; instead of ions from the solar wind, the environment was dominated by ions originating from the comet. When Giotto met Halley, this region extended over 100 000 kilometres from the nucleus; for Grigg-Skjellerup it is likely to reach only 5000 kilometres away and to have been detected about 7 minutes before the encounter. Closer to the nucleus there may be an ionopause, the outer boundary of a ‘cavity’ in space containing no magnetic field. Models predict that this region may stretch only 90 kilometres from the nucleus, so Giotto may miss it completely. If Giotto flew behind the comet, it may have instead probed the plasma and magnetic field structure in the mysterious region where the comet’s tail forms.
Giotto was also trying to examine the comet’s dust. It has a dust impact detector system, consisting of sensors mounted around the dust shield that was intended to protect the spacecraft during its encounter with Halley. As Grigg-Skjellerup is much less dusty than Halley, and Giotto’s relative velocity was lower this time, the energy of each impact, and its chances of detection, were reduced. In addition, Giotto flew through the cloud of gas and dust that makes up the comet’s coma with the dust shield at a glancing angle, further reducing the sensitivity of the detector. The number of impacts recorded also depended on the amount of dust around the nucleus, itself an unknown quantity, and how close Giotto approached. Scientists were estimating that they would see between 10 and 100 impacts.
If all goes well, the researchers hope that Giotto will reveal the ratio of gas to dust in a low-activity comet. They also hope to glean information on the performance of the craft and its instruments that could be useful in future missions to penetrate close to the nucleus of a comet.
John K. Davies is an astronomer at the Royal Observatory, Edinburgh.