Giotto fly-by of Halley's Comet: An interview with Gerhard Schwehm
While ESA’s next comet-chasing mission is being prepared, Gerhard Schwehm shares his memories of past triumphs such as when the intrepid Giotto spacecraft swept past the nucleus of Halley's Comet, while also looking forward to new revelations from Rosetta.
Gerhard Schwehm
Head of Planetary Missions Division, ESTEC, Noordwijk
In 1986, Gerhard was deputy project scientist for Giotto at the time of the Halley fly-by. His main task was to coordinate instrument operations. Later he became the project scientist for the Giotto Extended Mission. This led to a second fly-by in 1992 of Comet Grigg-Skjellerup. Currently he is Rosetta Project Scientist.
ESA: In the Giotto mission, where were you at the time of the Halley fly-by?
Gerhard Schwehm
I was at the heart of the excitement, at the European Space Operations Centre (ESOC) in Darmstadt, in the room where the scientists had their data displayed on their screens. Around me were all the experiment representatives with what we today would consider to be primitive computers. We were watching the raw images from Halley building up on the TV screen as the data were received.
It was a very exciting time, but also a time of tension because we didn't know whether the spacecraft would operate properly. The actual encounter lasted only a few hours and there was no time for recovery if anything went wrong. I am very proud to say that everything worked perfectly right up until 15 seconds before closest approach - the moment when the spacecraft was knocked spinning by a tiny dust particle and started to wobble.
This was not too much of a surprise. We thought something might happen because of the dust environment. The frequency of dust impacts was building up and we knew that this could cause us to lose contact. Fortunately, the signal slowly returned and we were able to continue on to another comet. We celebrated by eating some chocolate Giottos that had been made by a local baker!
ESA: What new discoveries resulted from the Giotto encounter with Halley?
Gerhard Schwehm
Giotto revolutionised our understanding of what a comet is like. It obtained the first images of a comet from a range of only a few thousand kilometres, and these confirmed that there was a 'solid' nucleus at the heart of the comet. After processing the images for a few hours, we realised that the nucleus was very irregular in shape. There were a number of jets - areas of high gas and dust emission - on the nucleus, while the rest of it was inactive. We also found that the nucleus was extremely dark - something that had been predicted shortly before the encounter but not really taken very seriously.
The object was very primitive. The abundances of chemical elements observed by Giotto's mass spectrometers showed that we were dealing with the most primitive matter ever encountered in our Solar System, probably going back to the primeval nebula from which our star and its planets were formed. Giotto also posed a lot of new questions, which is why it is so important to have a new mission such as Rosetta.
ESA: What contribution did the two Russian Vega spacecraft make to the success of the mission?
Gerhard Schwehm
The Vegas were our 'pathfinders' and were very important for the Giotto mission. Images of Halley from their cameras allowed us to determine very accurately the position of the comet and the trajectory we wanted Giotto to follow. There was great collaboration between ourselves and our colleagues in the Soviet Union, at JPL in the United States and at ESOC to calculate the orbit of the comet nucleus. Their hard work meant that Giotto was able to fly past the nucleus at a distance of only about 600 km.
ESA: Was this closer or further than you had hoped?
Gerhard Schwehm
We could have gone closer. 600 km was a compromise by the science teams. Some were interested in the dust and gas environment and wanted to get as close as possible. The imaging team wanted to stay further away because the camera would have to slew sideways much faster if the fly-by was at close range and this would make imaging much more difficult. We had a long discussion a few days before closest approach and agreed to settle for 600 km.
ESA: Earlier you mentioned ESA's Rosetta mission. Why is this new mission to a comet so important?
Gerhard Schwehm
Giotto was a reconnaissance mission. It was done in a hurry because we had a once-in-a-lifetime opportunity to visit Halley's Comet. We learned about many things for the first time from close range - the nucleus, the dust environment and the interaction between the solar wind and the comet - but this whetted our appetite to return to another comet and do it even better.
With Rosetta we plan to study the nucleus of another comet from a distance of only one or two kilometres, so we will obtain pictures with a much higher spatial resolution. Rosetta will also stay with the nucleus for over a year instead of making a quick fly-by, so we will discover for the first time what happens to a comet as it is warmed during its approach to the Sun. It also worth remembering that, although Rosetta's instruments are so much more advanced than Giotto's, many of them have their roots in the technology that we first used 15 years ago.
ESA: Rosetta will drop a lander onto a comet for the first time, what challenges does this present?
Gerhard Schwehm
Yes, the Rosetta lander will be the first man-made object to land on a comet. Firstly, we don't know anything about how rough the surface is. It could be covered with fluffy snow like the Alps or it could be hard rocks and craters. We can, however, be sure that it will not be smooth and flat, resembling parking lots. We’ve designed Rosetta's landing gear to cope with most nasty surprises as soon as it touches down. Two harpoons will anchor the probe to the surface. The self-adjusting landing gear will ensure that it stays upright, even on a slope. The lander's feet will drill into the ground. These devices will help counteract the fact that there is no gravity on a comet.