ESA Science & Technology - A night to remember: the Giotto flyby of Halley's comet

(Editor's note: This interview was conducted before the target comet for the Rosetta mission was changed from comet Wirtanen to comet 67P/Churyumov-Gerasimenko.)

Q. Tell us about your involvement in the Giotto mission.

A. I was the Deputy Project Scientist for the Giotto mission at the time of the Halley flyby. My main task was to coordinate the instrument operations and to help and support the scientists. A few months later, when we started to work on preparations to wake up the spacecraft again, I became the Project Scientist for the Giotto Extended Mission. This led to a second flyby, this time of Comet Grigg-Skjellerup, in 1992.

Q. Where were you at the time of the Halley flyby?

A. I was at the heart of the excitement, at the European Space Operations Centre (ESOC) in Darmstadt, in the room where the experimenters 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 the Halley Multicolour Camera building up on the TV screen as the data were received at ESOC.

It was a very exciting time, but also a time of tension because we didn't know whether the spacecraft would operate properly. It was vital that it did, because 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!

Q. What new discoveries resulted from the Giotto encounter with Halley?

A. 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.

Giotto confirmed that the object was very primitive. The abundances of chemical elements observed by the mass spectrometers on Giotto showed that we were dealing with the most primitive matter ever encountered in our Solar System. Its origins probably go back to the primeval nebula from which our star and its planets were formed.

The dust mass spectrometer also showed that there were two major classes of particles. One type was almost entirely dominated by the light CHON elements (organic grains made of carbon, hydrogen, oxygen and nitrogen). The other group was rich in mineral-forming elements such as magnesium, calcium and silicon.

Giotto also posed a lot of new questions, which is why it is so important to have a new mission such as Rosetta.

Q. What contribution did the two Russian Vega spacecraft make to the success of the mission?

A. 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.

Q. Was this closer or further than you had hoped?

A. We could have gone closer. The 600 km distance 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 flyby 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.

Q. Earlier you mentioned ESA's Rosetta mission. Why is this new mission to a comet so important?

A. 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 Comet Wirtanen 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 1 = years instead of making a quick flyby, so we will discover for the first time what happens to a comet as it is warmed during its approach to the Sun.

However, it is 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.

Giotto Key Facts:
Original Mission: Comet Halley flyby
Launch Vehicle: Ariane 1
Launch Date/Time: 2 July 1985 at 11:23:13 GMT
Launch Site: Kourou (French Guiana)
Spacecraft Mass: 960 kg
On-orbit dry mass: 582.7 kg

Key Dates:
14 March, 1986 - Giotto's Comet Halley flyby (Closest approach 00:03.01.34 +/- 0.20s GMT )
2 July, 1990 - Giotto's first Earth flyby
10 July, 1992 - Giotto flyby of Comet Grigg-Skjellerup
1 July 1999 - Giotto's second Earth flyby