Exactly 15 years after the intrepid Giotto spacecraft swept past the
nucleus of Halley’s Comet, ESA scientist Gerhard Schwehm shared his
memories of past triumphs while looking forward to new revelations from
the Rosetta mission.

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 1/2 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

13 January 2003 — Launch of Rosetta

June-July 2012 — Rosetta enters orbit around Comet Wirtanen and releases a lander onto its surface.

USEFUL LINKS FOR THIS STORY

* Giotto home page

http://sci.esa.int/home/giotto/
* More about Rosetta

http://sci.esa.int/rosetta

* The Halley flyby: 13 March 1986

http://sci.esa.int/content/doc/cb/13515_.htm

* Comets

http://sci.esa.int/structure/content/index.cfm?aid=13&cid=2127

IMAGE CAPTIONS:

[Image 1:

http://sci.esa.int/content/searchimage/searchresult.cfm?aid=15&cid=12&oid=26418&ooid=21451]
Gerhard Schwehm.

[Image 2:

http://sci.esa.int/content/searchimage/searchresult.cfm?aid=15&cid=12&oid=26418&ooid=13765]
The Giotto spacecraft during integration activities. The solar panels have
been removed to provide access to the spacecraft. The white object at the
intersection of the struts is the Halley Multicolour Camera.

[Image 3:

http://sci.esa.int/content/searchimage/searchresult.cfm?aid=15&cid=12&oid=26418&ooid=26428]
Image of the nucleus of Comet Halley as viewed by Giotto. Giotto’s encounter
with Comet Halley provided the first ever opportunity to take images of a
comet nucleus. The images were obtained with the Halley Multicolour Camera
on Giotto.

[Image 4:

http://sci.esa.int/content/searchimage/searchresult.cfm?aid=15&cid=12&oid=26418&ooid=26425]
Image of the nucleus of Comet Halley as viewed by Vega 2. The Vega mission
was an international project conducted by the former USSR with several
other countries within the framework of Intercosmos. The mission comprised
two identical spacecraft, Vega 1 and Vega 2, which first delivered a Venus
lander into the Venusian atmosphere. Using the gravity of Venus to propel
them towards Comet Halley, the two spacecraft encountered Halley in early
March 1986. The closest approach of Vega 1 to Halley was 8890 km while
Vega 2 had a close encounter at 8030 km.

[Image 5:

http://sci.esa.int/content/searchimage/searchresult.cfm?aid=15&cid=12&oid=26418&ooid=24280]
An artist’s impression of the Rosetta Spacecraft scheduled for launch in
2003. Rosetta will rendezvous with Comet Wirtanen in summer 2012.