HUGHES: GIOTTO ANNIVERSARY
The Giotto–Halley C
20th anniversary
The first image of a cometary nucleus was obtained 20 years ago.
David Hughes celebrates this momentous event.
ABSTRACT
Comet Halley’s timely approach to the
Sun in 1986 was an opportunity that
European astronomers grasped with
both hands – and reaped the rewards, in
the form of the first close images and
measurements of a comet’s nucleus.
This noted success came about to a large
extent because of the right people in the
right places in ESA, around 30 years ago.
A commemorative first-day cover issued
on the day of the fly-by, 13 March 1986,
carries the signatures of some of the
most influential people involved.
1 (above): ESA’s 2.85 × 1.86 m Giotto probe.
2 (below): One of the closest images of Halley’s
nucleus (15 × 7.5 km)) taken from about 20000 km.
Giotto was the first to image a nucleus, in 1986.
(Max Plank Institute für Astronomie/H U Keller)
omet Halley is the world’s most famous
comet. The first reliable written record
of its appearance in our skies was made
by Chinese observers in 240 BC. Present-day
cometary astronomers were very lucky that 30
subsequent solar visits brought this comet back
to the Sun in 1986. This was 29 years after the
dawn of the Space Age, an era heralded by the
first orbit of the USSR’s Sputnik 1. The timing
was such that space agencies with both the will,
and the money, had the chance of a lifetime to
fly a multi-instrumented space-probe speedily
through the cometary coma.
Europeans were also very lucky that Dr Ernst
Trendelenburg’s mother had seen the comet
when she was a young girl in May 1910, and
been overawed by the sight. In the late 1970s,
when decisions had to be made about a possible
Giotto mission to Halley’s comet, Trendelenburg was the European Space Agency’s influential Director of Science. In September 1978 he
started the ball rolling and the joint ESA/NASA
International Comet Mission, a combination of
a Halley flyby (using a small ESA craft) and a
Comet Tempel 2 rendezvous (the “exciting”
mother-ship NASA part) got underway. Solar
electric propulsion was to be used to bring the
main spacecraft up to Tempel 2 gradually and
finally put it in an orbit around the comet. And
the last possible launch date was July 1985.
The crunch came in November 1979. The US
Office of Management and Budget decided to
stop funding solar electric propulsion. NASA
was always somewhat dismissive of a mere
cometary flyby. They dropped out of the mission. In early 1980 ESA took up the baton.
Luckily they were fit. The first test launch of the
Ariane three-stage rocket had just been a superb
success. Europe could go it alone.
Fleeting target
In the history of space astronomy Giotto was
unique: the target was fleeting. Observing bodies
such as the Sun, Moon, Mars and Venus is relatively easy: they are always there; slipping the
schedule by a month or so is immaterial. But
Comet Halley was different. It was on its way. In
January 1980 it was well inside the orbit of
Uranus. It passed through the ascending node on
21 November 1985 and the descending node on
20 May 1986. You were either ready for it, or not.
Thinking of bright ideas for space missions is
easy; selecting and building instruments for real
flights is much more complex. The European
way was that ESA built the basic spacecraft and
individual member states paid for and supervised individual instruments. When it came to
comets some choices were obvious. A camera
was essential as was a set of mass spectrometers
to measure the composition of the cometary gas
and dust. The emitted gas quickly becomes
photoionized and then interacts with the ambient solar wind. Instruments to measure the
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HUGHES: GIOTTO ANNIVERSARY
FIRST DAY COVER, 13 MARCH 1986
3: The cartouche on the envelope is for the European Space Agency’s
operations centre in Darmstadt, West Germany, the control centre for the
Giotto mission in 1986. The 80 pfennig stamp shows the Giotto spacecraft
nearing the comet. The commemorative first-day post mark is dated 13
March 1986, the day of flyby, but unfortunately shows the craft passing on
the wrong side of the nucleus. Reading from the left, the signatures on the
envelope are:
● Anny-Chantal Levasseur-Regourd, the French PI of the Optical Probe
Experiment;
● Ray L Newburn Junior, the Californian vice-chairman of The
International Halley Watch Science Working Group;
● Hans Balsiger, the Swiss PI of the Ion Mass Spectrometer;
● Fred L Whipple, the Harvard originator of the “dirty-snowball”
hypothesis and the inventor of the vital comet dust bumper shield;
● Fritz M Neubauer, the German PI of the magnetometer;
● Rüdeger Reinhard, the Giotto Project Scientist;
● Alan Johnstone, the British PI of the fast ion and implanted ion sensor;
● Horst Uwe Keller, the German PI of the multicolour camera;
● Tony McDonnell, the British PI of the dust impact detector;
● Peter Eberhardt, the Swiss Co-I on the neutral mass spectrometer team;
● Jack Brandt, the NASA-Goddard Space Flight Center expert on cometary
plasma tails and disconnection events;
● Roger M Bonnet, the French Chairman of ESA’s Solar System Advisory
Committee in late 1978 (the committee that chose to explore Comet Halley
mass, velocity and direction of motion of these
ions and their effect on magnetic fields were
clearly important. And these ions were much
more energetic in the turbulent region where the
comet interacted with the solar wind than they
were in the inner coma close to the nucleus. Ionization produces electrons and these suprathermal electrons were measured too. Cometary
dust scatters sunlight and also has a wide range
of particle sizes. Dust was measured by treating
the front of the spacecraft like the skin of a
drum. Three instruments measured the magnetic
field, the light level along the direction travelled
by the spacecraft and the Faraday rotation produced in the transmitted radio waves. Each
instrument was funded by an ESA member state
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not the Moon) who became ESA’s Director of Science, after Trendelenburg;
● Dave Dale, the Yorkshireman promoted from the post of ESA’s director of
future projects to become the project manager for the Giotto mission;
● Jochen Kissel, the Heidelberg PI of the dust mass spectrometer;
● Susan McKenna-Lawlor, the Irish PI from Maynooth in charge of the
energetic particles analyser;
● Ian Axford, Germany’s influential representative on ESA’s Science
Advisory Committee when the Giotto mission was in competition with
POLO, a polar orbiting lunar observatory satellite; Axford stressed that the
Moon was always there but miss Halley in 1986 and we would have to wait
until 2061;
● Hugo Fechtig, the Heidelberg expert on the space detection of cosmic
dust, and one of the main advocates on ESA’s Solar System Working Group
for a Halley flyby mission;
● Don Brownlee, from Seattle, the comet dust expert who has collected
dust from the Earth’s atmosphere using U2 aeroplanes and from comet
Wilt2 with the Stardust mission;
● Peter Edenhofer, the German PI of the Radio Science Experiment;
● Armand H Delsemme, the Belgian who went to the University of Toledo,
USA, and became an expert on cometary chemistry;
● Don Yeomans, Jet Propulsion Labs expert on cometary astrometry, and
history;
● Ernst Trendelenburg, ESA’s Director of Science in 1978, without whom
Giotto would not have happened.
and run by a principal investigator (PI) and an
accompanying team of scientists and engineers.
Politics also reared its head. The instrument
deck had to be multinational. After much heated
discussion it ended up four German, two
French, two British, one Swiss and one Irish.
Hazard warning
Much time was spent trying to estimate the hazard to the spacecraft as it flew by Halley. Scientists even worried as to whether the nucleus
would be visible, cocooned as it was in a dusty
gaseous coma.
The spacecraft flew within 600 km of the
cometary nucleus on 13 March 1986. It just survived the cometary dust impacts, although it
was hit hard and set wobbling. All the experiments worked. ESA’s first planetary mission was
a great success. And it was a watershed for
comets.
Before 13 March 1986 no-one had ever
resolved the nucleus of a comet. The “bumperbook of nuclei pictures” was transformed from
an empty tome to a volume containing pictures
of one object. But the comet race had started.
Orbiting comets, landing on nuclei and returning with samples of cosmic snow are for the
future, but we will get there. ●
David W Hughes, Department of Physics and
Astronomy, The University of Sheffield, S3 7RH.
[email protected]
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