Number 20: 24/05/2003
A scientific publication by SGF and NEODyS

NEOs' Spacemissions Special: mission GIOTTO

"The International Halley Watch" by Donald K Yeomans - NASA, JPL

Figure 10. The International Halley Watch logo. It was widely used during the 1986 return of comet Halley, including being featured on several postage stamps

Comet Halley is the only naked-eye periodic comet that can be routinely observed at each apparition.  It has been seen and recorded at every apparition since 240 BC.  Beginning with its first predicted return in 1758-59 (Halley, 1705), comet Halley has been anxiously awaited at every return.  Plans for a coordinated set of photographic observations were attempted in 1909 but the effort met with little success (Comstock et al. 1909).  A series of monographs did get published for the 1909-11 apparition but only many years after the apparition itself was complete (Bobrovnikoff 1931; Perrine 1934; Donn et al. 1986). 

For the 1986 return to perihelion, the International Halley Watch movement began in 1979 when JPL scientist Louis Friedman convinced NASA that a major international effort would be required to maximize the scientific return from ground-based and space-based observations of comet Halley during its upcoming apparition.  Friedman asked JPL astronomer Ray Newburn Jr. to generate scientific goals for ground-based observations and outline the type of organization that could meet these goals.  Don Yeomans was asked to provide cometary ephemeris predictions and Jay Bergstrahl was asked to outline the types of comet Halley studies that could be carried out from Earth orbital spacecraft including the Shuttle, the Hubble Space Telescope, and the International Ultraviolet Explorer.  Friedman himself was promoting a U.S. mission to comet Halley and while that was not to be, his International Halley Watch idea grew into an international success story.By the fall of 1981, Ray Newburn and Jürgen Rahe had been appointed co-leaders of the International Halley Watch (IHW) and a competition for Discipline Specialists was established.  Although they were not all elected initially, eventually, there would be eight separate disciplines within the IHW (Astrometry, Infrared Spectroscopy and Radiometry, Large-Scale Phenomena, Meteor Studies, Near-Nucleus Studies, Photometry and Polarimetry, Radio Studies, Spectroscopy and Spectrophotometry).  An Amateur Observing Network, coordinated by S.J. Edberg, was also very active.  The goals of the IHW organization were to encourage and support any scientifically valid means of studying the comet, to coordinate the ground-based and space-based observations to maximize the scientific return, to set useful standards for observing within each discipline, and to properly docuAstrometry (The Discipline Specialists were D.K. Yeomans, R.M. West, R.S. Harrington, and B.G. Marsden).

Members of this discipline provided the positional observations of comet Halley that are used in the orbit determination process.  These data were used to continually update comet Halley's orbit with the resulting ephemerides being made available to the wide community of researchers who made observations using ground-based, Earth orbital, and interplanetary instruments.  Astrometric data were received via telegram, Telex, electronic mail message, computer-to-computer links as well as by telephone and postal delivery.  Many observers went to extraordinary efforts to see that their data were reduced and transmitted to the orbit determination center within the shortest possible time.  Some of these observations were less than a day old when received at the data collection center at the Jet Propulsion Laboratory (JPL).  These outstanding efforts were most obvious just prior to the spacecraft encounters of comet Halley in early March 1986.  As an example of the personal efforts that made the Astrometry Network a success, we note that an English amateur astronomer often took astrometric photographs, measured the images, reduced the measurements into positional data and telephoned the information to JPL before finally going to bed. 

A total of 6475 astrometric observations of comet Halley covering the interval from October 16, 1982 through January 9, 1989 were reported to the Astrometry Network.  These observations resulted from efforts by 430 observers working at 148 observatories throughout the world.  It is a credit to IHW Astrometry Net members that 90% of the data received were used in the most recent orbit updates for comet Halley. 

Infrared Spectroscopy and Radiometry (R.F. Knacke, T. Encrenaz)

In the early 1980's, infrared studies of comets were still a relatively young field.  Filter sets, when available, were not standardized.  The infrared region was loosely defined as 1 to 1000 micrometers and this discipline de\ith photometry, spectroscopy, polarimetry, and imaging.  As was the case for most of the disciplines, infrared observations were requested for certain "Halley Watch days", some of which were clustered near the times of the Giotto and VEGA spacecraft encounters.  Infrared science objectives included the photometric study of the 10-micrometer silicate emission feature, the evolution of the temperature, the dust size spectrum, composition information and the gas-to-dust ratio.  Some 600 infrared files of data were contributed to the IHW archive.  The time of the Halley perihelion return marked the beginning of infrared imaging arrays (rather than scanning) and comets IRAS-Araki-Alcock, Giacobini-Zinner and Halley were the first comets to be imaged in the infrared. 

Large-Scale Phenomena (J.C. Brandt, M.B. Niedner, J. Rahe)

With cometary gas and dust tails having the largest extent of any solar system object, wide field imaging was necessary to study the evolution, chemical composition and dynamical history of the dust tail grains.  Plasma tails reflect the nature of the interplanetary solar wind and solar magnetic field lines.  Extensive temporal coverage was required to capture large-scale phenomena such as solar magnetic field lines wrapping around the comet's head to form a plasma tail and then detaching from the head, perhaps as a result of a magnetic field reversal in the solar magnetic field.  Because the most impressive plasma tails develop near the sun, the observing time per person is often limited to a few hours per night.  Hence the coordinated approach of the IHW, with complete temporal coverage provided by many observers in different locations, was key to the success of these investigations.  Some 103 international observatories  contributed more than 3500 images of comet Halley's tails.  Of this amount, 3383 images were archived with about half of these being digitized before being archived.  An impressive book of images arranged in chronological order ( a real collector's item!) was also produced (Brandt et al., 1992).  

Meteor Studies (B. McIntosh, P.B. Babadzhanov, A Hajduk, B. Lindblad)

In May and late October each year, the Earth passes through a ribbon of dust debris from comet Halley.  The May and late October meteor showers, the Eta-Aquarid and Orionid showers respectively, typically last about 5 days.  Depending upon the circumstances and the observer's latitude, the zenith hourly rates can reach about 20 meteors per hour.  In an effort to determine the mass distribution of particles in the meteor streams, the composition of the dust particles, and their spatial structure and evolution with time, radar and visual observations of the two streams were solicited and archived.  Some 5662 hours of radar data over the 1984-1988 interval were archived as were 1600 hours of visual observations over the 1984-1986 interval.  Given the evolution of the Orionid meteor stream particles, the 1994-1998 events are likely caused by particles that left the comet more than 5 thousand years ago (McIntosh, 1990).  The recent meteor stream data was not expected to be influenced by the 1986 return of the comet.

Near-Nucleus Studies (S. Larson, Z. Sekanina, J. Rahe)

The goals of the Near-Nucleus Studies discipline were to understand the processes taking place in the coma and relate them, to the extent possible,  to the active areas on a rotating solid nucleus. Observations of individual coma features over long periods of time can constrain the rotational motion of the nucleus.  Some 3540 near-nucleus images, over the 1982-1989 interval, were ultimately archived.  The observed dust jet curvatures suggest the nucleus rotates in a prograde sense with an instantaneous, apparent period of about 2 days.  The dust morphology repeats with a period of 7.4 days.  These differing periods indicate the nucleus may be in a complex rotation state.  That is, the rotation axis (like a poorly thrown American football) wobbles and is not fixed in space.

Photometry and Polarimetry (M.F. AHearn, V. Vanysek)

The use of narrow band filters to investigate the chemical composition of the cometary coma was undertaken with the knowledge that several sites would be necessary to study the long-term and short-term variability of the cometary coma with a set of standard filters.  Sets of 8 standard filters were distributed to members of the network for their use.  These filters included spectral band passes to investigate the dust continuum radiation and the molecular emissions due to OH, NH, CN, C3, CO+, C2, and H2O+.   Data was taken by 116 observers using 53 different instruments in 16 countries.  Cyanogen (CN) and oxygen (OI) were first detected in February 1985 when the comet was 5.4 AU from the sun.  A periodicity of 7.4 days was noted in the C2 production rates and for the same heliocentric distance, the comet was brighter post-perihelion.

Radio Studies (W.M. Irvine, F.P. Schloerb, E. Gerard, R.D. Brown, P.Godfrey)

Some 36 different observing groups participated in the radio study of comet Halley with most of the work being carried out using the 18 cm line of OH.  OH was detected in July 1985 at a heliocentric distance of 4.3 AU.  An Arecibo radar experiment was attempted during the comet's closest approach to Earth in November 1985 (0.6 AU) and an echo was recorded but no range or Doppler information was determined. 

Spectroscopy and Spectrophotometry (W. Wyckoff, P.A. Wehinger, M.C. Festou)

This discipline area covered the spectral range from the ultraviolet to the visible region (3000 to 10000 Angstoms).  Some 3500 spectra were taken and archived from 150 observers at more than 80 observatories in 16 countries.  The first spectroscopic evidence for the onset of cometary outgassing was detected on February 17, 1985 when the 6300 A line of oxygen and the cyanogen (CN) line at 3880 A were detected.  The C2 carbon molecule was first detected on August 23, 1985 and singly ionized water (H2O+) was detected in October 1985 when the comet was 2.2 AU from the sun.  On the outbound journey, the last emission bands that indicate outgassing were the cyanogen (CN) and the carbon molecule (C3) on January 30, 1987 when the comet was 5 AU from the sun.  The outbound gas production rates were 15 times the rates inbound for the same heliocentric distance.

Amateur Network of observers (S.J. Edberg)

From the onset of the International Halley Watch, the participation of the amateur astronomer community was solicited and it became clear that some of the better ones were amateurs in name only.  A total of 863 amateur contributed data to the archives of the IHW with activity in the areas of astrometry, photometry, spectroscopy, meteor studies, drawings, photographs, and magnitude estimation. 

History will record that International Halley Watch was the largest coordinated international effort to observe any celestial object.  In what was termed the "pathfinder project", the ground-based positional measurements of the comet were combined with observations from the Soviet VEGA spacecraft to provide improved comet position predictions for the following Giotto spacecraft.  The cooperative efforts were extraordinary.  From a personal point of view, I recall it was a time when information on the comet, the spacecraft and the instrumentation seemed to be freely shared among the international group of scientists and engineers - a time before the current oppressive import-export restrictions and technology transfer issues.  In the pre-glasnost era of the cold war, we worked closely with our colleagues in the Soviet Union.  Direct, real-time, information links were established between our orbital computation activities in the US and the flight projects of the Soviet Union, the European Space Agency and Japan.  Political differences were put aside and everyone worked toward making the return of comet Halley a scientifically rewarding one.  The IHW data archives, on a set of CDs, will stand as a lasting tribute to the enormous amount of work that took place to capture the scientific importance of this once-in-a-lifetime experience.

The public and media response to the comet Halley phenomena was often overwhelming and the IHW responded with tens of thousands of brochures and a telephone "hotline" service.  Posters, information sheets and photographic slide sets were distributed to teachers and countless interviews and lectures were given by IHW personnel.  Entrepreneurs sold comet Halley hats, jackets, T-shirts, books, coffee cups, pins, medals and coins (available in bronze, silver, gold and platinum).  Expensive Halley telescopes were sold to folks who would have been better off using their binoculars.  The IHW refused to be part of any commercial venture but the IHW logo could be used for non-commercial purposes such as postage stamps.  One scoundrel sold an "official" Halley logo to many governments for use on their postage stamps when they could have used the real IHW logo for free. Comet Halley position predictions included in the 1981 IHW Comet Halley Handbook were used to recover comet Halley at Palomar mountain in October 1982 and in this booklet's foreword, the following paragraph attempts to explain the extraordinary interest in comet Halley.  Comet Halley's fame is not due to its superior brightness alone: its periodic returns every 76 years act like a clock counting time in units of human lifetimes.  Thus the ever-returning comet marks transitions from one era to the next.  Our parents and grandparents have told us about the great comet's visit in 1910, and we will tell our children and grandchildren about its return in 1986.  Once every 76 years, nearly everyone in recorded history has had the opportunity to view the comet.Before comet Halley's next return to perihelion in late July 2061, the IHW observing campaigns and their archiving activities will ensure that future generations will benefit from the careful and coordinated study of its 1986 return.

References:

Comstock, G.C., E.E. Barnard, E.B. Frost, C.D. Perrine, and E.C. Pickering (1909).  Circular respecting observation of Halley?s comet 1910.  Astronomy and Astrophysics Society of America, 12 pp.

Bobrovnikoff, N.T. (1931).  Halley's comet in its Apparition of 1909-1911.  Publications of the Lick Observatory, vol. 17, pp. 309-482.

Brandt, J.C., M.B. Niedner Jr., and Jurgen Rahe (1992).  The International Halley Watch Atlas of large-Scale Phenomena.  University of Colorado-Boulder, Johnson Printing Co., 736 pp.

Donn, B., J. Rahe, and J.C. Brandt (1986).  Atlas of comet Halley 1910 II.  NASA SP-488.  US Government Printing Office.

Halley, E. (1705).  A Synopsis of the astronomy of comets, London.

McIntosh, B.A. (1990).  The structure and evolution of the comet Halley meteor stream.  In Comet Halley: Investigations, Results, Interpretations.  Ellis Horwood, vol. 2, pp. 121-132.

Perrine, C.D. (1934).  Observaciones Del cometa Halley Durante Su Apparicion en 1910.  Results of the Observatory Nacional Argentino, vol. 25, 108 pp + 99 plates.

ment and archive the scientific data.