Comet Hale-Bopp over the Subaru and Keck Telescopes on Mauna Kea. Photo by John Davies.

IRAS Comets.

As is often the way in science I became interested in comets almost by chance. In 1983 I was working at the ground control station for the IRAS satellite with the task of looking for Earth approaching asteroids. On April 25th we detected a moving object which was rather brighter than we expected for an Earth approaching asteroid but after checking for any technical problems we asked some ground based telescopes to have a look. Optical confirmation was needed since all we got from IRAS was a stream of numbers on a piece of paper, not images that were simple to interpret. What happened next is a bit of a saga of confusion, missed messages and chance conversations (for an unbiassed account look up Sky and Telescope July 1983 p26-29) but in the end it turned out that the object was a comet, independently found by two amateur astronomers, and soon named Comet IRAS-Araki-Alcock (IAU rules didn't allow it to be called Davies-Araki-Alcock). The comet came unusually close to the Earth and was a naked eye object for a few days so I had my 15 minutes of fame on TV and radio. Comet IRAS-Araki-Alcock was actually a small comet and was only bright because it passed within 5 million km of the Earth, this was the closest approach of a comet since Comet Lexell about 200 years ago. The moving object team (me, Simon Green and Brian Stewart) went on to discover a total of 6 new comets, a couple of asteroids and a previously unknown infrared trail associated with comet Tempel-2. You can get the details from some of the papers in my publications list. The best summary of the programme is in Davies J.K., Green S., Stewart, B., Meadows, Aumann, H.H., (1984). IRAS and the search for fast moving objects. Nature 309 p315-319, 24 May 1984.

Infrared Spectroscopy of Comets from UKIRT

After IRAS stopped operating I began making infrared observations of comets from the ground using the United Kingdom Infrared telescope UKIRT. Using the near infrared spectrometers on UKIRT various collaborators and I studied a number of comets in the 3 micron region of the spectrum. We detected a variety of emission features in comets Austin, Levy and Swift-Tuttle and found that the spectra of each comet was subtly different. If you want the technical details of these observations look up the papers given in my publications list

Comet Hale-Bopp in the dawn sky of Mauna Kea. Photo by John Davies.

Comet Hale-Bopp

Shortly after comet Hale-Bopp was discovered I used UKIRT to observe it while it was still over 6 astronomical units from the Sun. These observations were combined with data taken by some friends of mine about the same time and showed evidence for particles of water ice in a halo around the comet. Although comets have long been known to have a lot of water ice in them detecting an icy grain halo has proved to be very difficult since usually, once the comet is close enough to the Sun to be bright, it is so warm that the ice grains have already evaporated. It was only the combination of new, much improved, detectors and the brightness of this particular comet that made the detection possible. You can read about the details here We went on to make other observations of the comet but these are still being worked on ready for publication.

Comet Hale-Bopp rising over trees in my garden. Photo by John Davies.

As well as UKIRT, I was also involved in observations of Comet Hale-Bopp using the JCMT sub-mm telescope. These observations allowed a team of people, including a group from the Observatoire de Paris in France, to monitor the evolution of various molecules as the comet got close to the Sun. For details look up the paper by Nicolas Biver and others called "The evolution of the outgassing of comet Hale-Bopp from radio observations" which was published in the magazine "Science" on 28 March 1996

Comet Hale-Bopp setting over the JKT telescope building on La Palma in May 1997. The cone of bright sky poking up from the horizon and surrounding the comet is real, it is the zodical light, caused by tiny dust particles in interplanetary space reflecting sunlight back towards Earth. Photo by John Davies.

Infrared Image of Comet Spacewatch. Taken in the H band (1.65 micron) on 25 February 1998 by John Davies using the UKIRT Telescope, Mauna Kea. The comet is right of centre, the stars are trailed left to right since the images was made tracking the comet's motion.

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John Keith Davies
Astronomy Technology Centre, Royal Observatory, Blackford Hill, Edinburgh, EH9 3HJ
tel: (44) 0131 668 8348 / fax: (44) 0131 662 1668/