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A joint American-Canadian observing program has produced some of the first photometrically calibrated images of Halley's Comet on its return trip to the inner solar system.
Astronomers from the Jet Propulsion Laboratory and from Canada used the 3.6-meter Canada-France-Hawaii Telescope on Mauna Kea, Hawaii, to obtain the Halley images during observations in December. Recent processing at JPL's Multimission Image Processing Laboratory has produced black- and-white and false-color photographs showing Halley's nucleus. (The nucleus is the small core of ice, dust, frozen gases and possibly rock from which comet's most striking features emanate: the halo-like coma and extended tail.)
The images were recorded by high-sensitivity charge-coupled-device electronic camera system. They were the first images taken through interference filters designated by the International Halley Watch (IHW), worldwide organization that is coordinating Earth-based observations of the comet during its current apparition.
According to JPL astronomer Bruce Goldberg, the images show the bare (but unresolved) Halley nucleus, which can be seen only when the comet is essentially inactive. few of these images also show hint of coma, which is consistent with other observations that show early signs of sporadic activity.
For comets such as Halley, which develop extensive comas and tails as they approach the Sun, the nucleus is visible only when the comet is far from the Sun and very faint. At these distances, the nucleus appears only as point-source of light. Its true character is therefore hidden either by its remoteness from the observer or by gas and dust when the comet is nearer the Sun.
At the time of the observations, Halley was about 795 million kilometers from the Sun and 65O million km. from Earth. The nucleus had an apparent visual magnitude of about 21, which made it more than 1 million times dimmer than the faintest star visible to the naked eye.
Collaborating on the JPL-Canada program are astronomers Bruce Goldberg of JPL; Ian Halliday, Allan Cook and Bruce McIntosh of the Herzberg Institute of Astrophysics in Ottawa; and Chris Aikman of the Dominion Astrophysical Observatory in Victoria, British Columbia.
The primary purpose of their program is to obtain better understanding of the nature of cometary nuclei. The observers are pursuing two approaches: direct imaging of the nuclei of faint and relatively inactive comets through interference filters; and monitoring the nucleus regions of brighter, active comets with high time and spatial resolution to study the structure, motions and production rates of dust and gas in their vicinity.
Use of the IHW filters will allow for direct correlation of the results with the findings of other astronomers throughout the world. The various Earth-based observations will also provide framework for interpreting data from the five international spacecraft that will fly by Halley in March 1986.
The outstanding observing conditions on Mauna Kea together with the large aperture and excellent tracking capabilities of the Canada-France-Hawaii Telescope offer unique advantages to this study. Funding for this program is provided by the National Aeronautics and Space Administration in the United States and the National Research Council of Canada.
Halley is now roughly 54O million km. from the Sun, on its way to perihelion, or close approach, of 86 million km. next Feb. 9. The comet will make close approaches to Earth this coming Nov. 27, at 93 million km., and the following April 11, at 63 million km.
Amateur observers with small telescopes will be able to see Halley this fall. Naked-eye viewing from dark- sky sites should be possible by late 1985 or early 1986. This 3Oth recorded appearance of Halley will not be very favorable to the casual observer but will yield wealth of scientific information because of the advent of modern instrumentation.