Ordinary Meeting and Exhibition Meeting, 2005 June 25
Spinning Asteroids
Mr Dymock explained that in the following talk he would be describing how even comparatively modestly equipped amateurs could conduct scientifically useful observations of asteroids. He explained that all asteroids rotated about some axis as they drifted through space, and while surface details on them were invariably irresolvable, their changing faces as they rotated could potentially give rise to variability in their brightnesses, following some pattern which repeated on each rotation.
The extent of this depended upon a number of factors, including the object's shape, and the degree of variation in its surface albedo. However, a critical factor was the orientation of its axis of rotation with respect to our line of sight. Asteroids which rotated about an axis closely aligned to our line of sight would, if they could be imaged, appear to rotate, but their unresolved brightnesses would remain unchanged in the absence of new features rotating into view. At the opposite extreme, asteroids whose rotation axes lay perpendicular to our line of sight revealed a maximal area of new surface as they rotated, and thus their unresolved brightnesses varied maximally.
This orientation would vary over time. As the Earth travelled around its orbit, and the asteroid along its, the direction of our line of sight to it would change, having the direct consequence that its apparent position in the celestial sphere would change. But, more subtly, the asteroid's rotation axis would remain in a fixed spatial direction throughout its orbit and hence, as our line of sight's direction changed, so too would its orientation with respect to the asteroid's spin axis.
Consequently, a wealth of information could be gleaned from an asteroid's light curve. The period of any observed modulation gave its rotation period. The amplitude of the modulation gave an indication of the asteroid's shape, increasing with the aspherilicity of the object. The variability of the modulation over the period of the object's orbit depended upon the orientation of its spin axis. And, from the spin period could also be estimated the asteroid's approximate size, as larger objects tended to rotate more slowly than their smaller counterparts.
Moving on to discuss the practicalities of asteroid photometry, Mr Dymock explained that there were two possible approaches: differential or all-sky photometry. In the former case, the target was placed in a CCD frame which also contained one, or preferably two, reference stars of known brightnesses. Photometric measurements could then be calibrated by comparison with these reference objects. In the latter case, calibration measurements of reference stars all around the sky were made prior to making any asteroid observations, to correct not only for instrumental effects, but also for the transparency and weather conditions of the particular night. Measurements from across the sky were required to determine how the zero-point and extinction of the sky varied with altitude due to the changing air-mass along the line of sight.
For beginners, the speaker urged the use of the former procedure, which was very simple to use. Although it had the downside of only being applicable to asteroids with steady reference stars nearby, and tended to be less accurate because fewer reference objects were used in the calibration, the results were still scientifically of great value.
He recommended newcomers to use Alan Harris' online database of targets of known period as a source of objects to practice on; he suggested selecting targets which were comparatively bright, and were up for a substantial portion of the night. To begin with, it was rewarding to select objects with spin periods less than eight hours, so that a whole period might be imaged in a single night.
Moving on to observing techniques, Mr Dymock explained that focussing and polar alignment, whilst needing to be reasonable, were less critical for photometry than for imaging. So long as the light from each source was contained within a relatively compact region of the frame, accurate photometric measurements could be made. However, well-chosen exposure times, to prevent saturation, and thorough calibration of the CCD array using flat-frames and dark-frames, were vital. The speaker recommended taking images as frequently as possible during observation, in order to get as many points on the light-curve as possible. He went on to provide a brief overview of how to perform the photometry using his choice of software, Canopus4.
For observers who felt ready to attempt the greater challenge of making scientifically valuable observations of asteroids of unknown periods, Mr Dymock recommended selecting sources from the lists published in the online Minor Planet Bulletins of the Minor Planet Section5 of the Association of Lunar and Planetary Observers (ALPO). In closing, he wished to emphasise that asteroid photometry was a very rewarding activity, and that even amateurs possessing only modest equipment could make very valuable observations.
Following the applause for Mr Dymock's talk, the President adjourned the meeting until the Out of London Meeting, to be held in Cambridge on September 3.
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Dominic Ford.