David H. Levy

Hubble Space Telescope Observations of Comet P/Shoemaker-Levy 9 (1993e)

The Hubble Space Telescope observed the fragmented comet P/Shoemaker-Levy 9 (1993e) (P indicates that it is a periodic comet) on 1 July 1993. Approximately 20 individual nuclei and their comae were observed in images taken with the Planetary Camera. After subtraction of the comae light, the 11 brightest nuclei have magnitudes between ∼23.7 and 24.8. Assuming that the geometric albedo is 0.04, these magnitudes imply that the nuclear diameters are in the range ∼2.5 to 4.3 kilometers. If the density of each nucleus is 1 gram per cubic centimeter, the total energy deposited by the impact of these 11 nuclei into Jupiters atmosphere next July will be ∼4 x 1030 ergs (∼108 megatons of TNT). This latter number should be regarded as an upper limit because the nuclear magnitudes probably contain a small residual coma contribution. The Faint Object Spectrograph was used to search for fluorescence from OH, which is usually an excellent indicator of cometary activity. No OH emission was detected, and this can be translated into an upper limit on the water production rate of ∼2 x 1027 molecules per second.

Comet P/Halley near-nucleus phenomena in 1986

Computer enhanced high resolution groundbased images taken during Halley’s most active phases in 1985–86 exhibit all of the coma structures seen in 1910 photographs. Improved detector technology compensated for the poor observing circumstances of the current apparition yielding a wealth of complex coma structure. This structure resulted mostly from discrete dust jet activity producing spirals, arcs, and jagged-edged envelopes. Several episodes of the “multiple nucleus” and antisunward jet phenomenon were also observed. The variation in dust jet activity during the Vega and Giotto flybys is easily seen on the groundbased images. Our images of jets in the 388 nm CN (0,0) band show that they usually do not correlate very well with visible dust jets. As in 1910, the observed expansion velocities and jet curvature indicate a rotation period near 2.2 days. Because the short period dominates the shape of the jets, detailed jet modeling will be required to detect longer term rotation components such as the proposed 7.4 day period.

The Collision of Comet Shoemaker-Levy 9 with Jupiter

The discovery of Comet Shoemaker-Levy 9 in March 1993 opened an extraordinary few years in the study of the history of impacts in the solar system. This review paper offers a background that attempts to set the events of 1993 and 1994 into a historical context, and describes events leading to the discovery and the mounting of a unique and unprecedented international effort to observe the comets collision with Jupiter. A selection of the results is presented to explore how the fate of Comet Shoemaker-Levy 9 has affected scientific and popular understanding of impacts in the solar system.

The Modern Role of Visual Observations of Comets

This paper reviews the role of past and future visual observations in cometary research. The strengths and limitations of visual observations are explored for the benefit of both investigators who might have to use them and observers who wish to make real contributions to the field. We consider the characteristics of the eye-brain combination as a detector and compare them with those of modern detectors. We specifically evaluate visual discoveries, magnitude estimates, and drawings.