Mosaic of HST Comet Images

The above figure is a mosaic showing images of all the comets observed with HST since the repair of its optics in December 1993, EXCEPT that I left out comet Shoemaker-Levy 9. (The latter consisted of over 20 "comets'', all of which plunged into Jupiter's atmosphere in July 1994.)

( Click here to see the postscript version of the above mosaic. The GIF version is shown above.)

All of the comets along the top row were imaged as part of HST programs led by Philippe Lamy of the Space Astronomy Laboratory in Marseilles, France. All of the comets along the bottom row were imaged as part of my HST programs, except that Schwassmann-Wachmann 1 (SW1) was imaged during a program led by Paul Feldman of Johns Hopkins University. The image of comet Encke is the first comet image ever obtained by the Space Telescope Imaging Spectrograph (STIS), which is a "second generation'' HST instrument installed during the servicing mission in February 1997. All of the other images were taken by the Wide Field Planetary Camera 2 (WFPC2), which was installed during the HST servicing mission in December 1993.

This portrait shows the diversity in morphology of the inner comae of these comets. However, the really exciting aspect of these images is that we can use them to estimate the sizes of the cometary nuclei that are embedded within the comae. (The "nucleus'' is the single, solid body at the center of the comet. As the comet passes close to the Sun, ices in the nucleus are heated up and sublime from the surface. The molecules in these ices then escape into space, dragging with them fine dust. Most of the light that you see from a comet is sunlight reflected from these dust grains.) Only an instrument with very high spatial resolution, like HST, can hope to pick out the signature of the nucleus when it is surrounded by the coma. The HST's spatial resolution was particularly good during the observations of comets Honda-Mrkos-Pajdusokova (H-M-P), Hyakutake, and Encke, all of which passed within 0.2 AU of the Earth ("AU'' stands for "Astronomical Unit'' and is the average distance between the Sun and the Earth, which is 93 million miles or 150 million kilometers) . Comets Hale-Bopp and SW1 could only be observed beyond the orbit of Jupiter, so the spatial resolution is not as good for those cases. For all the images, the scale bars indicate sizes at the comet's distance. (Remember, we're viewing the comets in projection against the sky background. For a comet that is 1 AU from us, a 1 arcsec angular field of view [1 arcsec = 1/3600th of a degree] subtends a projected linear distance at the comet of 725 km = 450 miles.) Each pixel in the WFPC2 has an angular field of view of 0.0455 arcsec, while each pixel in the STIS has an angular field of view of 0.050 arcsec. Typically, the resolution of most ground-based images is about 1 arcsec, or even larger. Thus, HST improves our spatial resolution by more than a factor of 10.

Analyses of the above images provide the following estimates for the effective diameters of the nuclei:

Borrelly --- 7.8 km

H-M-P --- 0.44-0.68 km

Kopff --- 3.6 km

Wirtanen --- 1.2 km

Hyakutake --- smaller than 15 km

Hale-Bopp --- 30-40 km

SW1 --- 30-40 km

Encke --- 2-4 km

( Click here to see a table of information on the sizes, shapes and active areas of the above comets in postscript format.)

Hale-Bopp and SW1 are just about the largest comets known. Only the unusual cometary object Chiron, whose orbit is well outside of Jupiter's, is bigger.

At the other end of the scale, we see that there are also some very TINY comets, whose diameters are about 1 km, or even less. These HST images are providing important information on the size distribution of cometary nuclei, data that have not been previously accessible. Many more images need to be acquired before we can make definitive statements about the sizes of cometary nuclei as a population, but these new HST results indicate that at least some nuclei are significantly smaller than originally imagined.