C/1989 X1 (Austin)

Lore & Background

Rodney Austin discovered the comet on December 6, 1989, through an 8-inch Schmidt–Newtonian telescope; it was his third comet discovery after C/1982 M1 and C/1984 N1. At discovery, the comet was more than 350 million kilometres from the Sun yet shone at 11th magnitude, indicating a large object. Its orbit showed it would pass perihelion on April 9, 1990, inside Mercury's orbit, and come within 38 million kilometres of Earth on May 25. The high eccentricity above 1 indicated it was a dynamically new object from the Oort cloud. The comet was at 83 degrees elongation from the Sun at discovery and nearly circumpolar for the southern hemisphere. It moved northwards, crossing the equator on March 21, 1990, but its elongation around 25 degrees made it difficult to observe. After perihelion, it moved away from the Sun and at closest approach to Earth was 100 degrees away in the constellation Aquila, moving southward. A long ion tail was observed in late February 1990, but the brightening rate slowed when the comet was 1.56 to 1.27 AU from the Sun. Although activity increased again in April, the comet ended up about 6 magnitudes dimmer than expected, with most observations reporting a maximum brightness between +4 and +6. It was visible with the naked eye between May 5 and May 25, 1990, before fading.

Reader's Guide

C/1989 X1 (Austin) is significant in amateur astronomy as a cautionary example of a comet that was widely predicted to become a brilliant naked-eye object but failed to do so, peaking at magnitude +4 to +6 instead. Its discovery by an amateur astronomer using modest equipment underscores the ongoing contributions of amateurs to comet hunting. The comet's spectrum revealed unusual features: it showed hydroxyl emission and continuum from dust, but dust was unusually low; it also contained gases like cyanide, CO2+, and diatomic carbon, and the millimetre spectrum detected hydrogen cyanide, formaldehyde, hydrogen sulphide, and methanol—the first time the latter two were observed in a comet's spectrum. Infrared observations by the COBE satellite on May 6, 1990, showed a dust tail 6 degrees long with complex structure, and the nucleus was estimated to be covered mainly by 20-micrometer particles. Polarimetric observations revealed jets and shells, and dust particles had higher albedo than usual. On May 13, 1990, the comet's tail passed in front of the quasar 3C 441, enhancing scintillations in radio waves. The comet's legacy lies in its rich scientific data despite its visual disappointment, and it remains a reference for studies of dynamically new comets from the Oort cloud.

Did You Know?

• The comet was discovered by New Zealand amateur Rodney R. D. Austin on December 6, 1989, using an 8-inch Schmidt–Newtonian telescope.
• It was the first comet in which hydrogen sulphide and methanol were detected in the millimetre spectrum.