23P/Brorsen–Metcalf

Lore & Background

The comet was first discovered by Theodor Brorsen at the Altona Observatory on 20 July 1847, and independently by Kaspar Schweizer in Moscow on 11 August 1847. Predictions indicated it would reappear between 1919 and 1922. On 21 August 1919, Joel Hastings Metcalf recovered the comet at Camp Idlewild, Vermont, as 8th magnitude. Additional recoveries were made by Edward Barnard, Michel Giacobini, Ostrovlev, and Selavanov. By the end of September 1919 it was confirmed as Brorsen's comet. It became visible to the naked eye as a small hazy spot, reaching magnitude 4.5 on 6 October 1919, with a slender tail 8.5 degrees long. Photographs on 22 October 1919 showed a tail disconnection event that probably started on 20 October.

The comet was recovered by the Palomar Observatory on 4 July 1989 at magnitude 15, while Alan Hale estimated magnitude 11.5 on 7 July. It brightened rapidly, reaching magnitude 7–7.5 by the end of July with a short tail. Perigee occurred on 6 August at 0.62 AU, and perihelion on 11 September. Between those dates the comet was magnitude 5–6 and visible to the naked eye. In September the tail grew to about 7 degrees. The comet faded in the second half of September as solar elongation diminished. During this apparition, it became the first comet definitely detected in submillimeter wavelengths. Spectroscopic and infrared photometric observations from the NASA Infrared Telescope Facility on Mauna Kea between 28 August and 6 September 1989 set an upper limit on the nucleus radius of 5.0 km.

Reader's Guide

23P/Brorsen–Metcalf is significant as a Halley-type comet with a 70-year orbital period, linking 19th-century visual discoveries with modern multi-wavelength astronomy. Its 1919 apparition was notable for a tail disconnection event captured photographically, an early observation of such a phenomenon. The 1989 apparition marked a milestone as the first comet definitely detected in submillimeter wavelengths, opening a new observational window for studying cometary dust and gas. The upper limit on its nucleus radius (5.0 km) derived from infrared observations provided constraints on its physical size. The comet's independent discoveries by Brorsen and Schweizer in 1847, and its recovery by multiple astronomers in 1919, illustrate the collaborative nature of cometary astronomy. Its visibility to the naked eye during both the 1919 and 1989 apparitions made it accessible to a wide range of observers. The comet's legacy lies in bridging classical cometography with modern astrophysical techniques, particularly submillimeter astronomy.

Did You Know?

• The comet was independently discovered by Theodor Brorsen and Kaspar Schweizer in 1847.
• In 1919, a tail disconnection event was photographed on 22 October, likely starting on 20 October.
• It was the first comet definitely detected in submillimeter wavelengths, during its 1989 apparition.