Comet Bennett

Lore & Background

The comet was discovered by John Caister Bennett on 28 December 1969 from Pretoria, South Africa, during his comet seeking routine. It was located in the constellation Tucana, at 65° south declination, with an estimated magnitude of 8.5. At that time the comet was about 1.7 AU from both the Sun and Earth. The orbit was computed by M. P. Candy of the Perth Observatory, and it became apparent that the comet could become a bright object by the end of March, during its perihelion at a distance of 0.54 AU (81 million km), as it moved northwards. The comet became visible to the naked eye in February, and by the first week of that month it had a magnitude of 5 and its tail measured about one degree in length. By the end of February the comet had brightened to magnitude 3.5 while its tail was about two degrees long. The comet continued to brighten during March, and by the middle of the month it was a first magnitude object with a prominent curved tail about 10 degrees long. The comet reached perihelion on 20 March and crossed the equator on 25 March, becoming better visible in the morning sky of the northern hemisphere. On 26 March was the perigee of the comet, when it approached Earth at a distance of 0.69 AU (103 million km). The comet was then at the square of the Pegasus and continued moving northwards until it reached its maximum north declination of 83° in August, when it was in the constellation of Camelopardalis. The comet at the start of April had a magnitude of 1, but as it receded both from the Sun and Earth, it had dimmed to magnitude 3-4 by the end of April, when it was circumpolar, located in Cassiopeia. The comet had in April two tails, with the longest being 20-25 degrees long. Although by the start of May the comet head had faded to magnitude 5, its tail was still 10-15 degrees long, but by the end of the month it was only 2.5 degrees long. It was last seen by naked eye around mid May. The comet was observed to fade during summer, autumn and winter. By the start of July it was around magnitude 10 and by September it was magnitude 12. In January 1971 it was photographed as an 18.9 magnitude object. It was last photographed by Elizabeth Roemer on 27 February 1971, when the comet was 4.9 AU (730 million km) from the Sun and 5.3 AU (790 million km) from Earth. In 1979, Ichiro Hasegawa tentatively identified a comet observed in September 363 AD as a previous apparition of C/1969 Y1 (Bennett).

Reader's Guide

Comet Bennett was notable for being the most photographed and most thoroughly researched comet at the time of its appearance, due to its favorable characteristics: a short perihelion distance, a short distance from Earth, and high intrinsic brightness. Numerous research projects were initiated, including the first ultraviolet observations of a comet from space. In February 1970, the comet was systematically observed with the Orbiting Astronomical Observatory 2 (OAO-2) and the Orbiting Geophysical Observatory (OGO-5) from mid-March to mid-April. From OAO-2 photometric data, the production rates of OH and H were derived, confirming that gas production at small solar distances is determined by evaporation of water from the nucleus; the total water loss during its passage was estimated at 200 million tonnes. OGO-5 detected hydrogen emissions up to several million km from the nucleus, yielding a hydrogen mass of about 2 million tonnes, and produced twelve intensity maps of Lyman-α emission by April 30. In visible light, images were taken at Goddard Space Flight Center, Hamburg Observatory, University of Western Ontario, University of Toledo, Cerro Tololo Inter-American Observatory, and Osservatorio Astrofisico di Asiago, evaluating emission lines of CN, C2, CO+, Na, H2O+, and oxygen. A brightness profile of the 'forbidden' oxygen line at 630 nm suggested that comet Bennett contained CO2 in excess of water. On 3/4 April, the comet's gas tail was observed to have been torn off the coma, and a prominent kink in the gas tail on April 4 was correlated with simultaneous measurements of solar activity and solar wind from OGO-5, Vela 5, and HEOS-1.

Did You Know?

• Comet Bennett was discovered by John Caister Bennett on December 28, 1969, from Pretoria, South Africa.
• It was the first comet to be systematically observed in the ultraviolet from space, using OAO-2 and OGO-5.
• The comet's total water loss during its passage through the inner solar system was estimated at 200 million tonnes.