Question

Comet Bennett (1969 Y1) came to perihelion (closest approach to the Sun) on April 4, 1970. The

period of this comet is 1678 years and its orbital eccentricity is 0.9962. Calculate, in AU, the semi-major

axis, perihelion distance, and aphelion distance (farthest recession from the Sun) of the orbit of this

comet.

** All work must be shown to recieve full points!

Answer 1

Historical Highlights

M. P. Candy (Perth Observatory) computed the first orbit for this comet. Using more than three accurate positions obtained over a 4-day arc, he computed a parabolic orbit which was first published on January 6, 1970. The perihelion date was determined as 1970 March 20.30 and the perihelion distance was 0.542 AU. These figures changed only slightly by the time observations were completed. Ultimately the orbit was found to be elliptical with an orbital period of about 1700 years.

During January of 1970 the comet steadily brightened as it approached the sun and Earth. Magnitude estimates at the beginning of the month were generally around 9, while estimates at the end of the month were slightly brighter than 7. The tail continued to be absent until nearly the end of the month, when observatories began photographing a tail about one degree long. Bennett himself reported a possible tail seen visually through his telescope on the 31st.

The comet brightened from about magnitude 6 to about 3 as February progressed. Further brightening occurred during March as the comet continued to approach both the sun and Earth. The tail became rather striking by about mid-March as it was curved and contained filaments. The length was generally estimated as 10 to 12 degrees. Meanwhile, close scrutiny of the coma revealed short faint jets extending from the sunward side of the nucleus.

The comet passed closest to Earth on 1970 March 26 (0.69 AU). At that time the average brightness was around 0, while estimates of the tail length ranged from 5 to 10 degrees.

The comet was widely observed in April as its morning sky position improved. The fact that the comet was moving away from the sun and Earth, and was therefore fading, did not prevent it from being mentioned in newspapers and on television news broadcasts. The comet began the month near magnitude 1 and faded to about 5 by month's end. Although the tail was generally estimated as about 10 or 12 degrees, a few estimates went as long as 25 degrees around mid-April. The tail continued to show a curved appearance and contained numerous streamers that were most prominent within about 5 degrees of the coma.

The comet was no longer a naked-eye object after the first few days of May. It began this month slightly brighter than mangitude 6 and finished the month fainter than magnitude 9. Although the tail streamers continued to be reported throughout the month, the tail length was widely reported as around 2.5 degrees as May came to a close.

The comet faded throughout the fall and into winter. The tail was no longer reported as August progressed and the brightness finally dropped below 13 during late September. During January 1971 the comet was photographed as a well-condensed object of magnitude 18.9. It was last detected on February 27, when Elizabeth Roemer photographed it at the Lunar and Planetary Laboratory. The comet was then 4.9 AU from the sun and 5.3 AU from Earth.