C/1979 Y1 (Bradfield)
Long-period comet, parent of July Pegasids meteor shower.
C/1979 Y1 (Bradfield), also designated Comet 1979X and 1979l, is a long-period comet discovered by amateur astronomer William A. Bradfield on 24 December 1979. Its orbital period is 308 ± 6 years, and it last reached perihelion on 21 December 1979. The comet is considered the parent body of the July Pegasids meteor shower and is expected to return to perihelion around 2287.
Bradfield spotted the comet using a 150mm f5.5 refractor at Dernancourt, South Australia. It was his tenth comet discovery, following 67 hours of searching after his previous find. The comet was found three days after perihelion, with an estimated apparent magnitude of 5 and a tail over a degree long. At the time, it lay in the constellation Scorpius and was moving southward.
In January 1980, the comet stayed near magnitude 5 as it approached Earth. On 8 January, its tail stretched three degrees long as seen through 15×80 binoculars; by 17 January, it had shortened to two degrees. The comet moved south until 23 January, reaching a declination of -80°, then swung rapidly northward. It became visible from the northern hemisphere on 26 January, when it was 0.2 AU from Earth. On 6 February, its ion tail undulated, shifting its axis by about 10 degrees in 27 minutes due to a solar wind disturbance. As the comet receded from Earth in February, it faded and its tail shrank to 5 arcminutes by 18 February. The comet was last detected on 17 March 1980, at an apparent magnitude of 11.7.
The comet’s favorable position allowed observation by the International Ultraviolet Explorer, making it the third comet—after C/1975 V1 (West) and C/1978 T1 (Seargent)—to have its ultraviolet spectrum obtained from space. Observations ran from 10 January 1980 (heliocentric distance 0.71 AU) to 3 March 1980 (1.55 AU). The most prominent feature was the overexposed Lyman-alpha line of hydrogen. Other detected lines included OI λ1302, CI λλ1561, 1637, and 1931, and SI λ1813, with the CII complex and CO just above noise. The ultraviolet spectrum closely resembled that of comet West, despite differences in gas-to-dust ratio. At longer wavelengths, CO+, CO₂+, CS, and OH bands appeared, though CO+ (0,0) and (1,0) lines were very weak or absent. CS and SI lines concentrated near the nucleus.
Quick Facts
- Discovery Ref
- IAUC_3437
- Discoverer
- William A. Bradfield
- Discovery Site
- Dernancourt, South Australia
- Discovery Date
- 24 December 1979
- Designations
- 1979 X, 1979lICQ1
- Orbit Ref
- jpldata · Horizons2287
- Epoch
- 25 January 1980 (JD 2444263.5)
- Observation Arc
- 75 days
- Obs
- 127
- Aphelion
- ~90 AU
- Perihelion
- 0.5453 AU
- Semimajor
- 46 AU
Facts from the source article.
Lore & Background
The comet was discovered by amateur astronomer William A. Bradfield on 24 December 1979 using a 150mm f5.5 refractor at Dernancourt, South Australia. It was his 10th comet discovery, found after 67 hours of searching following his previous comet. Discovered three days after perihelion, it had an apparent magnitude of 5 and a tail over a degree long, located in Scorpius and moving southwards. In January 1980 the comet remained around magnitude 5 as it approached Earth; its tail was reported to be three degrees long on 8 January and two degrees long by 17 January. It moved southwards until 23 January, reaching a declination of -80°, then rapidly moved northwards, becoming visible from the northern hemisphere on 26 January when it was 0.2 AU from Earth. On 6 February the ion tail undulated, shifting its axis by about 10 degrees in 27 minutes due to a solar wind disturbance. The comet faded in February, its tail shortening to 5 arcminutes on 18 February, and was last detected on 17 March 1980 at magnitude 11.7.
Reader's Guide
C/1979 Y1 is significant as the parent body of the July Pegasids meteor shower, though comet C/1771 A1 has also been suggested as a possible parent, and both may have originated from a single progenitor comet. Modeling of the comet's meteor streams indicates it may also produce a daylight meteor shower with a radiant near alpha Microscopii, and possibly the γ-Bootids shower, though the latter's existence is questionable as it was identified by only two meteors. The streams causing the Pegasids and daylight showers are dragged away from Earth's orbit by the Poynting–Robertson effect, while the γ-Bootids are dragged into it. The comet could also produce a meteor shower on Mars, visible in the southern hemisphere morning. Its ultraviolet observations by the International Ultraviolet Explorer from 10 January to 3 March 1980 revealed prominent Lyman-alpha hydrogen lines, oxygen, carbon, sulfur lines, and the first unambiguous detection of CII Mulliken bands in a comet. The water production rate at 0.71 AU was estimated at 1–2.4 × 10^29 mol/s, decreasing at a rate of r^−3.7. Searches for X-ray emission, millimeter-wavelength HCN, CS, and CH3OH, and radio emissions from hydroxyl, formaldehyde, water, and ammonia were unsuccessful.
Did You Know?
- The comet was discovered three days after perihelion on 24 December 1979 by amateur astronomer William A. Bradfield.
- It was the third comet whose ultraviolet spectrum was obtained from space, after comets West and Seargent.
- The comet's ion tail undulated by about 10 degrees in 27 minutes on 6 February 1980 due to a solar wind disturbance.
- It has a lower minimum orbit intersection distance with Mars (0.0497 AU) than with Earth.
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