C/1979 Y1 (Bradfield)
Parent body of the July Pegasids meteor shower.
C/1979 Y1 (Bradfield), also designated Comet 1979X and 1979l, is a long-period comet. Amateur astronomer William A. Bradfield spotted it on 24 December 1979 using a 150mm f5.5 refractor from Dernancourt, South Australia. It was his tenth comet discovery, coming after 67 hours of searching following his previous find. The comet passed perihelion three days before its discovery, on 21 December 1979, and had an orbital period of 308 ± 6 years. Its next perihelion is expected around 2287. At discovery, its apparent magnitude was estimated at 5, and Bradfield noted a tail over a degree long. The comet then lay in Scorpius, moving southward.
In January 1980, the comet stayed near magnitude 5 as it approached Earth. On 8 January, observers using 15×80 binoculars reported a tail three degrees long; by 17 January, it had shortened to two degrees. The comet moved south until 23 January, reaching a declination of -80°, then swung rapidly north. 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, likely due to a solar wind disturbance. As the comet receded from Earth in February, it faded and its tail shrank, measuring 5 arcminutes on 18 February. The last detection came on 17 March 1980, at an apparent magnitude of 11.7.
The comet’s position made it a good target for the International Ultraviolet Explorer (IUE). It was the third comet observed from space in ultraviolet light, after C/1975 V1 (West) and C/1978 T1 (Seargent). IUE observations ran from 10 January 1980 (heliocentric distance 0.71 AU) to 3 March 1980 (1.55 AU). The hydrogen Lyman-alpha line dominated the spectrum, appearing overexposed. Other detected lines included OI λ1302, CI λλ1561, 1637, and 1931, and SI λ1813; the CII complex and CO were just above the noise level. 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, but the CO+ (0,0) and (1,0) lines were very weak or absent. CS and SI lines were concentrated near the nucleus. The OH and hydrogen lines stem from water photodissociation by solar radiation, with other molecules possibly contributing to hydrogen but not significantly. OH further breaks down into hydrogen and oxygen.
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. The comet was discovered three days after perihelion, with an estimated apparent magnitude of 5 and a tail over a degree long, located in the constellation 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 appeared to undulate, 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 (Bradfield) holds significance 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 morning in the southern hemisphere, as it has a lower minimum orbit intersection distance with Mars (0.0497 AU) than with Earth. Scientifically, it was the third comet observed in ultraviolet from space, revealing prominent Lyman-alpha hydrogen lines and other species, and its 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. The absence of some CO+ lines was surprising, while CII Mulliken bands were unambiguously present for the first time in a comet.
Frequently Asked Questions
Who discovered C/1979 Y1 (Bradfield) and under what circumstances?
Australian amateur astronomer William A. Bradfield found it on 24 December 1979 with a 150mm f5.5 refractor while observing from Dernancourt, South Australia. It was his tenth comet discovery, arriving just 67 hours after he had identified his previous one.
What is C/1979 Y1 (Bradfield)'s orbital period?
This long-period comet completes one full orbit around the Sun roughly every 308 years, with an uncertainty of about ±6 years. Its last perihelion passage occurred on 21 December 1979, just three days before Bradfield spotted it.
When will C/1979 Y1 (Bradfield) next return to the inner solar system?
Based on its approximately 308-year orbital period, the comet is expected to swing back through perihelion around the year 2287. That means no one alive today will likely witness its return under normal circumstances.
What meteor shower is linked to C/1979 Y1 (Bradfield)?
The July Pegasids meteor shower is believed to originate from debris shed by this comet as it passes through the inner solar system. Earth crosses the trail of those particles each July, producing the brief shower.
How bright was C/1979 Y1 (Bradfield) at discovery?
Bradfield estimated its apparent magnitude at around 5, making it visible to the unaided eye under dark skies. He also recorded a tail stretching over one degree in length at the time of observation.
More in Named Comets, Part 3 1-24
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