During a routine comet-sweeping session, the Reverend Leo Boethin (Abra, The Philippines) discovered this comet on 1975 January 4.52. He determined the magnitude as 12.3. Boethin obtained additional observations on January 5.48, January 7.48, and January 8.50 (estimating the magnitude as 12.0 on each date), before he sent a letter to the Central Bureau for Astronomical Telegrams announcing his discovery. While the letter was in transit, Boethin continued observing the comet during the next week. The letter finally reached the Central Bureau on January 17, at which time bright moonlight made confirmation impossible. Boethin next observed the comet on January 29.47, and on February 1.51, and having heard no news of his discovery he sent a cablegram to the Central Bureau which gave details of his February 1st observation. The cablegram arrived on February 3, and B. G. Marsden computed a tentative ephemeris and sent it to a handful of observers. The first person to report a confirmation was C. Scovil, who obtained a suspected visual observation on February 4.04, with the 56-cm Maksutov telescope at the Stamford Museum, but no convincing photographic confirmation was acquired. The first definite confirmation came on February 5.40, when Takeshi Urata (Nihondaira Observatory) estimated the magnitude as 10 (15-cm f/6 reflector). Additional confirmations were reported during the next few days.
Even though Brian G. Marsden (Central Bureau for Astronomical Telegrams) had computed a rough orbit from Boethin's rough positions during January to aid in searches, the first published orbit came from Marsden on February 12. Even though this was a parabolic orbit, Marsden noted, "It is not impossible that the comet is of short period." On February 25 Marsden published both a parabolic and an elliptical orbit. Both indicated the comet probably passed perihelion in early January and the elliptical orbit indicated an orbital period of 7.3 years. Marsden commented that the elliptical orbit was "in closer agreement with the rough January positions." The problem Marsden faced was that no precise positions were available for January, so there was still some uncertainty as to the comet's true motion. By early March enough observations had been acculmulated to enable Marsden to compute a more precise orbit. On March 13 he published an elliptical orbit with a period of 12.41 years. Ultimately, the period proved to be 11.2 years.
Although there were brightness estimates of magnitude 10 during the 1975 apparition, precise brightness determinations by experienced observers indicated the comet was near magnitude 11 during the first half of February and faded thereafter. The comet was closest to the sun on January 5 (1.09 AU) and was closest to Earth on January 17 (1.13 AU).
Apparition of 1986: The comet next arrived at perihelion on 1986 January 16 and passed closest to Earth on January 25 (1.32 AU). Even though the comet was only expected to reach magnitude 12.4, it surprised observers by brightening faster than expected following its recovery by A. C. Gilmore and P. M. Kilmartin (Mount John University Observatory) on 1985 October 11. (The positions indicated the prediction had been in error by -3.5 days.) The comet was already brighter than 11 as November began and then surpassed magnitude 10 by the end of that month. The comet was at its brightest during 1986 January, when observers found it slightly brighter than magnitude 8. The comet held this brightness throughout January and into February before fading set in. By the end of February it was only slightly brighter than magnitude 10. The comet was last seen on March 1.
Although the comet was next expected at perihelion on 1997 April 17, no observations were reported.
On 2006 October 30, NASA announced that it had approved a proposal by the University of Maryland to send the Deep Impact spacecraft to this comet. Deep Impact sent an impactor into period comet Tempel 1 during 2005 July, but the main craft, which carried a wide variety of instruments, continues to orbit the sun. The plan was to redirect the spacecraft to fly passed periodic comet 85P/Boethin during 2008 December, but early attempts to recovery the comet failed and the spacecraft was direct toward a different comet. 85P/Boethin was supposed to become fairly bright during 2008 December and 2009 January, but extensive searches by amateur and professional astronomers failed to locate it. In lieu of these extensive searches, it is believed that the comet may have broken up.
Close approaches to planets: The comet experienced two close approaches to Jupiter during the 20th century. It makes two close approaches to Earth and two close approaches to Jupiter during the first half of the 21st century. (From the orbital work of Kazuo Kinoshita)
- 0.67 AU from Jupiter on 1921 October 17
- decreased perihelion distance from 1.18 AU to 1.12 AU
- decreased orbital period from 11.74 to 11.24 years
- 0.63 AU from Jupiter on 1995 August 18
- increased perihelion distance from 1.11 AU to 1.16 AU
- increased orbital period from 11.23 to 11.63 years
- 0.44 AU from Jupiter on 2007 June 2
- increased perihelion distance from 1.16 AU to 1.15 AU
- increased orbital period from 11.63 to 11.54 years
- 0.87 AU from Earth on 2008 December 23
- 0.80 AU from Jupiter on 2019 February 25
- increased perihelion distance from 1.15 AU to 1.13 AU
- increased orbital period from 11.54 to 11.30 years
- 0.54 AU from Earth on 2031 December 12
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