International Comet Quarterly

The 1892/3 and 2007 Outbursts of Comet 17P/Holmes

Comet 17P/Holmes was first discovered by Edwin Holmes in London in early Nov. 1892 while undergoing a massive outburst when it was passing near the naked-eye nearby galaxy M31 in our skies; it then rivaled M31 itself in brightness and was near total visual magnitude 4. The famous comet observer Edward Barnard photographed comet 17P with the wide-field 6-inch Willard portrait lens at Lick Observatory, and published the plates in Publications of the Lick Obs. 11 (1913) as plates 102-105. James McGaha of Tucson, AZ, has kindly sent us his 600-dpi digital scans of these plates and they are available here:

  • Plate 102: 3-hr exposure made on 1892 Nov. 10; coma diameter measured by Barnard as 8', with an outer coma of diameter 19'. [1.5-MB jpeg file]
  • Plate 103: 3-hr exposure made on 1892 Nov. 10 (enlargement of Plate 102), showing the faint, distorted gas tail. [1.7-MB jpeg file]
  • Plate 104: 1.25-hr exposure made on 1892 Nov. 21; coma diameter measured by Barnard as 19'. [1.2-MB jpeg file]
  • Plate 105: 1.25-hr exposure made on 1892 Dec. 8; coma diameter measured by Barnard as 25'. [653-kB jpeg file]

Comet 17P gradually faded below naked-eye visibility in late 1892, and by early Jan. 1893, it had faded to mag 9-10 before having another outburst to faint naked-eye visibility again (mag perhaps 5). The comet then had an orbital period of about 6.9 years, and it was observed following predictions at its return to perihelion in 1899 and 1906 (its perihelion distance is around 2.1-2.2 AU, and at aphelion the comet is at about Jupiter's distance from the sun, though away from the ecliptic due to the comet's 19-deg orbital inclination with respect to the orbits of the major planets).

However, the comet then went lost for several revolutions, until Brian Marsden re-analyzed positional data on comet 17P with early electronic computer programs in the early 1960s and published search ephemerides to guide observers. As a result of Marsden's efforts, Elizabeth Roemer recovered comet 17P in July 1964, and the comet has been observed at every return to perihelion (every seven years or so) since then.

The comet was recovered earlier in 2007 and was predicted to be around mag 16-17 through most of the year -- a brightness prediction that held true until 2007 Oct. 24, when it burst from mag 15 or 16 to mag 3 within about 48 hours. The outburst is likely to have been produced by a build-up of sublimated volatile ices just beneath the comet's surface, whereby the crust that was holding these high-pressure gases suddenly gave way, leading to an explosive eruption that contained a very large amount of dust and a smaller amount of visible gaseous material. Indeed, the 2007 outburst made the comet look similar in appearance to its appearance during its 1892 outburst (see the Barnard images above), as one can see from these images taken during the 2007 outburst:

Observers would be wise to continue following the comet, in case it experiences a second significant outburst, as it did in 1893. Other useful current information on comet 17P/Holmes:

[NOTE on nomenclature: The proper designation for this comet is "17P/Holmes". In everyday conversation, people will naturally casually just call it "comet Holmes", or sometimes even just "Holmes" without the word "comet", but in written remarks and formal talks, it is always best to use the designation whenever a comet is referred to, for clarity. The designation of this comet is "17P", so that it is best in writing to refer to this as either "comet 17P/Holmes" or abbreviated to just "comet 17P". Details on the nomenclature system for comets is given elsewhere at this website.]

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