20 February 2021
Perihelion Distance (q)
Aphelion Distance (Q)
Click for NASA orbit diagram
Very faint but monitor - brightened to 6th magnitude in 2007
Hills Observatory: 1 January 2013 to 26 May 2020
Comet Holmes was discovered by Edwin Holmes on November 6, 1892, while he was conducting
regular observations of the Andromeda Galaxy (M31). Its discovery in 1892 was possible
because of an increase in its magnitude similar to the 2007 outburst; it brightened to an
approximate magnitude of 4 or 5 before fading from visibility over a period of several weeks.
The comet's discovery was confirmed by Edward Walter Maunder (Royal Observatory, Greenwich,
England), William Henry Maw (Kensington, London, England), and B. Kidd (Bramley, Surrey,
England). Independent discoveries were made by Thomas David Anderson (Edinburgh, Scotland)
on November 8 and by Mike Brown (Wilkes, USA) and by John Ewen Davidson (Mackay,
Queensland, Australia) on November 9.
The first calculations of the elliptical orbit of 17P/Holmes were done independently by Heinrich
Kreutz and George Mary Searle. Additional orbits eventually established the perihelion date as
June 13 and the orbital period as 6.9 years. These calculations proved that the comet was not a
return of Biela's Comet.
The 1899 and 1906 appearances were observed, but the comet was lost after 1906 until it was
recovered on July 16, 1964, by Elizabeth Roemer (US Naval Observatory Flagstaff Station,
Arizona, USA). Aided by the computer predictions of Brian G. Marsden, the comet has been
observed on every subsequent return.
During its 2007 return, Holmes unexpectedly brightened from a magnitude of about 17 to about
2.8 in a period of only 42 hours, making it visible to the naked eye. This represents a change of
brightness by a factor of about half a million and is the largest known outburst by a comet. The
outburst took place from October 23 to 24, 2007. The first person reportedly to notice a change
was J. A. Henríquez Santana on Tenerife in the Canary Islands; minutes later, Ramón Naves in
Barcelona noticed the comet at magnitude 7.3. It became easily visible to the naked eye as a
bright yellow "star" in Perseus and by October 25 17P/Holmes appeared as the third brightest
"star" in that constellation.
Although large telescopes had already shown fine-scale cometary details, naked-eye observers
saw Holmes as merely star-like until October 26. After that date, 17P/Holmes began to appear
more comet-like to naked-eye observers. This is because during the comet's outburst, its orbit
took it to near opposition with respect to Earth, and because comet tails point away from the
Sun, Earth observers were looking nearly straight down along the tail of 17/P Holmes, making
the comet appear as a bright sphere.
Based on orbital computations and luminosity before the 2007 outburst, the comet's nucleus
was estimated at 3.4 km.
Comet Holmes not only became brighter, but it also swelled in size as its coma expanded. In
late October 2007 the coma's apparent diameter increased from 3.3 arcminutes to over 13
arcminutes, about half the diameter of the Moon in the sky. At a distance of around 2 AU, this
means that the true diameter of the coma had swelled to over 1 million km, or about 70% of
the diameter of the Sun. By comparison, the Moon is 380,000 km from Earth. Therefore, during
the 2007 outburst of Comet Holmes the coma was a sphere wider than the diameter of the
Moon's orbit around Earth. On 2007 November, the coma had dispersed to a volume larger than
the Sun, briefly giving it the largest extended atmosphere in the Solar System.
The cause of the outburst is not definitely known. The huge cloud of gas and dust may have
resulted from a collision with a meteoroid, or, more probably, from a build-up of gas inside the
comet's nucleus that eventually broke through the surface. However, researchers at the Max
Planck Institute suggest in a paper published in Astronomy and Astrophysics that the brightening
can be explained by a thick, air-tight dust cover and the effects of H2O sublimation, with the
comet's porous structure providing more surface area for sublimation, up to one order of
magnitude greater. Energy from the Sun (insolation) was stored in the dust cover and the
nucleus within the months before the outburst.
The comet remained visible in February 2008 though it had become a challenging target at
about magnitude +5 in the constellation Perseus. It had expanded to greater than 2 degrees
of arc as seen from Earth, and thus had very little surface brightness.
18 Sep 13
18 Nov 13