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17P/Holmes
	Comet 17P/Holmes and its blue ion tail; (taken on November 4, 2007)
Discovery
Discovery date	November 6, 1892
Designations
Alternative designations	1892 V1; 1892 III;; 1892f; 1899 L1;; 1899 II; 1899d;; 1906 III; 1906f;; 1964 O1; 1964 X;; 1964i; 1972 I;; 1971b; 1979 IV;; 1979f; 1986 V;; 1986f; 1993 VII;; 1993i
Orbital characteristics
Epoch	October 27, 2007 (JD 2454400.5)
Aphelion	5.183610 AU
Perihelion	2.053218 AU
Semi-major axis	3.618414 AU
Eccentricity	0.432564
Orbital period	6.882994 a
Inclination	19.1126°
Last perihelion	February 19, 2021; March 27, 2014; May 4, 2007
Next perihelion	January 31, 2028

Comet Holmes /ˈhoʊmz/ (official designation: 17P/Holmes) is a periodic comet in the Solar System, discovered by the British amateur astronomer Edwin Holmes on November 6, 1892. Although normally a very faint object, Holmes became notable during its October 2007 return when it temporarily brightened by a factor of a million, in what was the largest known outburst by a comet, and became visible to the naked eye.^[5] It also briefly became the largest object in the Solar System, as its coma (the thin dissipating dust ball around the comet) expanded to a diameter greater than that of the Sun (although its mass remained minuscule).^[6] Between 1857–2106 perihelion remains between 2.05–2.36 AU.^[7]

Discovery

10 November 1892, near the Andromeda Galaxy

Comet Holmes was discovered by Edwin Holmes on November 6, 1892, while he was conducting regular observations of the Andromeda Galaxy (M31).^[8]^[9] 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.^[10]

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).^[8]^[9] 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.^[11]

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 (see Lost comet) after 1906 until it was recovered on July 16, 1964, by Elizabeth Roemer (US Naval Observatory Flagstaff Station, Arizona, United States). Aided by the computer predictions of Brian G. Marsden, the comet has been observed on every subsequent return.

2007 outburst

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 a million and is the largest known outburst by a comet thus far.^[5] The outburst took place from October 23 to 24, 2007.^[5]^[12]^[13] 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.^[13] It became easily visible to the naked eye as a bright yellow "star" in Perseus,^[14]^[15] and by October 25 17P/Holmes appeared as the third-brightest "star" in that constellation.^[13]

Although large telescopes had already shown fine-scale cometary details, naked-eye observers saw Holmes as merely star-like until October 26.^[14] After that date, 17P/Holmes began to appear more comet-like to naked-eye observers.^[14] 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 17P/Holmes, making the comet appear as a bright sphere.

Holmes's nucleus is estimated at 3.4 km.^[16]

Comet Holmes not only became brighter, but its coma (nebulous envelope around the nucleus) expanded. In late October 2007 the coma's apparent diameter increased from 3.3 arcminutes to over 13 arcminutes,^[17] 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,^[18] 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. In November 2007, the coma had dispersed to a volume larger than the Sun, briefly giving it the largest extended atmosphere in the Solar System.^[6]^[19]

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.^[20] 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.^[21]

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. Notably the comet 17P/Holmes dust trail from the 2007 outburst repeatedly converges at the original site.^[5]

An outburst of 3–4 magnitudes occurred in January 2015, but still required a large telescope to be seen.^[22]

On October 25 the comet looked liked a bright new star in the constellation of Perseus.
This photo composite shows the comet's size and motion in the constellation Perseus from October 25, 2007 through March 9, 2008.
Motion with expanding dust cloud
A simulation showing the angular diameter of the expanding dust cloud for 120 days past the initial event on October 24. The surface brightness decreased over time.
17P/Holmes is a periodic comet in an inclined and elliptical orbit between Mars and Jupiter.^[23] The comet was closest to the Sun on May 4, 2007.
Animation of Comet Holmes's orbit from 1 January 2011 to 31 December 2017
Comet Holmes · Earth · Mars · Jupiter

References

^ Seiichi Yoshida. "17P/Holmes". Seiichi Yoshida's Comet Catalog. Retrieved 2010-02-24.
^ Syuichi Nakano (2011-05-19). "17P/Holmes (NK 2100)". OAA Computing and Minor Planet Sections. Archived from the original on 2015-09-11. Retrieved 2012-02-18.
^ "17P/Holmes Orbit". Minor Planet Center. Retrieved 2021-06-12.
^ "Horizons Batch for 17P/Holmes (90000285) on 2028-Jan-31" (Perihelion occurs when rdot flips from negative to positive). JPL Horizons. Retrieved 2023-02-11. (JPL#K212/32 Soln.date: 2023-Jan-30)
^ ^a ^b ^c ^d Gritsevich, M.; Nissinen, M.; Oksanen, A.; Suomela, J.; Silber, E. A. (June 2022). "Evolution of the dust trail of comet 17P/Holmes". Monthly Notices of the Royal Astronomical Society. 513 (2): 2201–2214. doi:10.1093/mnras/stac822. hdl:10995/117894.
^ ^a ^b Jewitt, David (2007-11-09). "Comet Holmes Bigger Than The Sun". Institute for Astronomy at the University of Hawaii. Retrieved 2007-11-17.
^ Kinoshita, Kazuo (2019-05-22). "17P/Holmes past, present and future orbital elements". Comet Orbit. Archived from the original on 2011-07-10. Retrieved 2023-07-19.
^ ^a ^b Holmes, Edwin (1892). "Discovery of a New Comet in Andromeda". The Observatory. 15: 441–443. Bibcode:1892Obs....15..441H.
^ ^a ^b "Meeting of the Royal Astronomical Society, Friday, November 11, 1892". The Observatory. 15: 417–424. 1892. Bibcode:1892Obs....15..417.
^ Editors. "Comet Holmes Stays Bright, Enlarges in the Evening Sky Archived 2007-10-27 at archive.today", Sky and Telescope, 27 October 2007. Retrieved 29 October 2007.
^ Davidson, J. E. "Comet e, 1889," The Observatory, July 1890, Vol. 13, pp. 247. Retrieved 27 October 2007.
^ Gunn, Angela. "Flash News Flash! Archived 2007-06-25 at the Wayback Machine," USA Today Tech Space, 24 October 2007. Retrieved 25 October 2007.
^ ^a ^b ^c Roger W. Sinnott (October 24, 2007). "Comet Holmes Undergoes Huge Outburst". Sky & Telescope. Archived from the original on 2007-10-27. Retrieved 2007-10-25.
^ ^a ^b ^c Fischer, Daniel. "Incredible comet eruption: from under 17th to 3rd magnitude in hours!," The Cosmic Mirror, #306, 24 October 2007. Retrieved 25 October 2007.
^ Skymap: late October 2007, Northeast, after sunset, Spaceweather.com. Retrieved 28 October 2007
^ Primary measurements, Chris L. Peterson Cloudbait Observatory, Colorado Archived 2011-05-25 at the Wayback Machine, The coma size values plotted at the bottom of this page are primary measurements. They were obtained using conventional methods: individual short CCDs images were made in order to avoid saturation, and these were then calibrated with bias, flat, and dark frames and summed to increase the image dynamic range. Each stacked image (for the 5 nights of data) was astrometrically calibrated (using Pinpoint) for scale, and the intensity profile of the coma measured with a standard tool (in this case, the line profile tool in MaximDL). The resulting profiles were exported to Excel, normalized to the same gain, and the width measured against the noise floor. The best reference is the plotted data itself.
^ Primary measurements, (see luminosity graph; bottom of page) Cloudbait Observatory, Colorado Archived 2011-05-25 at the Wayback Machine
^ 2 AU×(~150 Gm/AU)×sin(13 arcmin) ≈ 1.1 million km
^ Britt, Robert (2007-11-15). "Incredible Comet Bigger than the Sun". Space.com. Retrieved 2008-04-30.
^ "Comet Holmes brightens in retreat", BBC NEWS, 30 October 2007
^ Altenhoff, W. J.; Kreysa, E.; Menten, K. M.; Sievers, A.; Thum, C.; Weiss, A. (2009). "Why did Comet 17P/Holmes burst out? Nucleus splitting or delayed sublimation?". Astronomy and Astrophysics. 495 (3): 975–978. arXiv:0901.2739. Bibcode:2009A&A...495..975A. doi:10.1051/0004-6361:200810458.
^ Comet 17P/Holmes: report on brightest outburst since 2007
^ Nasa 3D simulation of orbit for 17P/Holmes (Java Applet)