 Comet 17P/Holmes and its blueion tail (taken on November 4, 2007) | |
| Discovery | |
|---|---|
| Discovery date | November 6, 1892 |
| 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 | 3.618414AU |
| Eccentricity | 0.432564 |
| Orbital period | 6.882994a |
| Inclination | 19.1126° |
| Last perihelion | February 19, 2021[1][2] March 27, 2014 May 4, 2007 |
| Next perihelion | January 31, 2028[3][4] |
Comet Holmes/ˈhoʊmz/ (official designation:17P/Holmes) is aperiodic comet in theSolar System, discovered by the British amateur astronomerEdwin 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 thelargest object in the Solar System, as itscoma (the thin dissipating dust ball around the comet) expanded to a diameter greater than that of theSun (although itsmass remained minuscule).[6] Between 1857–2106 perihelion remains between 2.05–2.36 AU.[7]
Comet Holmes was discovered byEdwin Holmes on November 6, 1892, while he was conducting regular observations of theAndromeda Galaxy (M31).[8][9]Its discovery in 1892 was possible because of an increase in itsmagnitude 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 byEdward Walter Maunder (Royal Observatory,Greenwich,England),William Henry Maw (Kensington,London,England), and B. Kidd (Bramley, Surrey,England).[8][9]Independent discoveries were made byThomas David Anderson (Edinburgh, Scotland) on November 8 and by Mike Brown (Wilkes, USA) and byJohn Ewen Davidson (Mackay,Queensland, Australia) on November 9.[11]
The first calculations of the elliptical orbit of 17P/Holmes were done independently byHeinrich Kreutz andGeorge Mary Searle. Additional orbits eventually established theperihelion date as June 13 and theorbital period as 6.9 years. These calculations proved that the comet was not a return ofBiela's Comet.
The 1899 and 1906 appearances were observed, but the comet was lost (seeLost comet) after 1906 until it was recovered on July 16, 1964, byElizabeth Roemer (US Naval Observatory Flagstaff Station,Arizona, United States). Aided by the computer predictions ofBrian G. Marsden, the comet has been observed on every subsequent return.
During its 2007 return, Holmes unexpectedly brightened from amagnitude 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 onTenerife in theCanary Islands; minutes later, Ramón Naves inBarcelona noticed the comet at magnitude 7.3.[13] It became easily visible to thenaked eye as a bright yellow "star" inPerseus,[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 nearopposition 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'snucleus 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 thecoma'sapparent 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 2AU, this means that the true diameter of the coma had swelled to over 1 million km,[18] or about 70% of the diameter of theSun. By comparison, theMoon is 380,000 km fromEarth. Therefore, during the 2007 outburst of Comet Holmes the coma was a sphere wider than the diameter of the Moon's orbit aroundEarth. 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 ameteoroid, or, more probably, from a build-up of gas inside the comet'snucleus that eventually broke through the surface.[20] However, researchers at theMax Planck Institute suggest in a paper published inAstronomy and Astrophysics that the brightening can be explained by a thick, air-tight dust cover and the effects of H2Osublimation, 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 constellationPerseus. 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]
![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.](/mt/?noimg=&dark=on&url=https%3a%2f%2fwikipedia.org%2fwiki%2fFile%3aComet_Holmes_orbit_2007.png)
| Numbered comets | ||
|---|---|---|
| Previous 16P/Brooks | Comet Holmes | Next 18D/Perrine–Mrkos |
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