 Comet Lovejoy photographed byDan Burbank aboard theISS, 21 December 2011 | |
| Discovery | |
|---|---|
| Discovered by | Terry Lovejoy |
| Discovery site | Queensland, Australia |
| Discovery date | 27 November 2011 |
| Orbital characteristics[1][2] | |
| Epoch | 6 December 2011 (JD 2455901.5) |
| Observation arc | 40 days |
| Orbit type | Kreutz sungrazer |
| Aphelion | 157.36 AU |
| Perihelion | 0.0056 AU |
| Semi-major axis | 78.683 AU |
| Eccentricity | 0.99993 |
| Orbital period | ~622 years (outbound) |
| Max.orbital speed | 565 km/s |
| Inclination | 134.36° |
| 326.37° | |
| Argument of periapsis | 53.509° |
| Mean anomaly | 359.99° |
| Last perihelion | 16 December 2011 |
| Next perihelion | ~2633? |
| TJupiter | 0.002 |
| EarthMOID | 0.490 AU |
| Physical characteristics[3][4] | |
| Dimensions | 0.2–7.7 km (0.12–4.78 mi) |
| Comet total magnitude (M1) | 15.3 |
| Comet nuclear magnitude (M2) | 20.6 |
| –4.0 (2011 apparition) | |
Comet Lovejoy,formally designatedC/2011 W3 (Lovejoy), is along-period comet andKreutz sungrazer. It was discovered in November 2011 by Australianamateur astronomerTerry Lovejoy. The comet'sperihelion took it through the Sun'scorona on 16 December 2011, after which it emerged intact, though greatly impacted by the event.[ambiguous]
As Comet Lovejoy was announced on the 16th anniversary of theSOHO satellite's launch it became known as "The Great Birthday Comet of 2011", and because it was visible from Earth during theChristmas holiday it was also nicknamed "The Great Christmas Comet of 2011".[5][6] Lovejoy was retroactively dubbed theGreat Comet of 2011.[7]
Comet Lovejoy was discovered on 27 November 2011 by Terry Lovejoy ofThornlands, Queensland, during a comet survey using a 20 cm (7+7⁄8 in)Schmidt–Cassegrain telescope and a QHY9CCD camera.[8] It is the third comet discovered by Terry Lovejoy. He reported that it was "a rapidly moving fuzzy object" of 13thmagnitude, and additional observations were made by him over the next couple of nights.[8][9]
Independent confirmation of the comet did not come until 1 December, when it was observed byAlan Gilmore andPamela Kilmartin at theMount John University Observatory in New Zealand, using its 100 cm (39 in) McLellan Telescope. Upon confirmation, an official report was made to theCentral Bureau for Astronomical Telegrams, and the comet's existence was announced by theMinor Planet Center on 2 December.[8] It is the first Kreutz-group comet discovered by ground-based observation in 40 years.[10]
The firstorbital elements using an assumedparabolic trajectory were published by Gareth Williams of the Minor Planet Center on 2 December, with an estimated perihelion of 0.0059AU occurring near midnightUTC of 15/16 December. Further refinements were published during subsequent days, including one on 5 December, estimating perihelion at 0.0056 AU just before midnight on 15 December. On 11 December the firstelliptical orbit was published, estimating perihelion at 0.0055 AU just after midnight on 16 December.[8][11]
In space, the comet first became visible to theSTEREO-A spacecraft on 3 December, and to theSOHO spacecraft on 14 December. As the comet approached the Sun it was observed by eighteen instruments on six satellites: STEREO-A and -B, SOHO, SDO,Hinode andPROBA2.[12][13]
A small companion comet, calledSOHO-2203,[14] was detected in SOHO images on 14 December by Zhijian Xu, and later spotted by the twin STEREO spacecraft.[5][15] It is believed to be a fragment of Comet Lovejoy that broke away several decades ago. This discovery was not unexpected, as Kreutz-group comets are often found with smaller companions.[5][16]
At its brightest, Comet Lovejoy had anapparent magnitude of between–3 and–4, which is about as bright as the planetVenus.[12][17] It is the brightestsungrazing comet ever observed by SOHO, and the brightest comet to appear sinceComet McNaught of 2007, which shone at visual magnitude–5.5.[5][18] Nevertheless, Lovejoy was largely invisible to the naked eye during its peak brightness due to its proximity to the Sun.[6][16]
Comet Lovejoy reachedperihelion on 16 December 2011 at 00:17UTC, as it passed approximately 140,000 kilometres (87,000 mi) above the Sun's surface[2][19] at a speed of 536 km/s (333 mi/s; 1.93 million km/h; 1.20 million mph), or 0.18% thespeed of light.[20] It was not expected to survive the encounter due to extreme conditions in the corona, such as temperatures reaching more than one millionkelvins and the exposure time of nearly an hour. However, theSolar Dynamics Observatory (SDO), as well as other Sun-monitoring spacecraft, observed the comet emerge from the corona intact.[21][22][23] The STEREO and SOHO spacecraft continued to observe the comet as it moved away from the Sun.[23]
Before perihelion, thenucleus of Comet Lovejoy had been estimated to be between 100 and 200 metres (330 and 660 ft) in diameter. Since the comet survived perihelion, it is thought that the nucleus must have been larger, perhaps up to 500 metres (1,600 ft).[24] During the coronal passage, it is believed that a significant fraction of the comet's mass evaporated.[23]
The first ground-based observation of Comet Lovejoy post-perihelion occurred on 16 December at 19:55 UTC, when it was seen by Rick Baldridge and Brian Day of theFoothill Observatory. Baldridge estimated the comet atmagnitude −1.[8] The comet's discoverer,Terry Lovejoy, made a pair of observations on 17 December at 01:12 and 20:24 UTC, with apparent magnitudes estimatedat −1.2and −0.8, respectively.[8]
Images taken on 20 December around 08:00 UTC suggested that the comet had undergone significant changes. Taken by Czech astronomer Jakub Černý using the robotic FRAM telescope atPierre Auger Observatory, the images indicated that "the nucleus had apparently become bar-shaped and was accompanied by a bright tail ray."[8][25]
Comet Lovejoy re-emerged as anaked eye object in theSouthern Hemisphere around 21–22 December,[26] when astronaut Dan Burbank photographed it from theInternational Space Station.[27][28] Ground-based photographers continued to capture images of the comet, which had dimmed to around 4th magnitude.[27] While Lovejoy would have continued to be visible to Southern Hemisphere observers into early 2012, large telescopes would have been required to see the comet by the time it crossed into the Northern Hemisphere in February.[6]
Some concern was expressed after perihelion that the stresses induced in the comet by its close approach to the Sun might result in its disintegration.[12] That observers were unable to locate a distinct nucleus amidst the more visible tail furthered this idea;[6] using observations from thePierre Auger Observatory,Zdeněk Sekanina and Paul Chodas determined that, while the nucleus did survive perihelion for several days, following a significant outburst of dust on 19 December, the nucleus underwent a "cataclysmic fragmentation" event on 20 December and completely disappeared.[29]
| Date and time of closest approach | Earth distance (AU) | Sun distance (AU) | Velocity relative to Earth (km/s) | Velocity relative to Sun (km/s) | Uncertainty region (3-sigma) | Solar elongation |
|---|---|---|---|---|---|---|
| 7 January 2012 ≈18:56 | 0.499 AU (74.6 million km; 46.4 million mi; 194 LD) | 0.877 AU (131.2 million km; 81.5 million mi; 341 LD) | 53.8 | 44.8 | ± 6300 km | 63° |
In the event that some portion of the nucleus did survive, the eccentricity and inclination of Comet Lovejoy's orbit avoids significantperturbation from planets, which leaves the possibility that the comet may return for another perihelion.[12] Using anepoch 2050 solution, Comet Lovejoy is estimated to have about a 622-year orbit and a return to perihelion around the year 2633.[1]
An elliptical orbit calculated by Sekanina and Chodas in 2012 indicates that Comet Lovejoy is a fragment of an unrecordedsungrazer that reached perihelion around 1329. The fragmentation history suggested by these authors is that a parent sungrazer, likely the comet observed in 467 CE, split near the Sun due totidal forces during its pass. The principal fragment – or a non-tidally fragmented portion of it – returned as theGreat Comet of 1106, but a secondary fragment was imparted a longer orbital period and returned about 1329. This secondary also split at perihelion and its principal fragment will return around 2200, likely as a cluster of further fragments. A secondary fragment of this event left on a shorter period that theoretically should have brought it back to the inner solar system during the early years of the 21st century. At some point after perihelion, this secondary fragment broke apart due to non-tidal forces and one of the resulting fragments became Comet Lovejoy.[clarification needed] Other similar fragments may also exist and might return as sungrazers in the near future.[29]
.gif)
2011 in space | ||
|---|---|---|
2012 » | ||
Space probe launches |
|   .jpg) |
| SelectedNEOs | ||
| Exoplanets |
| |
| Discoveries | ||
| Novae | ||
| Comets | ||
| Space exploration | ||
.png)
