Observing Comet 17P/Holmes with the WISE Space Telescope.

Comet 17P/Holmes is a Jupiter Family Comet with a 6.89 year orbit that takes it from 2.06 AU from the Sun (2.06 times the average distance at which the Earth orbits the Sun, considerably outside the orbit of Mars) to 5.18 AU from the Sun (slightly inside the orbit of Jupiter). Since its discovery in 1892 it has been observed to undergo three massive outbursts when it increased dramatically in brightness; the last of these was in October 2007, when its brightness increased by a factor of almost a million, and it briefly became visible to the naked eye. Each of these outbursts has come 6-9 months after the comet reached its perihelion (the closest point in its orbit to the Sun).

Image of 17P/Holmes taken on 2 November 2007. Syuichi Nakano/IQC/Havard Cometary Science Archive.

In a paper published in on the arXiv database at Cornell University Library on 22 April 2014, a team of scientists led by Rachel Stevenson of the Jet Propulsion Laboratory at the California Institute of Technology, discuss the results of a study of a series of images of 17P/Holmes taken by the Wide-Field Survey Explorer (WISE) Space Telescope on 14-15 May 2010, when 17P/Holmes was 5.1 AU from the Sun (i.e. 5.1 times the average distance between Earth and the Sun), and roughly five months short of reaching its aphelion (the point in its orbit when it is furthest from the Sun).

Based upon the data gathered by the WISE telescope, Stevenson et al. were able to determine that 17P/Holmes has a core with an equivalent diameter of 4.135 km (i.e. a spherical body with the same volume as the comet would have a diameter of 4.135 km). It is surrounded by a cloud of material (halo) with a temperature of 134 K (-139.15˚C) which, while very cold, is slightly warmer than would be predicted for particles at this distance from the Sun, suggesting that the particles are either too small to radiate heat effectively, have a rough surface which does not radiate heat evenly, or are large enough to maintain thermal gradients internally (the cloud is probably made up by a mixture of particles showing all three properties). Dynamical modelling of the material in the observed tail of 17P/Holmes suggests that it was mostly made up of material produced by the October 2007 eruptive episode.

Comet 17P/Holmes as observed by the WISE mission on 14-15 May 2010 at a wavelength of 22 µm. The nucleus is located in the south-east corner of the image. Celestial north (N) and east (E) are marked, as are the solar ( ) and velocity (v) vectors. (Note celestial east is reversed with respect to normal maps, since the observer is looking up). Stevenson et al. (2014).

This still leaves the question as to why 17P/Holmes undergoes periodic spectacular outbursts, unlike those seen in other Jupiter Family Comets, which typically only undergo moderate outgassing at perihelion. None of the properties uncovered by WISE are atypical for such a comet, and previous studies have shown that its chemical composition is also typical. Stevenson et al. suggest that the outbursts may be related to eccentricities in the orbit of 17P/Holmes, which at perihelion in 2007 was 0.12 AU closer to the Sun than it had been at its previous perihelion in 2000. This might potentially cause solar heat to reach deeper into the comet, potentially reaching pockets of highly volatile gasses such as carbon monoxide or carbon dioxide, which would result in a more spectacular outburst than at the previous perihelion. However Stevenson et al. also note that this explanation would require 17P/Holmes to have a remarkably high internal tensile strength to survive such a process. Estimates of internal tensile strengths for the Jupiter Family Comets 16P/Brooks 2 and Shoemaker-Levy 9, which were both broken apart by tidal forces produced by the planet Jupiter, suggest that they had internal tensile strengths of 0.1-0.38 kPa (kilopascals), while the Stevenson et al. explanation for the outbursts on 17P/Holmes would require it to have an internal tensile strength in the 10-100 kPa range.

The orbit of 17P/Holmes. JPL Small Body Database Browser.

See also…

 The breakup of Main Belt object P/2013 R3 (Catalina-Pan STARRS)

 The nature and history of 'Quasi-Hilda Object' 2000 YN30.

 The perihelion of Comet ISON (C/2012 S1).

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The nature and origin of the July 2009 Jovian Impactor.

In 2009 the remains of the comet Shoemaker-Levy 9 were observed impacting the Jovian atmosphere, the first time a body had been directly observed colliding with a planet other than the  Earth, and the first time a comet had ever been seen impacting a planet. At the time this was thought to be an extremely rare event, possibly happening as infrequently as once every 500 years. However in July 2009 a scar similar to that caused by the Shoemaker-Levy 9 impact was observed in the southern hemisphere of Jupiter, suggesting that a similar impact had occurred once again.

In a paper published in the Astrophysics Journal Letters on 12 May 2010 and on the arXiv database at Cornell University Library on 13 May 2010, a team of scientists led by Augustin Sánchez-Lavega of the Universidad del País Vasco in Bilbao discuss the July 2009 Jovian impact, and try to determine the nature and origin of the impactor.

Composite image of Jupiter, with the impact scar as seen at the times indicated at beside the boxes; note this is one scar seen at different times in different positions, not a series of scars as seen with the Shoemaker-Levy impact. Sánchez-Lavega et al. (2010).

The impact scar was first detected at 1.02 am GMT on 20 July 2009, as a dark spot as it rotated into view from the west. The most recent previous image of the same spot was taken at 7.40 am GMT on 19 July 2009, constraining the time of the event that caused it to a 17 hour 22 minute window. Images taken by NASA’s Infrared Telescope Facility at 10.13 am GMT on 20 July 2009 showed a bright spot in the methane and hydrogen absorption bands, reaching high above the surrounding clouds; this was the same pattern observed after the Shoemaker-Levy 9 impacts, suggesting this was another impact of a similar nature.

The July 2009 impact scar seen (a) in visible wavelengths, and (b) in infrared. Sánchez-Lavega et al. (2010)

The scar extended 4800 km east-west and 200 km north-south, though it was tilted at 12˚ to true latitude. This is more elongate than the Shoemaker-Levy 9 scars, which Sánchez-Lavega et al. interpret as a sign that the impacting body had a shallower incidence angle relative to the horizon. A thin debris crescent extended 4800 km northwest of the western edge of the scar; a similar crescent structure was seen after the Shoemaker-Levy 9 impacts, and was interpreted as being the result of Coriolis force on the falling material plus a sliding in the atmosphere that conserves the tangential velocity. 

Working from the size of the impact scar, Sánchez-Lavega et al. calculate the original body to have been between 500 m and 1 km in diameter, and that it struck Jupiter at a speed of between 54.52 and 55.1 kms¯¹. Attempts to model the previous path of the impactor prior to the collision by working backwards from the impact site suggest there was a 47% chance that the body was on its original, Sun orbiting path when it hit Jupiter, and a 53% chance that it had previously been captured by the planet into  a Jovicentric orbit, probably more recently than 1989, as was the case with Shoemaker-Levy 9. Furthermore the previous orbit of the body was equally likely to have been a Main Asteroid Belt body (either a Hilda Group Asteroid or a Quasi-Hilda Comet), or a Jupiter Family Comet. Shoemaker-Levy 9 is thought to have originally been a Quasi-Hilda Comet.

See also…

 Ripples in the rings of Jupiter.

 The nature of the Chicxulub impactor.

 The breakup of Main Belt object P/2013 R3 (Catalina-Pan STARRS).

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The breakup of Main Belt object P/2013 R3 (Catalina-Pan STARRS)

Main Belt object P/2013 R3 (Catalina-Pan STARRS) was discovered on 15 September 2013, by both the University of Arizona's Catalina Sky Survey in the Catalina Mountains north of Tucson, and the Pan-STARRS telescope at the University of Hawaii's Institute for Astronomy. It is located in the Main Asteroid Belt, with an orbit that takes it from 2.20 AU from the Sun (i.e. 2.20 times the average distance between the Earth and the Sun) to 3.86 AU from the Sun, and which is tilted to slightly less than 1 degree from the plain of the Solar System. At the time of its discovery it had a dust envelope resembling that of a comet, and was therefore classified as a Main Belt Comet (a comet that orbits in the Main Asteroid Belt, and which produced a small tail at the closest point in its orbit to the Sun), rather than an asteroid.

 

In a paper published on the online arXiv database at Cornell University Library on 5 March 2013, a team of scientists led by  David Jewitt of the Department of Earth and Space Sciences and Department of Physics and Astronomy at the University of California Los Angeles describe a series of follow up observations made by the Keck 10 m Telescope in Hawaii, which revealed that P/2013 R3 had recently broken up, and was in the process of dispersing as a number of smaller objects.

 

The asteroid is thought to have broken up into at least 10 smaller objects, none larger than 10 m in diameter, between February and September 2013. The resultant debris cloud had expanded to cover a volume with a cross sectional diameter of between 21 and 29 km, at the time of observation. Around 800 000 tonnes of dust was released during this volume, spectographic analysis of which suggested that the parent body was a chondrite stony asteroid rather than a comet; no traces of comet-type outgassing was discovered.

 

Objects in space have previously been observed to break up for a variety of reasons; they can be torn apart by a close encounter with a larger object such as a planet or the Sun, directly smashed in a collision with another object, be torn apart by the pressure of internal gasses sublimating (turning directly from solids to gasses) as they warm, or be torn apart by rotational stresses.

 

P/2-13 R3 does not have an orbit which takes it close to any body large enough to produce a tidal force which could have caused its breakup, so this can be ruled out as the source of its disruption. Similarly its breakup appears to have been gradual, over a period of several months, which would appear to rule out a collision with another object, since such an event would be expected to produce a single sudden breakup. It does appear to have contained some water ice, but this is unlikely to have produced enough pressure to disrupt the parent body by sublimating at these distances from the Sun; such an effect could be produced by the sublimation of more volatile compounds, such as carbon monoxide, at this distance, but there is no evidence for such compounds having been present. For these reasons Jewitt et al. conclude that the breakup of P/2013 R3 was caused by rotational stresses, leading to a profound structural failure in the parent body.

Four epochs of R3 imaging from 2013 shown as raw images (left column) and spatially altered to suppress diffuse coma (right column). October 01 data are from Keck, all the rest from HST. Each panel has North to the top, East to the left and has dimensions 14" X12". The projected anti-solar direction is shown by a yellow arrow marked "-S". Projected negative velocity vector is indicated by a green arrow marked "-V". Jewitt et al (2014).

See also The origin of the Bosumtwi Impactor, The nature and history of 'Quasi Holda Object' 2000 YN30, The Eulalia and Polana asteroid families, The dust tail of 3200 Phaeton and The perihelion of Comet 103P/Hartley 2.

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The ejecta of Main-Belt Comet P/2012 T1 (PANSTARRS).

Main-Belt Comets are bodies bodies within the Main Asteroid Belt that occasionally produce tails similar to those of comets. Most comets have highly elliptical orbits and spend much of their time in the Outer Solar System, far from the influence of the Sun. When they make brief visits to the Inner Solar System frozen gasses at their surface are heated by the Sun and sublimate (pass directly from a solid to a gas, liquids do not usually exist in a vacuum), freeing dust particles that drift away from the comet and are then blown away by the Solar Winds (ionized particles and photons streaming out from the Sun, thus the tail of a comet always points away from the Sun, not back along the path of the comet as we would instinctively predict). Main-Belt Comets do not have such extreme orbits, but this does not mean their orbits are circular, they still get closer to and further from the Sun, and is is thought that some gasses are sublimating from them during their closer approaches, though this is open to dispute in the case of individual objects without repeat observations of the phenomena, which could be ejecting material as a result of a collision rather than solar heating.

In a paper published on the arXiv online database at Cornell University Library on 19 May 2013, a team of scientists led by Fernando Moreno of the Instituto de Astrofísica de Andalucía discus the results of a series of observations of the ejecta of Comet P/2012 T1 (PANSTARRS) made at Observatorio del Roque de los Muchachos on La Palma in the Canaries, between November 2012 and February 2013.

Image of Comet P/2012 T1 (PANSTARRS) made by the 10.4m Gran Telescopio Canarias at Observatorio del Roque de los Muchachos. Moreno et al. (2013).

Based upon these observations Moreno et al. conclude that the ejection of material from Comet P/2012 T1 (PANSTARRS) was a sustained event lasting 4-6 months, inconsistent with a one-off event caused by a collision (the body has a period of 5.6 years); it was estimated that between 6 and 25 million kg of material was ejected, with particle sizes of up to 10 cm. The pattern of material ejecting from the object was compatible with grains being freed by the sublimation of gasses, though it was unclear whether this was a sudden event near the perihelion (closest point in a bodies orbit to the body which it is orbiting), or a more sustained event that began earlier in the orbit. The most likely source for the material was close to one of the poles (dubbed arbitrarily the south), which could be permanently in the sun at the time of the perihelion, as the poles of the Earth are around the solstices.

The orbit of P/2012 T1 (PANSTARRS). Image created using the JPL Small Body Database Browser.

See also The Main-Belt Comet P/2012 T1 (PANSTARRS), Comet C/2011 L4 (PANSTARRS) to reach its closest point to Earth this week, Comet C/2012 S1 (ISON) to pass within 60 000 000 km of the Earth, The origin of Comet P/2006 VW₁₃₉ and Comet C/2011 W3 (Lovejoy) survives a close encounter with the sun.

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The Main-Belt Comet P/2012 T1 (PANSTARRS).

Main-Belt Comets are bodies within the Main Asteroid Belt that show some similarities with comets, notably the production of a halo (dust tail) during the part of their orbit that takes them closest to the Sun. The tail of a typical comet is a distinctive object; comets have icy surfaces which evaporate away as they pass through the Inner Solar System, carry a trail of mineral specks and chunks of ice that form the visible tail. For most of a comets life it is in the Outer Solar System, far from the Sun's influence and produces no tail. Main Belt Comets spend their entire lives between the orbits of Mars and Jupiter. They are thought to be compositionally similar to regular comets, but only to produce much smaller halos at the innermost part of their orbit's, when they are warmed marginally more by the Sun. Such objects are less obvious than regular comets, and have only been known about since 1996.

In a paper published on the arXiv database at Cornell University Library on 23 May 2013, a team of scientists led by Henry Hsieh of the Institute for Astronomy at the University of Hawaii describe a new Main-Belt Comet, discovered on 6 October 2012 by the Pan-STARRS1 survey telescope on the Haleakala volcano in Hawaii, and named P/2012 T1 (PANSTARRS); where 'P/' implies a periodic comet, '2012' is the year of discovery, 'T1' implies the object was the first such object discovered between 1 and 16 October that year, and '(PANSTARRS)' is the discoverer (historically this would be a single astronomer, but this is not appropriate for large modern instruments operated by teams of scientists).

Image of P/2012 T1 (PANSTARRS). The comet is a point, while the elongate objects are stars; this is because the telescope is tracking the moving comet, making the (non-moving) stars in the background appear to be in motion. G.V. Schiaparelli Astronomical Observatory.

After the initial discovery follow-up observations of the new comet were made with University of Hawaii (UH) 2.2 m and the 10 m Keck I telescopes, both on Mauna Kea, the 6.5 m Baade and Clay Magellan telescopes at Las Campanas in Italy, the 2.0 m Faulkes Telescope South at Siding Spring in New South Wales, the 1.8 m Perkins Telescope at Lowell Observatory in Arizona, and the Southern Astrophysical Research Telescope on Cerro Pachon in Chile.

P/2012 T1 (PANSTARRS) roughly doubled in brightness in the 40 days following its discovery (between 6 October and mid-November 2012), remained constant in brightness till late December, then declined in brightness by 60% over the period till mid-February. The steady brightening over the initial part of this cycle and steady dimming over the second part is taken to be representative of sublimation from the surface of P/2012 T1 (PANSTARRS), rather than a sudden ejection of matter following a collision.

Orbital diagram for P/2012 T1 (PANSTARRS). The point at the centre is the Sun, the four inner rings, the orbits of Mercury, Venus, Earth and Mars respectively, the outermost ring showing only at the corners of the diagram is the orbit of Jupiter. The point marked 'P' is the comet's perihelion, the point at which it is closest to the Sun; the point marked 'A' is it's aphelion,  when it is furthest from the Sun. The points (1-6) represent observations of the comet; (1) 2012 October 6-8, (2) 2012 October 12-25, (3) 2012 November 8-14, (4) 2012 December 18-20, (5) 2013 January 8, and (6) 2013 February 4. The scale is in Astronomical Units (AU), where 1 AU is the average distance between the Earth and the Sun. Hsieh et al. (2013).

Hsieh et al. calculate that P/2012 T1 (PANSTARRS) ejected water molecules at an average rate of 5 × 10²⁵ mol s-¹ during the eruptive part of it's cycle, which is equivalent to just under 2 100 000 000 megatonnes of water per  day. All the ejected material appears to be water; a spectrographic analysis could find no sign of hydrated minerals; this does not imply that the temperatures are high enough to sublimate water (turn directly from a solid to a gas; liquids cannot exist in a vacuum), it is more likely that it is due to the sublimation of carbon dioxide (at a lower temperature) creating a halo of water-ice molecules (snowflakes) freed from the matrix.

P/2012 T1 (PANSTARRS) sits within a cluster of asteroids known as the Lixiaohua Family, which are believed to have originated in the breakup of a larger body about 155 million years ago, though it is not possible to tell if it shares a common origin with these objects or whether its position is coincidental; it could be an object from the Outer Solar System that has only recently been captures or it could potentially be a member of the much older Thetis group of asteroids.

See also Asteroid 1998 QE2 to pass Earth at about 5 800 000 km on Friday, Second meteorite from New Haven County, Connecticut, The Eta Aquarid Meteors, Connecticut house struck by meteorite and The Lyrid Meteors.

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The origin of Comet P/2006 VW₁₃₉.

Comet P/2006 VW₁₃₉ is a Main-Belt Comet, one of a recently discovered group of objects, which inhabit the main Asteroid Belt, but which produce a tail similar to that of a comet when at their closest to the Sun. Asteroids and comets have traditionally been viewed as very different types of objects; asteroids being rocky objects of the inner Solar System, Comets being icy objects from the outer system that occasionally get thrown into the inner system, melting as they do so (strictly speaking sublimating, the solid ices turning directly to gasses in the vacuum of space) and generating snowy tails that point away from the Sun. 

Main-Belt comets do not fit into either of these neat categories, they inhabit the Main Asteroid Belt, but have eccentric orbits (i.e. orbits that are not perfect circles centered on the Sun) and when they are closest to the Sun produce comet-lie tails, suggesting that they are primarily icy objects, similar to comets.

The orbit of P/2006 VW₁₃₉. Green squares are sightings; (a) 30 August 2011, (b) 5 November 2011, (c) 12-14 November 2011, (d) 22 November-4 December 2011, (e) 16-19 December 2011, (f)  7 January 2012. (A) Represents the aphelion, the point in the orbit where the orbiting object is furthest from the Sun. (P) Represents the perihelion, the point in the orbit closest to the Sun. Also shown are the orbits of Mercury, Venus, Earth, Mars and Jupiter. The scale is in Astronomical Units (AU), one AU being equal to the average distance between the Earth and the Sun. Hsieh et al. (2012).

In a paper published on the arXiv database at Cornell University Library on 22 May 2012, and also due to be published in the Monthly Notices of the Royal Astronomical Society, Bojan Novaković of the Department of Astronomy at the Faculty of Mathematics at the University of Belgrade, Henry Hsieh of the Institute for Astronomy at the University of Hawaii, and Alberto Cellino of the INAF–Osservatorio Astronomico di Torino, discuss the origin of P/2006 VW₁₃₉, and its relationship to other objects in similar orbits.

Novaković et al. compared the orbit of P/2006 VW₁₃₉ to that of 24 other objects in similar orbits, calculating their orbits backwards to look for a common origin. 19 of these objects were found to be in stable orbits; the remaining 5 were in unstable orbits, and therefore unlikely to have been there very long. These objects are most likely to have been shifted into their current orbits in the recent past (though this might mean a hundred thousand years or so) by encounters with other objects, and will most probably be shifted out of their current orbits within the next hundred thousand years or so. Of the remaining 19 objects, 11 could be traced backwards to a likely common origin with P/2006 VW₁₃₉, about 7.5 million years ago, in the breakup of a larger body, probably as the result of a collision. 

Furthermore the parent body probably belonged to a group of asteroids known as the Themis Family, which are thought to have originated in the breakup of a larger body about 2.5 billion years ago. This suggests that both the objects of the 'P/2006 VW₁₃₉ Family' and the Themis Family may also be substantially comet-like in composition, i.e. icy rather than rocky in composition.

See also Asteroid 2012 KP24 passes Earth at a distance of 51 000 km, Fragments of 22 April meteor found in California, Fireball over Nevada and California, The Lyrid Meteors and Asteroid 2012 DA14 may pass within 21 000 km of the Earth.

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