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 of the Chicxulub impactor.

65 million years ago, at the end of the Cretaceous, the Earth underwent the last of the five great mass extinctions recorded in the fossil record. While this is by no means the largest of these events, it is the most familiar to the general public, as it was responsible for the extinction of, amongst other things, the non-Avian Dinosaurs and the large marine Reptiles of the Mesozoic Era. For many years the exact nature of this event was a mystery to scientists, and while many theories were proposed, prior to the 1980s few of these were grounded in any actual data. 

In 1980, a team of scientists led by Luis Alvarez of the Lawrence Berkeley Laboratory at the University of California, Berkeley proposed in a paper in the journal Science that the extinction might have been brought about by the extinction could have been caused by the impact of a large extra-terrestrial body with the Earth, based upon the discovery of a distinct layer of iridium-rich sediments at the top of Cretaceous strata in several parts of the world (iridium is rare in terrestrial rocks, but present at much higher levels in many meteorites). This is a dramatic theory, and quickly caught the imagination of the world’s media and the non-scientific public. What is more, unlike many other theories proposed for the end-Cretaceous extinction, it was possible to look for evidence to either support or undermine the theory, an important test in the eyes of the scientific community. Since this time the impactor theory has become one of two main rival explanations for the end-Cretaceous mass extinction (the other being flood-volcanism in the Deccan Traps in India).

An artists impression of the theoretical end-Cretaceous impact event. Don Davis.

In order to make calculations about the energy released by a collision with an extra-terrestrial object, the size and nature of this object need to be estimated with some degree of accuracy (exact details about an object destroyed 65 million years ago in a huge explosion are unlikely to be forthcoming), something which was not possible in the 1980s, though a number of theories were put forward. In 1983 Alvarez proposed in a paper in the Proceedings of the National Academy of Sciences of the United States if America that this object was a large asteroid, while in 1984 David Raup and John Sepkoski of the Department of Geophysical Sciences at the University of Chicago proposed in a paper in the Proceedings of the National Academy of Sciences of the United States of America that the repeated nature of mass extinctions in the fossil record might periodic in nature, and that this periodicity might have an extra-terrestrial cause and in 1987 a team of scientists led by Piet Hut of the The Institute for Advanced Study in Princeton, New Jersey proposed in a paper in the journal Nature that this repeated nature of mass extinctions in the fossil record might be due to repeated encounters with a cometary cloud. While the idea that the Earth’s mass extinctions have a regular and predictable nature with an extra-terrestrial cause is no longer taken seriously, the question of whether such an impact could have been caused by an asteroid or a comet is still debatable.

In 1991 a team of scientists led by Alan Hildebrand of the Department of Planetary Sciences at the University of Arizona published a paper in the journal Geology in which they announced the discovery of a large impact crater, between 180 and 200 km in diameter, buried beneath Tertiary deposits near Chicxulub on the Yutican Peninsula in Mexico, which they proposed might be direct evidence of an Alvarez-type impact at the end of the Cretaceous (though some geologists still dispute that this crater does actually date from the end of the Cretaceous; if it is simply of Late Cretaceous origin, pre-dating the end of the period by hundreds of thousands of years, then it is irrelevant).

Gravity map of the Chicxulub Crater. Virgil Sharpton/Lunar and Planetary Institute.

A simplified section through the geology of the Chicxulub Crater. David Kring/NASA/University of Arizona Space Imagery Center.

 In a paper published on the arXiv database at Cornell University Library on 19 March 2014, Hector Javier Durand-Manterola and Guadalupe Cordero-Tercero of the Instituto de Geofísica at the Universidad Nacional Autonoma de México, attempt to calculate the nature and size of the object which caused the Chicxulub Crater, based upon the calculating the amount of energy necessary to cause a crater of this size, and the concentration of iridium in the sedimentary layer that marks the end of the Cretaceous.

Using four different methods to calculate the mass of the object which caused the Chicxulub crater, all of which rely on scaling up the levels of energy known to have been released from nuclear explosions which caused craters of known sizes, Durand-Manterola and Cordero-Tercero calculate  that the object must have had a mass of between 5 700 000  and 460 000 000 megatons, and a diameter of between 5.1 and 80.9 km. 

Within this range exactly how much energy would have been needed to cause the crater depends upon the nature of the object involved in the impact; Durand-Manterola and Cordero-Tercero considered three possibilities, an iron asteroid, a stony asteroid and an (icy) comet, with the most energy being needed to cause the crater with a comet and the least with an iron asteroid. This is because an iron meteorite would be more than three times as dense as the limestone which the object is thought to have impacted, a stony asteroid slightly denser than the limestone and an icy comet considerably less dense (it would take more energy to break a window with a snowball than with a stone of equal mass). Thus if the object was an icy comet it would need to have been either considerably larger or considerably faster than if it was a stony or iron asteroid.

Durand-Manterola and Cordero-Tercero consider that the ratio of iridium to dust in the terminal Cretaceous boundary layer is closer to that found in comets than in either iron or stony asteroids, and therefore propose that the object was a comet (comets are thought to contain considerably less iridium by mass than either type of asteroid), moreover they suggest that the overall levels of iridium are low enough to suggest the object was towards the smaller end of the calculated possible range for the size of the original object, suggesting that the impact must have occurred at an exceptionally high velocity. For this reason they suggest that the impact may have been caused by a long period comet originating in the Öpik-Oort cloud.

This last set of calculations seem slightly optimistic from a geological point of view; it is highly unlikely that the iridium:dust ratio in the terminal Cretaceous layer would reflect that seen in the original impactor, and quite possible that the level of iridium in the layer could have been either concentrated or diluted by sedimentary processes; the iridium and dust are both thought to have passed from the impact site into the atmosphere, then the hydrosphere, then the sedimentary record. Passage through the atmosphere and hydrosphere are both known to sort particles my mass and surface area (feathers fall through the atmosphere more slowly than stones, and more massive particles sink more rapidly through water than less massive ones). Furthermore a considerable amount of debris is likely to have originated at the impact site, rather than in the impactor. 

It should also be noted that volcanism is also known to produce iridium, although at rather lower concentrations than would be predicted from an asteroid impact. While the iridium layer at the end of the Cretaceous is rather more concentrated than would be expected from a volcanic eruption, which is generally considered to be evidence for an extra-terrestrial origin for the layer, Durand-Manterola and Cordero-Tercero’s calculations suggest that the layer is considerably less dense than would be predicted from an asteroid impact, and therefore, if accurate, would tend to suggest that the iridium layer should be seen as rather less conclusive evidence.

See also…

 The origin of the Bosumtwi Impactor.

 The origin of the Chelyabinsk Meteor.

 The End of the Cretaceous.

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Knots in the Plasma Tail of Comet C/2013 R1(Lovejoy).

Comet C/2013 R1 (Lovejoy) was discovered on 7 September 2013 by talented Australian amateur astronomer Terry Lovejoy (who to date has discovered for new comets). It is a long period comet, thought to visit the inner Solar System roughly once every 20 years, when it reaches 0.58 AU from the Sun (58% of the distance between the Earth and the Sun, between the orbits of Venus and Mercury), before returning to the outer Solar System, eventually reaching 1063 AU from the Sun (1063 times as far from the Sun as the Earth, or more than 35 times as far from the Sun as Neptune, the outermost planet).

In a paper published on the online arXiv database at Cornell University Library on 6 March 2014, a team of scientists led by Masafumi Yagi of the National Astronomical Observatory of Japan describe the results of an observation of C/2013 R1 (Lovejoy) made on 4 December 2013, using the Subaru Telescope on Mauna Kea, Hawaii, which detected two knots (patches of denser material) in the tail of the comet, roughly 300 000 km from the head, moving away from the head at speeds of 21 and 24 km per second respectively.

Such knots have been described in the tails of a number of comets previously, but all have been much further from the head, and moving at far greater speeds. It ha previously been thought that such knots accelerated as they moved away from the comet, which this new observation would appear to confirm.

Image of Comet C/2013 R1 (Lovejoy) produced using the Subaru Telescope on Mauna Kea. Yagi et al. (2014).

See also The breakup of Main Belt object P/2013 R3 (Catalina-Pan STARRS), The nateure and hostory of 'Quasi-Hilda Object' 2000 YN30, The perihelion of comet ISON (C/2012 S1), The perihelion of Comet 103P/Hartley 2 and The origin of Comet P/2006 VW139.

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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 Lyrid Meteors.

The Lyrid Meteors will be at peak visibility between 21 and 22 April  this year, though this is close to a full moon (which occurs on the 25th), so the display will not be good. The meteors, which appear to radiate from the constellation of Lyra, have been visible since the 16th, and will continue till 26 April. At its peak the Lyrid Meteor shower typically produces about 20 meteors per hour, though higher rates have been recorded.

The origin point of the Lyrid Meteors. SpaceWeather.com.

The Lyrid Meteors are comprised of debris from the comet C/1861 G1 Thatcher (named after the astronomer A. E. Thatcher, not the politician). This is a long-period comet that spends most of its time in the Oort Cloud, only visiting the inner Solar System once every 415 years, the last occasion being in 1861. When the comet visits the inner Solar System it is heated by the Sun, melting the ices that make up its surface and releasing a trail of dust, which continues to follow the path of the comet. The Earth passes through this trail in April each year, creating a light show as the dust particles burn in the upper atmosphere.

The orbit and current position of C/1861 G1 Thatcher. Image created using the JPL Small-Body Database Browser.

See also Fireball over Wyoming, Comet C/2011 L4 (PANSTARRS) to reach its closest point to Earth this week, The Earth approaches its perihelion, The Southern Solstice and Possible second meteor shower to coincide with the Geminids.

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Comet C/2011 L4 (PANSTARRS) to reach its closest point to Earth this week.

Comet C/2011 L4 (PANSTARRS) will come within 1.1 AU of the Earth (1.1 times as distant as the Sun) on Tuesday 5 March 2013. The comet is visible with binoculars in the Southern Hemisphere, and should become visible in the Northern Hemisphere from 7 March, and possibly become bright enough to be seen with the naked eye. The comet is already closer to the Sun than the Earth, and indeed Venus, but not close to us and traveling at a very high angle to the plane of the Solar System. It will reach its perihelion (closest point to the Sun) on 10 March, when it will potentially be at its brightest (though comets are notoriously hard to predict), and should remain visible for the rest of the Month. It will be easiest to locate on 12-14 March, when it is closest to the Moon, though it might be hard to see at this time due to the Moon's brightness.

The passage of comet C/2011 L4 (PANSTARRS) in February-April 2013. Eagle Eye on the Sky.

Comet C/2011 L4 (PANSTARS) is thought to be a non-periodic comet on its first visit to the inner Solar System, having been disturbed from its previous orbit within the Oort Cloud some time within the last few million years. It is likely that it will revisit the inner Solar System every 110 000 years from now on. It was discovered in June 2011 by the Pan-STARRS (Panoramic Survey Telescope and Rapid Response System) array of telescopes on Haleakala, Hawaii. It will move away from the (Northern Hemisphere) horizon during April, becoming fainter and disappearing towards the North Pole as it moves away from the Sun. It will not be visible from the Southern Hemisphere during this time.

The orbit of comet C/2011 L4 (PANSTARRS). Image created using the JPL Small-Body Database Browser.

See also 99942 Apophis to fly by the Earth, The Earth approaches its perihelion, Possible second meteor shower to coincide with the Geminids, Asteroid 4179 Toutatis/1989 AC to fly past the Earth and The Geminid Meteors.

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Comet C/2012 S1 (ISON) to pass within 60 000 000 km of the Earth.

Comet 2012/S1 (ISON) was discovered by Vitali Nevski and Artyom Novichonok at the International Scientific Optical Network near Kislovodsk, Russia on 21 September, and its discovery rapidly confirmed by follow-up observations from the Remanzacco Observatory in Italy, and detection of the (previously unnoticed) comet in images from other telescopes dating back as far as December 2011. 

Photograph of 2012/S1 (ISON). RAS Observatory.

The comet is currently approaching the Inner Solar System on a parabolic orbit that suggests that it might have recently been disturbed from an orbit which kept it entirely within the Oort Cloud. If this is the case, and 2012/S1 (ISON) has not previouly visited the Inner Solar System, then the commet is likely to produce a substantial icy tail (each time a comet visits comes close to the Sun then some of its ice is lost, so that 'fresher' comets that have not made to many visits, will have more ice to shed), and consequently appear brighter to observers.

2012/S1 (ISON) will pass within 0.07 AU of Mars on 1 October 2013 (0.07 Au = 7% of the distance between the Earth and the Sun, or about 10 000 000 km). It will reach its perihelion (closest point to the Sun) on 28 November, coming within 0.00735 AU (1 100 000 km) of the surface of the Sun, and pass by the Earth on 26 December 2012, at a distance of 0.4 AU (60 000 000 km).

The position of Comet 2012/S1 (ISON) on 1 October 2012, and it's orbit relative to the plane of the Solar System. NASA/JPL Small-Body Database Browser.

It has been widely reported that 2012/S1 may outshine the full Moon when at its closest to the Sun, though predictions about the brightness of comets should be made with caution, and it may be difficult to see the comet when it is this close to the Sun. It is quite likely it will be easier to observe as it passes the Earth, even if it is not as bright in the sky.

See also The Perseid Meteors, Asteroid 2002 AM₃₁ flies past the Earth, 2012 LZ1; bigger than we thought, Asteroid 2012 LZ1 flies by the Earth and The origin of Comet P/2006 VW₁₃₉.

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