This image of Comet Lulin taken Jan. 28 merges data acquired by Swift's Ultraviolet/Optical Telescope (blue and green) and X-Ray Telescope (red). At the time of the observation, the comet was 99.5 million miles from Earth and 115.3 million miles from the sun. Credit: NASA/Swift/Univ. of Leicester/Bodewits et al.
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The Swift spacecraft is doing double duty these days. Normally, the Gamma-ray Explorer satellite is on the lookout for high-energy outbursts and cosmic explosions. But now Swift is also monitoring Comet Lulin as it comes closer to Earth. For the first time, astronomers are seeing simultaneous ultraviolet and X-ray images of a comet. “The comet is releasing a great amount of gas, which makes it an ideal target for X-ray observations,” said Andrew Read, also at Leicester. And the ultraviolet data shows that Lulin is also shedding a huge amount of water, about 800 gallons of water each second!
“We won’t be able to send a space probe to Comet Lulin, but Swift is giving us some of the information we would get from just such a mission,” said Jenny Carter, at the University of Leicester, U.K., who is leading the study.
Comets are called “dirty snowballs,” as they are clumps of frozen gases mixed with dust. As comets venture near the sun, gas and dust are released. Comet Lulin, which is formally known as C/2007 N3, was discovered last year by astronomers at Taiwan’s Lulin Observatory. The comet is now faintly visible from a dark site. Lulin will pass closest to Earth — 38 million miles, or about 160 times farther than the moon — late on the evening of Feb. 23 for North America.
On Jan. 28, Swift trained its Ultraviolet/Optical Telescope (UVOT) and X-Ray Telescope (XRT) on Comet Lulin. “The comet is quite active,” said team member Dennis Bodewits, a NASA Postdoctoral Fellow at the Goddard Space Flight Center in Greenbelt, Md. “The UVOT data show that Lulin was shedding nearly 800 gallons of water each second.” That’s enough to fill an Olympic-size swimming pool in less than 15 minutes. Comet Lulin was passing through the constellation Libra when Swift imaged it. This view merges the Swift data with a Digital Sky Survey image of the star field. Credit: NASA/Swift/Univ. of Leicester/DSS (STScI, AURUA)/Bodewits et al.
Swift can’t see water directly. But ultraviolet light from the sun quickly breaks apart water molecules into hydrogen atoms and hydroxyl (OH) molecules. Swift’s UVOT detects the hydroxyl molecules, and its images of Lulin reveal a hydroxyl cloud spanning nearly 250,000 miles, or slightly greater than the distance between Earth and the moon.
The UVOT includes a prism-like device called a grism, which separates incoming light by wavelength. The grism’s range includes wavelengths in which the hydroxyl molecule is most active. “This gives us a unique view into the types and quantities of gas a comet produces, which gives us clues about the origin of comets and the solar system,” Bodewits explains. Swift is currently the only space observatory covering this wavelength range.
In the Swift images, the comet’s tail extends off to the right. Solar radiation pushes icy grains away from the comet. As the grains gradually evaporate, they create a thin hydroxyl tail.
Farther from the comet, even the hydroxyl molecule succumbs to solar ultraviolet radiation. It breaks into its constituent oxygen and hydrogen atoms. “The solar wind — a fast-moving stream of particles from the sun — interacts with the comet’s broader cloud of atoms. This causes the solar wind to light up with X rays, and that’s what Swift’s XRT sees,” said Stefan Immler, also at Goddard.
This interaction, called charge exchange, results in X-rays from most comets when they pass within about three times Earth’s distance from the sun. Because Lulin is so active, its atomic cloud is especially dense. As a result, the X-ray-emitting region extends far sunward of the comet.
“We are looking forward to future observations of Comet Lulin, when we hope to get better X-ray data to help us determine its makeup,” noted Carter. “They will allow us to build up a more complete 3-D picture of the comet during its flight through the solar system.”
The Bejar bolide photographed from Torrelodones, Madrid, Spain. The incoming fireball is the streak to the right of the floodlit house. The bright light at the top is the overexposed Moon. Credit: J. Perez Vallejo/SPMN.
Astronomers have analyzed the cometary fireball that blazed across the sky over Europe last year and concluded it was a dense object, about a meter (3.2 feet) across and with a mass of nearly two tons — large enough that some fragments probably survived intact and fell to the ground as meteorites.
Last July, people in Spain, Portugal and France watched the brilliant fireball produced by a boulder crashing down through the Earth’s atmosphere. In a paper to be published in the journal Monthly Notices of the Royal Astronomical Society, astronomer Josep M. Trigo-Rodríguez, of the Institute of Space Sciences in Spain, and his co-authors present dramatic images of the event. The scientists also explain how the boulder may originate from a comet which broke up nearly 90 years ago, and suggest that chunks of the boulder (and hence pieces of the comet) are waiting to be found on the ground.
“If we are right, then by monitoring future encounters with other clouds of cometary debris, we have the chance to recover meteorites from specific comets and analyse them in a lab,” Dr Trigo-Rodríguez said. “Handling pieces of comet would fulfil the long-held ambitions of scientists — it would effectively give us a look inside some of the most enigmatic objects in the Solar System.”
Fireballs (or bolides) are the name given by astronomers to the brightest meteors, popularly referred to as shooting stars. On the afternoon of July 11, a brilliant fireball was recorded over southwestern Europe. At maximum intensity, the object was more than 150 times brighter than the full Moon. It was first picked up at a height of 61 miles (98.3 km) and disappeared from view 13 miles (21.5 km) above the surface of the Earth, tracked by three stations of the Spanish Fireball Network above Bejar, near Salamanca in Spain. At the same time, a professional photographer took a picture of the fireball from the north of Madrid.
A close-up image of the Bejar bolide, photographed from Torrelodones, Madrid, Spain. Credit: J. Perez Vallejo/SPMN.
From these images, the astronomers have demonstrated that before its fiery demise, the boulder traveled on an unusual orbit around the Sun, which took it from beyond the orbit of Jupiter to the vicinity of Earth. This orbit is very similar to that of a cloud of meteoroids known as the Omicron Draconids, which on rare occasions produces a minor meteor shower and probably originates from the breakup of Comet C/1919 Q2 Metcalf in 1920. The authors suggest the boulder was once embedded in the nucleus of that comet.
Comet C/1919 Q2 Metcalf was discovered by Joel Metcalf from Vermont in August 1919, and was visible until February 3, 1920. The orbit was not well determined and no subsequent appearances are known. The Omicron Draconids meteor stream was discovered to be following a similar orbit to this comet by Allan F. Cook in 1973. The stream characteristically produces bright fireballs and rare meteor outbursts.
In the mid-1980s, the astronomers Tamas I. Gombosi and Harry L.F. Houpis first suggested that the nuclei of comets consist of relatively large boulders cemented together by a ‘glue’ of smaller particles and ice. If the rocky and icy nucleus of a comet disintegrates, then these large boulders are set loose into space. If the Bejar bolide was formed in this way, it confirms the glue model for at least some comets.
When Chi-Sheng Lin of Taiwan’s Institute of Astronomy captured three images on July 11, 2007 with something strange in them, it was first believed he’d picked up just another asteroid. But, by July 17 astronomers in Table Mountain Observatory, California were noticing a coma 2-3″ across, with a bright central core. That’s not an asteroid… That’s a comet! And now it’s a comet that’s doing something very strange…
By the end of 2008, Comet C/2007 N3 Lulin had steadily began to brighten and now is within easy reach of binoculars for all observers. How bright is it? At last estimate it is between magnitude 6 and 7. That means just a little too faint to be seen unaided, but bright enough to be spotted easily with just the slightest of visual aids. Our own Nancy A. did an article on this not long ago!
But there’s something going on with N3 Lulin, right now… Something very different. There’s a twist in the tail! Check this out…
Comet C/2007 N3 Lulin (negative luminance) - J. Brimacombe
While imaging N3 Lulin for UT Readers, Dr. Joe Brimacombe used a negative luminance frame to take a closer look at what’s going on and discovered something quite out of the ordinary. First off, you’ll notice an anti-tail – quite rare in itself – but if you take a look about halfway down the ion/dust tail, you’ll see a very definite twist in the structure. It it rotating? Exactly what’s causing it? Torsional stress? Is it possible that the kink in the tail is an instability resulting from currents flowing along the tail axis? Right now there’s absolutely no information available about what’s going on in the tail – because what you’re seeing is perhaps one of the most current pictures of the comet that can be found!
Chart Courtesy Heaven's AboveIf you’re interested in viewing Comet C/2007 N3 Lulin for yourself and would like some help locating it, there’s a wonderful resource that’s easy to use. Just go to Chris Peet’s Heaven’s Above website and make use of the tools there. It will give you easy to follow charts and all you need is just a pair of binoculars to spot this comet for yourself. Don’t sit inside… Do it!
My sincere thanks to Dr. Joseph Brimacombe of Northern Galactic for not only his superb imaging – but his sublime sense of curiosity which caught this anomaly!
Comet Lulin on January 11, 2009. Credit: Gregg Ruppel
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A new comet is swinging around the sun, and soon it will be more visible to stargazers, perhaps even with the naked eye. Both professional and amateur astronomers have been tracking this unusual comet, named Comet Lulin. Thanks to amateur astronomer Gregg Ruppel, who lives in the St. Louis, Missouri area for sharing images he has acquired of Comet Lulin. Gregg took the image above on January 11, 2009. The most interesting characteristic of this comet is its orbit. Lulin is actually moving in the opposite direction as the planets, so its apparent velocity will be quite fast. Estimates are it will be moving about 5 degrees a day across the sky, so when viewed with a telescope or binoculars, you may be able to see the comet’s apparent motion against the background stars. This is quite unusual! Today, January 14, the comet is at perihelion, closest to the sun. As it moves to its closest approach to Earth on February 24, Lulin is expected to brighten to naked-eye visibility in rural areas, (at best about magnitude 5 or 6) and will be observable low in the sky in an east-southeast direction before dawn.
Comet Lulin on January 8, 2009. Credit: Gregg Ruppel
The comet will pass 0.41 Astronomical Units from earth at its closest distance to Earth, about 14.5 times the distance between the Earth and the Moon. It has a parabolic trajectory, which means it may have never come this way before –this may be its first visit to the inner solar system
Lulin was jointly discovered by Asian astronomers in July of 2007. Quanzhi Ye from China first saw the comet on images obtained by Chi-Sheng Lin from Taiwan, at the Lu-lin Observatory.
The discovery of Comet Lulin (also known as C/2227 2007 N3) was part of the Lulin Sky Survey project to explore the various populations of small bodies in the solar system, especially objects that could be a hazard to the Earth.
It has both a tail and an anti-tail, visible in this image. Lulin's Tails. Credit: Gregg Ruppel.
Artist rendering of Stardust-NeXT spacecraft approaching Earth's gravitational pull, resulting in accelerating of spacecraft and bending of flight path. Courtesy: NASA
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Remember the Stardust mission that returned samples of comet dust back to Earth in 2006? The spacecraft dropped off a capsule containing samples of a comet’s coma and interstellar dust particles, but the spacecraft “bus” is still out there in an elongated orbit of the sun. It will come home again, swinging by Earth on January 14, at 19:40 UTC (12:40 pm PST), getting a gravity assist from the home planet as it flies approximately 5713 miles (9200 kilometers) from the Earth’s surface. But the spacecraft isn’t just wandering the solar system with nothing to do. It has a new job and a new mission. Called Stardust NExT, (New Exploration of Tempel 1) the spacecraft will re-survey comet Tempel 1 – the comet that the Deep Impact mission left a mark on — encountering the comet on Feb. 14, 2011.
And remember aerogel – the wispy material that collected the comet dust? Turns out this stuff can come home, too: into homes and other buildings as a super-insulating material. Engineers say using aerogel as an insulator can increase the thermal insulation factor of a wall by over 40%!
Lightweight, wispy aerogel. Credit: NASA
If you’ve ever had the chance to handle aerogel, you know it’s really weird stuff. It’s fragile, but it’s also strong. You can crush it easily in your hand, but it has just the right qualities to be able to capture dust particles zooming in space at extremely high speeds without breaking, and was “gentle” enough to preserve the particles. Engineers say the aerogel insulation technology developed by NASA, is the highest insulating material in existence, and the company Thermablok(TM) developed an amazing product that may soon become a requirement in the building industry.
Aerogel, also referred to as “frozen smoke,” has been difficult to adapt to most uses because it’s so fragile The patented Thermablok material however overcomes this by using a unique fiber to suspend a proprietary formula of Aerogel such that it can be bent or compressed while still retaining its amazing insulation properties. Aerogel-based insulation.
Aerogel material is 95% air, and just a 1/4″ x 1-1/2″ (6.25mm x 38mm) strip of Thermablok(TM) added to each stud in a wall before putting on drywall, breaks the “thermal bridging,” increasing the thermal insulation factor of a wall by 42%.
The U.S. Department of Energy has verified the findings on the producst’s thermal capability. Plus its recyclable, fire resistant and not affected by water (so no mold).
Speaking of recyclable, NASA’s plans for the Stardust spacecraft to revisit Tempel 1 will finish the investigation begun in 2005 when the Deep Impact mission blasted a crater into the comet. “The crater’s there,” said Joseph Veverka, Professor of Astronomy at Cornell University and Principal Investigator of Stardust-NExT, “but we’ve never seen it.” That’s because the cloud of material ejected from the crater obscured the Deep Impact spacecraft’s view. By the time the particles slowly settled back down to the comet’s surface, the spacecraft, traveling at about 10 km (about 6 miles) per second, was gone.
Looking into the crater with Stardust-NExT will provide mankind’s first view of a comet’s internal structure, information that is not only scientifically interesting, but vital to our future ability to keep a comet from hitting the Earth. Even the size of the crater will be revealing. “That will tell us the mechanical properties of the subsurface of the comet,” Veverka said. “In other words, how does the comet respond to impacts? And that’s one of the fundamental things that you’d need to know if you were trying to blow up a comet or push it out of the way.”
Stardust was originally launched in 1999, and in January 2004, the spacecraft performed a risky and historic flyby of Comet Wild 2 to capture the samples and take pictures of the comet’s nucleus.
[/caption]According to 2012 doomsday proponents, something big is out to get us. By “something big” I mean some uncontrollable cosmic entity (i.e. Planet X, Nibiru or a “killer” solar flare), and by “us” I mean the whole of planet Earth. Pinning 2012 doomsday scenarios on the end of the ancient Mayan “Long Count” calendar appears to be growing momentum amongst authors, websites, documentaries and (my personal favourite) YouTube videos. According to them, something bad is going to happen on or around December 21st 2012. Probably the most interesting difference between the 2012 doomsday scenario and the doomsday prophecies of the past is that almost every possible (and impossible… or implausible) harbinger of doom is being suggested as a planet killer.
So, in this sixth article addressing another astronomical doomsday scenario, I will look at the theory that there is a comet currently out there in deep space, slowly making its final approach on its parabolic orbit toward Earth. But before you get worried, you’ll be glad to hear that the 2012 cometary impact theory is as watertight as a teabag; there is no object observed out there and there is certainly no evidence to suggest there could be a comet impact in 2012… and here’s why…
Marketing Doomsday
In four years today (2012 21-December), the world will be coming to an end according to a few misguided individuals. Doomsayers always begin their arguments using an ancient calendar (plus a heavy dose of Bible Codes, I Ching and ancient Sumerian cuneiform scriptures) to support their new and inventive way the world may end. Alas, most doomsday theories are based on over-hyped scientific misinterpretation and outright lies. Usually there is a book to sell or website to promote. After all, there is nothing more profitable than fear.
Interestingly, I started writing for the Universe Today a year ago today, exactly five years before the end of the Mayan Long Count calendar. Don’t go reading too much into this little fact, pure coincidence, but I think it would be fitting to write the sixth in my series of 2012 articles exposing the myths surrounding this date.
You’ve probably seen the prolific ads for the “2012 Comet” across a range of websites, so I decided to delve into this particular theory to see if there is any truth behind the claims that a comet (or “comet planet”) is approaching Earth on a collision course. To cut a long story short, I can categorically say that no cometary impact is imminent. Any accusations of government cover-ups are to hide the poor science doomsayers are citing (much like the Planet X/Nibiru connection). If you want the long story, read on…
The Comet Threat NASA's Deep Impact probe hits Comet Tempel 1 (NASA)Before we look at the claims behind this doomsday scenario, we must first study Earth’s risk of actually being hit by a comet. We know we’ve been hit by comets in the past, and we will most definitely be hit by more in the future, but the coast is clear for at least a few decades from a marauding comet or asteroid. In fact, meteoroids in the form of chunks of rock are far more numerous than icy comets, and we are hit by several sizeable rocky meteoroids throughout the year (take 2008 TC3, the first predicted meteoroid atmospheric impact, for example).
Although rare, planetary impacts by comets do happen. As Shoemaker-Levy 9 showed us in 1994 when 2km-wide fragments of the comet bombarded the Jovian atmosphere, we mustn’t be complacent when considering a large impact event by comets or asteroids. The dazzling light show by Shoemaker-Levy 9 actually stimulated efforts to increase sky surveys for a possibly catastrophic impact event. Although a vast number of near-Earth objects (NEOs) have been identified, a very small number are considered to be a risk.
Fragments of Shoemaker-Levy 9 (NASA/HST)
The 270 meter-wide asteroid 99942 Apophis caused a stir in 2006 when it became the highest ranking asteroid on the Torino impact hazard scale. Apophis is now expected to glide safely past the Earth in 2029, but depending on the gravitational deflection caused by Earth in 2029, Apophis could pass through a gravitational “keyhole”, creating another impact possibility on April 13th, 2036. Still, the odds are not worth betting on; would you put money on a 1 in 45,000 chance of an Apophis 2036 impact?
There are other lumps of rock out there, but most are benign, and certainly not a threat to everyday life in 2012. However, we must be aware that asteroids are a very real future threat to humanity. As a result of this increased awareness, other NEOs have been discovered and tracked. Objects such as 2007 VK184, a 130 meter-wide asteroid may cause problems in the distant future, but the probability of impact is still extremely low. Astronomers from the Catalina Sky Survey estimate a few possible impact dates for 2007 VK184, but the odds never exceed a 0.037% chance of hitting Earth in the next 100 years. Other asteroids are currently being tracked and they may cause some concern over a century from now (although none surpass a Torino scale of Level 1, and if they do, all tend to fall back to the “normal” Level 0).
In short, the skies are clear from any imminent (certainly within the next 4 years) impact from an asteroid. Comets do not feature as a significant risk either. There is no astronomical evidence supporting otherwise.
This doesn’t stop organizations such as ex-NASA astronaut Rusty Schweickart’s B612 Foundation from planning for possible future asteroid/comet threats. While Hollywood movies would have us believe blowing a comet up with a nuclear bomb will be a very good idea, the B612 Foundation disagrees. In fact, it could be a very bad idea. The key thing to remember when reading about NEO surveys or asteroid/comet deflection techniques is that we need a lot of lead time to stand any hope of deflecting a possible catastrophic impact event. This does not indicate a concern in the near future, it is simply a prudent precaution to safeguard the distant future of our planet.
The 2012 Comet-Google Conspiracy The evidence for a comet... or Planet X... whichever. The void in Google Sky, are they hiding something? (Google)So, it looks like we are safe from any astronomical impact. That’s not to say we won’t be hit by a small meteoroid, large fireballs occur regularly (remember the November 21st Canada bolide, and the most recent December 6th Colorado fireball, the largest of which was possibly caused by a 10-tonne rocky meteoroid). Also, this is not to say we won’t discover more NEOs within the next four years (we could spot a threatening object tomorrow for all we know), but the point is that there is absolutely no evidence for any civilization-ending comet impact in 2012. Any claims to the contrary are completely false.
So why are we seeing ads touting the “2012 Comet” theory? As far as I can tell, it is based on one very flimsy piece of evidence. So, lets load up Google Earth to see where the problem is…
If you have Google Earth installed on your computer, you have the ability to look “up” rather than just down at the Earth’s surface. Switching the software to the night sky allows you to see the constellations and will guide you and a dazzling tour of the observable Universe. Despite this overload of information, is Google hiding something? Is the huge search-engine based company actively trying to hide observations of a comet from us?
Using Google Earth data for optical, infrared (IRAS) and microwave (WMAP) surveys (Google)
Guide Google Earth to RA:5h 54m 00s, Dec: -6° 00′ 00″ and zoom in. If you don’t have Google Earth, this region can also be found in the online version of Google Sky. You’ll see an ominous rectangular void (a.k.a. the “Google Anomaly” in the images above) right next to the Orion Nebula, south of Orion’s Belt.
Note: The Constellation of Orion and therefore the “Google Anomaly” is in a very conspicuous location of the night sky, observable from northern and southern hemispheres.
This void is only apparent in the optical data; if you switch the data set to the microwave survey carried out by the Wilkinson Microwave Anisotropy Probe (WMAP), you’ll find the void is replaced by data. Also, infrared data covers the region pretty nicely.
Note: This infrared view of the sky was observed by the Infrared Astronomical Satellite (IRAS).
So, the theory is that Google is hiding observations of an incoming comet. But there is an added twist to the comet conspiracy theory; the comet is also referred to as a “comet planet” and therefore a Planet X candidate (but I thought Planet X was a brown dwarf candidate?). Yes, Planet X seems to be at the root of all doomsday scenarios.
I’ll try to make this as quick as possible:
1) IRAS Data A popular image on Planet X websites. Is this Planet X, or is it simply a young galaxy? (NASA - possible source)The Infrared Astronomical Satellite (IRAS) was an orbiting telescope that lasted for 10 months in 1983. It performed an infrared survey of the entire sky, churning up some fantastic observations of ultra-luminous young galaxies and intergalactic “cirrus”. However, before these objects were formally identified, the media (in particular the Washington Post) hinted heavily that some of these objects could be the fabled “Planet X” in the outskirts of our Solar System. This is one of the key theories doomsayers cite as fact that Planet X exists. Using dubious logic, several authors claim these early observations prove that Planet X is in fact the Sumerian planet “Nibiru”. Nibiru is therefore a brown dwarf. In this theory, death and destruction quickly follows, including the appearance of an alien race called the Annunaki (our alien ancestors) who want their planet back. Wonderful science fiction, with no roots in science fact.
So, is this “2012 comet” actually Planet X? If it is (disregarding the obvious fact that a comet is not a planet, let alone a brown dwarf), why is the Google Anomaly only a void patch in optical data? If Google and NASA were trying to hide evidence of a “comet” (by removing a region of optical data), surely they’d remove the IRAS data too? In any case, the IRAS data shows no object within the anomaly. Besides, why would Google leave a very obvious patch of missing optical data, when they could have just airbrushed the object from the dataset?
In conclusion, the Google Anomaly is in fact missing data, pure and simple. There’s no comet there, and simply because there is missing data does not prove the existence of anything sinister.
2) Just Look Up
Just in case you needed a little more convincing that the 2012 comet/Planet X theory was complete bunkum, think about the location of this proposed comet. The Google Anomaly is in full view for most of the planet throughout the year as it is in the constellation of Orion, right in the neighbourhood of some of the most famous, and well-studied stars and nebulae (the Horsehead Nebula and the Great Orion Nebula for example). If anyone looks at the Google Anomaly with suspicion, why not look straight up and see for yourself? Amateur astronomers have access to very advanced optics, so I think that if there was a suspect “comet planet” in the region, it would have been spotted by now (without Google’s help).
In Conclusion
The truth is that the Planet X conspiracy theory is wrong, but the 2012 comet theory is even worse. The chances of a large planet swinging through the inner solar system in 2012 has the same odds as a comet impact on that date: nil.
We cannot predict the future, and no ancient prophecy will prove the existence of a modern astronomical “end of the world” scenario. I am sure 2012 will be a significant year for spiritual and religious reasons, I’m not debating that. However, for doomsayers to use modern science to prove their inaccurate doomsday creations for personal gain is not only irresponsible, it can be very damaging.
Swift's Ultraviolet/Optical Telescope (UVOT) captured Comet 73P/Schwassmann-Wachmann 3's fragment C as it passed the famous Ring Nebula (oval, bottom) on May 7, 2006 (NASA)
[/caption]Things appeared to get a little strange in the field of X-ray astronomy when the NASA/ESA ROSAT observatory started seeing emissions from a series of comets. This discovery in 1996 was a conundrum; how could X-rays, more commonly associated with hot plasmas, be produced by some of the coldest bodies in the Solar System? In 2005, NASA’s Swift observatory was launched to look out for some of the most energetic events in the observable Universe: gamma-ray bursts (GRBs) and supernovae. But in the last three years, Swift has also proven itself to be an expert comet hunter.
If X-rays are usually emitted by multi-million Kelvin plasmas, how can X-rays possibly be generated by comets composed of ice and dust? It turns out there is an interesting quirk as comets interact with the solar wind within 3AU from the solar surface, allowing instrumentation designed to observe the most violent explosions in the Universe to also study the most elegant objects closer to home…
“It was a big surprise in 1996 when the NASA-European ROSAT mission showed that comet Hyakutake was emitting X-rays,” said Dennis Bodewits, NASA Postdoctural Fellow at the Goddard Space Flight Centre. “After that discovery, astronomers searched through ROSAT archives. It turns out that most comets emit X-rays when they come within about three times Earth’s distance from the sun.” And it must have been a very big surprise for researchers who assumed ROSAT could only be used to glimpse the transient flash of a GRB or supernova, possibly spawning the birth of black holes. Comets simply did not feature in the design of this mission.
However, since the launch of another GRB hunter in 2005, NASA’s Swift Gamma-ray Explorer has spotted 380 GRBs, 80 supernovae and… 6 comets. So how can a comet possibly be studied by equipment intended for something so radically different?
As comets begin their death-defying sunward orbit, they heat up. Their frozen surfaces begin to blast gas and dust into space. Solar wind pressure causes the coma (the comet’s temporary atmosphere) to eject gas and dust behind the comet, away from the Sun. Neutral particles will be carried away by solar wind pressure, whereas charged particles will follow the interplanetary magnetic field (IMF) as an “ion tail”. Comets therefore can often be seen with two tails, a neutral tail and an ion tail.
This interaction between the solar wind and comet has another effect: charge exchange.
The principal of charge exchangeEnergetic solar wind ions impact the coma, capturing electrons from neutral atoms. As the electrons become attached to their new parent nuclei (the solar wind ion), energy is released in the form of X-rays. As the coma can measure several thousand miles in diameter, the comet atmosphere has a huge cross section, allowing a vast number of these charge exchange events to occur. Comets suddenly become significant X-ray generators as they get blasted by solar wind ions. The total power output from the coma can top a billion Watts.
Charge exchange can occur in any system where a hot stream of ions interact with a cooler neutral gas. Using missions such as Swift to study the interaction of comets with the solar wind can provide a valuable laboratory for scientists to understand otherwise confusing X-ray emissions from other systems.
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All comets are about the same, right? Not necessarily. Astronomer David Schleicher has been studying 150 comets, measuring the abundances of five different molecules in each comet. One of these comets, Comet 96P/Machholz 1 was different from all the rest, showing an extremely unusual chemistry. Schleicher believes the anomalous composition may reveal the existence of a new class of comets. What makes Machholz 1 different is that the molecule cyanogen, CN, is extremely depleted. In Machholz 1, CN is missing by about a factor of 72 from the average of other comets, i.e., only a little above one percent of normal. “This depletion of CN is much more than ever seen for any previously studied comet, and only one other comet has even exhibited a CN depletion,” said Schleicher. The cause of the chemical anomaly is unknown.
However Schleicher, a planetary astronomer at the Lowell Observatory has come up with three intriguing scenarios to explain origins of Machholz 1, and each one will yield important but differing new constraints on the formation or evolution of comets.
One possible explanation is that Machholz 1 did not originate in our Solar System, but instead escaped from another star. In this scenario, the other star’s proto-planetary disk might have had a lower abundance of carbon, resulting in all carbon-bearing compounds having lower abundances. “A large fraction of comets in our own Solar System have escaped into interstellar space, so we expect that many comets formed around other stars would also have escaped,” said Schleicher. “Some of these will have crossed paths with the sun, and Machholz 1 could be an interstellar interloper.”
Another possible explanation for Machholz 1’s anomalous composition is that it formed even further from the sun in a colder or more extreme environment than any other comet we have studied thus far. If this was the case, then the scarcity of such objects is likely associated with the significant difficulty of explaining how such comets moved into the inner solar system where they can then be discovered and observed.
A third possibility is that Machholz 1 originated as a carbon-chain depleted comet but that its chemistry was subsequently altered by extreme heat. While no other comet has exhibited changes in chemistry due to subsequent heating by the sun, Machholz 1 has the distinction of having an orbit that now takes it to well inside Mercury’s orbit every five years. (Other comets get even closer to the sun, but not as often). “Since its orbit is unusual, we must be suspicious that repeated high temperature cooking might be the cause for its unusual composition,” said Schleicher. “However, the only other comet to show depletion in the abundance of CN did not reach such high temperatures. This implies that CN depletion does not require the chemical reactions associated with extreme heat.”
Although comet 96P/Machholz 1 was first sighted in 1986 and orbits the sun with a period of slightly over five years, compositional measurements only took place during the comet’s recent 2007 apparition. Lowell Observatory’s program of compositional studies, currently headed by Schleicher, includes measurements of over 150 comets obtained during the past 33 years. This research is unique because it compares and contrasts Machholz 1 against this large database of 150 comets.
Currently there are two types of comets, these being identified by a program at the Lowell Obervatory in the early 1990s. One class, containing the majority of observed comets, has a composition called “typical.” Most members of this typical class have long resided in the Oort Cloud at the very fringes of our Solar System but are believed to have originally formed amidst the giant planets, particularly between Saturn, Uranus, and Neptune. Other members of this compositional class arrived from the Kuiper Belt, located just beyond Neptune.
The second compositional class of comets has varying depletions in two of the five chemical species measured. Since both depleted molecules, C2 and C3, are wholly composed of carbon atoms, this class was named “carbon-chain depleted.” Moreover, nearly all comets in this second class have orbits consistent with their having arrived from the Kuiper Belt. For this and other reasons, the cause of the depletion is believed to be associated with the conditions that existed when the comets formed, perhaps within an outer, colder region of the Kuiper Belt.
Comets are widely thought to be the most pristine objects available for detailed study remaining from the epoch of Solar System formation. As such, comets can be used as probes of the proto-planetary material that was incorporated into our Solar System. Differences in the current chemical composition among comets can indicate either differences in primordial conditions or evolutionary effects.
Although the location of origin cannot be definitively determined for any single comet, Machholz 1’s short orbital period means that astronomers can search for additional carbon-bearing molecular species during future apparitions. “If additional carbon-bearing species are also strongly depleted, then the case for its origin outside of our Solar System would be strengthened,” said Schleicher. The next opportunity for observations will be in 2012.
The study is published in the November issue of the Astronomical Journal.
Pan-STARRS 1 prototype, part of the Panoramic Survey Telescope and Rapid Response System, Haleakala mountain, Maui. Photo / MIT Lincoln Laboratory
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A prototype telescope with an enhanced ability to find moving objects will soon be operational, and its mission will be to detect asteroids and comets that could someday pose a threat to Earth. The system is called Pan-STARRS (for Panoramic Survey Telescope and Rapid Response System) located on Haleakala mountain in Maui,Hawaii, and is the first of four telescopes that will be housed together in one dome. Pan-STARRS will feature the world’s largest and most advanced digital camera, providing more than a fivefold improvement in the ability to detect Near Earth Asteroids and comets. “This is a truly giant instrument,” said University of Hawaii astronomer John Tonry, who led the team developing the new 1.4-gigapixel camera. “We get an image that is 38,000 by 38,000 pixels in size, or about 200 times larger than you get in a high-end consumer digital camera.” The Pan-STARRS camera will cover an area of sky six times the width of the full moon and it can detect stars 10 million times fainter than those visible to the naked eye.
The Lincoln Laboratory at the Massachusetts Institute of Technology (MIT) developed charge-coupled device (CCD) technology is a key enabling technology for the telescope’s camera. In the mid-1990s, Lincoln Laboratory researchers developed the orthogonal-transfer charge-coupled device (OTCCD), a CCD that can shift its pixels to cancel the effects of random image motion. Many consumer digital cameras use a moving lens or chip mount to provide camera-motion compensation and thus reduce blur, but the OTCCD does this electronically at the pixel level and at much higher speeds.
The challenge presented by the Pan-STARRS camera is its exceptionally wide field of view. For wide fields of view, jitter in the stars begins to vary across the image, and an OTCCD with its single shift pattern for all the pixels begins to lose its effectiveness. The solution for Pan-STARRS, proposed by Tonry and developed in collaboration with Lincoln Laboratory, was to make an array of 60 small, separate OTCCDs on a single silicon chip. This architecture enabled independent shifts optimized for tracking the varied image motion across a wide scene.
“Not only was Lincoln the only place where the OTCCD had been demonstrated, but the added features that Pan-STARRS needed made the design much more complicated,” said Burke, who has been working on the Pan-STARRS project. “It is fair to say that Lincoln was, and is, uniquely equipped in chip design, wafer processing, packaging, and testing to deliver such technology.”
The primary mission of Pan-STARRS is to detect Earth-approaching asteroids and comets that could be dangerous to the planet. When the system becomes fully operational, the entire sky visible from Hawaii (about three-quarters of the total sky) will be photographed at least once a week, and all images will be entered into powerful computers at the Maui High Performance Computer Center. Scientists at the center will analyze the images for changes that could reveal a previously unknown asteroid. They will also combine data from several images to calculate the orbits of asteroids, looking for indications that an asteroid may be on a collision course with Earth.
Pan-STARRS will also be used to catalog 99 percent of stars in the northern hemisphere that have ever been observed by visible light, including stars from nearby galaxies. In addition, the Pan-STARRS survey of the whole sky will present astronomers with the opportunity to discover, and monitor, planets around other stars, as well as rare explosive objects in other galaxies.
The astronomy world buzzed in the Fall of 2007 when Comet Holmes – a normally humdrum, run-of-the-mill comet — unexpectedly flared and erupted. Its coma of gas and dust expanded away from the comet, extending to a volume larger than the Sun. Professional and amateur astronomers around the world turned their telescopes toward the spectacular event. Everyone wanted to know why the comet had suddenly exploded. The Hubble Space Telescope observed the comet, but provided few clues. And now, observations taken of the comet after the explosion by NASA’s Spitzer Space Telescope deepen the mystery, showing oddly behaving streamers in the shell of dust surrounding the nucleus of the comet. The data also offer a rare look at the material liberated from within the nucleus. “The data we got from Spitzer do not look like anything we typically see when looking at comets,” said Bill Reach of NASA’s Spitzer Science Center at Caltech.
Every six years, comet 17P/Holmes speeds away from Jupiter and heads inward toward the sun, traveling the same route typically without incident. However, twice in the last 116 years, in November 1892 and October 2007, comet Holmes exploded as it approached the asteroid belt, and brightened a millionfold overnight.
In an attempt to understand these odd occurrences, astronomers pointed NASA’s Spitzer Space Telescope at the comet in November 2007 and March 2008. By using Spitzer’s infrared spectrograph instrument, Reach and his colleagues were able to gain valuable insights into the composition of Holmes’ solid interior. Like a prism spreading visible-light into a rainbow, the spectrograph breaks up infrared light from the comet into its component parts, revealing the fingerprints of various chemicals. The Spitzer Space Telescope. Credit: NASA
In November of 2007, Reach noticed a lot of fine silicate dust, or crystallized grains smaller than sand, like crushed gems. He noted that this particular observation revealed materials similar to those seen around other comets where grains have been treated violently, including NASA’s Deep Impact mission, which smashed a projectile into comet Tempel 1; NASA’s Stardust mission, which swept particles from comet Wild 2 into a collector at 13,000 miles per hour (21,000 kilometers per hour), and the outburst of comet Hale-Bopp in 1995.
“Comet dust is very sensitive, meaning that the grains are very easily destroyed, said Reach. “We think the fine silicates are produced in these violent events by the destruction of larger particles originating inside the comet nucleus.”
When Spitzer observed the same portion of the comet again in March 2008, the fine-grained silicate dust was gone and only larger particles were present. “The March observation tells us that there is a very small window for studying composition of comet dust after a violent event like comet Holmes’ outburst,” said Reach.
Comet Holmes not only has unusual dusty components, it also does not look like a typical comet. According to Jeremie Vaubaillon, a colleague of Reach’s at Caltech, pictures snapped from the ground shortly after the outburst revealed streamers in the shell of dust surrounding the comet. Scientists suspect they were produced after the explosion by fragments escaping the comet’s nucleus.
In November 2007, the streamers pointed away from the sun, which seemed natural because scientists believed that radiation from the sun was pushing these fragments straight back. However, when Spitzer imaged the same streamers in March 2008, they were surprised to find them still pointing in the same direction as five months before, even though the comet had moved and sunlight was arriving from a different location. “We have never seen anything like this in a comet before. The extended shape still needs to be fully understood,” said Vaubaillon.
He notes that the shell surrounding the comet also acts peculiarly. The shape of the shell did not change as expected from November 2007 to March 2008. Vaubaillon said this is because the dust grains seen in March 2008 are relatively large, approximately one millimeter in size, and thus harder to move.
“If the shell was comprised of smaller dust grains, it would have changed as the orientation of the sun changes with time,” said Vaubaillon. “This Spitzer image is very unique. No other telescope has seen comet Holmes in this much detail, five months after the explosion.”
“Like people, all comets are a little different. We’ve been studying comets for hundreds of years — 116 years in the case of comet Holmes — but still do not really understand them,” said Reach. “However, with the Spitzer observations and data from other telescopes, we are getting closer.”
