The Solar System often throws up surprises for astronomers, but the recent discovery of a 2- to 3-km wide asteroid called 2009 HC82 has sent observers in a spin. A retrograde spin to be precise.
This particular near-Earth asteroid (NEO) should have already been spotted as it has such a strange orbit. It is highly inclined, making it orbit the Sun backwards (when compared with the rest of the Solar System’s planetary bodies) every 3.39 years. What’s more, it ventures uncomfortably close (3.5 million km) to the Earth, making this NEO a potentially deadly lump of rock…
2009 HC82 was discovered on April 29th by the highly successful Catalina Sky Survey, and after independent observations by five different groups, it was determined that the asteroid has an orbit of 3.39 years and that its orbit is very inclined. So inclined in fact that the asteroid’s orbit takes it well out of the Solar System ecliptic at an angle of 155°. Inclined orbits aren’t rare in themselves, but if you find an asteroid with an inclination of more than 90°, you are seeing a very rare type of object: a retrograde asteroid.
The last time I wrote about a retrograde asteroid was back in September 2008 (Kuiper Belt Object Travelling the Wrong-Way in a One-Way Solar System), when a University of British Columbia researcher spotted a rather unique retrograde Kuiper belt object (called 2008 KV42) that had a large looping orbit with an inclination larger than 90°. It was nicknamed “Drac” after Dracula’s ability to walk on walls.
2009 HC82 is therefore not only rare, it is also very strange. It orbits the Sun the wrong way (therefore making it very inclined), it is a potentially hazardous NEO (it is smaller than the 10 km asteroid that is attributed to wiping out the dinosaurs, but it would cause significant devastation on a global scale if it did hit us) and it is very eccentric.
The orbit of 2009 HC82 (NASA)
All these orbital components have led to speculation that 2009 HC82 is in fact a “burnt out” comet. Comets originate from the Oort Cloud, a theoretical region cometary nuclei that occasionally gets nudged by gravitational disturbances when stars pass by. The Oort Cloud is not restricted to a belt along the ecliptic (like the asteroid belt or the Kuiper belt), it encapsulates our Solar System. Therefore, this may explain 2009 HC82’s bizarre trajectory; it was a comet, but all the ice has vaporized, leaving a rocky core to fling around the Sun on a death-defying orbit, buzzing the inner Solar System.
Brian Marsden of the Minor Planet Center agrees that some retrograde asteroids could be burnt-out comets. The size and shape of the new asteroid’s orbit “is very like Encke’s comet except for inclination,” he said, but the only difference is the fact that 2009 HC82 has no cometary tail.
More observations are needed before a definitive conclusion can be made, but Marsden is confused as to why this object has not been discovered before now. “It should have been easily observable in 2000,” says Marsden. “Why wasn’t it seen then?”
It is hoped further investigation may answer this question…
The black arrows indicate the orientation of chevrons along the southern coast of Madagascar, but the white arrows indicate what computer models say should have been the orientation if they were caused by the impact of a space body in the Indian Ocean. Credit: Robert Weiss
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Coastal formations called chevrons, large U- or V-shaped features found on coastlines around the world were originally thought to be evidence of ancient “megatsunamis” caused by asteroids or comets slamming into the ocean. However, new research using Google Earth and computer models to recreate large wave action refutes that school of thought.
The theory of chevrons being created by tsunamis was proposed in 2006 after the structures were found in Egypt and the Bahamas. Some were, at places, between several hundred meters- and a kilometer-wide. Since they were also found to exist in Australia and Madagascar, some geologists formed the hypothesis that they were sediment cones left behind by large tsunamis, perhaps up to ten times stronger than the devastating tsunami in the Indian Ocean in December 2005.
The theory propsed the only source for such a megatsunami was a meteor impact, occurring about 5,000 years ago.
But a new study, led by Jody Bourgeois, a geologist and tsunami expert at the University of Washington, argues that this theory is simply “nonsense. For example, she said, there are numerous chevrons on Madagascar, but many are parallel to the coastline. Models created by Bourgeois’ colleague Robert Weiss show that if they were created by tsunamis they should point in the direction the waves were travelling, mostly perpendicular to the shore. Landsat image of the Fenambosy Chevrons in Madagascar by USGS. The open side of these chevrons point directly at a crater at the bottom of the Indian Ocean. They suggest a gigantic meteor impact occurred about 4800 years ago. But new research says chevrons were likely formed by wind.
“And if it really was from an impact, you should find evidence on the coast of Africa too, since it is so near,” she said.
By using Google Earth, Bourgeois and her team searched for chevrons and surprisingly they found some in desert areas, well inland and away from the shores.
“The extraordinary claim of ‘chevron’ genesis by megatsunamis cannot withstand simple but rigorous testing. There are the same forms in the Palouse in eastern Washington state, and those are clearly not from a tsunami,” Bourgeois said.
She believes the structures were formed by wind.
The discovery of marine fossils in some chevron formations seems to support the idea that a wave created the deposit, but Bourgeois discounts that evidence also.
“Marine fossils can get into non-marine deposits. It’s not uncommon. You only have to change sea level a little bit or have them wash up on a beach in a storm,” she said. “And some marine organisms can be carried by the wind. I am convinced these are largely wind-blown deposits.”
She noted that similar deposits have been seen on the Kamchatka Peninsula on Russia’s east coast, where she has conducted research for more than a decade.
“Those are made of volcanic ash, and they are not near the coast at all, yet they look very similar to these coastal chevrons,” Bourgeois said.
In 1979, the huge Chicxulub crater, measuring about 180 km (112 miles) in diameter, was discovered on the northern Yucatan Peninsula, Mexico. Scientists made the obvious conclusion that something rather large had hit the Earth in this location, probably causing all kinds of global devastation 65 million years ago. At around the same time, 65% of all life on the face of the planet was snuffed out of existence. The dinosaurs that roamed the planet up to that point were no more.
The timing of asteroid impact and the time of the mass extinction was too much of a coincidence to be ignored. When particles from the asteroid impact were discovered just below the Cretaceous-Tertiary (K-T) boundary, there was a strong causal link: the effects of the asteroid impact had driven the dinosaurs to extinction.
However, a problem with this theory has come to light. It turns out the Chicxulub impact may pre-date the K-T boundary by 300,000 years…
A number of scientists have disagreed with the theory that the Chicxulub impact caused the death of the dinosaurs 65 million years ago, and this newest research appears to show the two events may not be linked after all.
Gerta Keller of Princeton University in New Jersey, and Thierry Adatte of the University of Lausanne, Switzerland, are set to publish this new work in the Journal of the Geological Society today, using data from the analysis of sediment from Mexico to prove the asteroid impact pre-dated the K-T boundary by as much as 300,000 years.
“We know that between four and nine meters of sediments were deposited at about two to three centimeters per thousand years after the impact,” said Keller. “The mass extinction level can be seen in the sediments above this interval.”
This means that the mass extinctions appeared to take place a long time after the impact. However, impact-extinction advocates point out that this inconsistency in sediment data is probably down to sediment disruption by tsunamis and geological upheaval immediately after the impact.
According to Keller, there is no indication that this could be the case. Deposition of impact sediment occurred over a huge time period, not the hours or days deposition would have taken if a tsunami affected sedimentary records.
Another problem with the impact extinction theory is that the Chicxulub impact may not have had the radical extinction effect on plants and animals as we previously thought. The researchers found a total of 52 fossilized species that appeared to be happily living before the layer of impact sediment… and the same 52 species appeared to by happily living after the layer of impact sediment.
“We found that not a single species went extinct as a result of the Chicxulub impact.” — Gerta Keller
Although this is some very interesting research, sure to turn dinosaur extinction theory on its head, if an asteroid didn’t kill the dinosaurs, what did?
Keller points the finger at volcanic activity. Massive amounts of dust and gas was released from eruptions at the Deccan Traps in India 65 million years ago, possibly plunging the planet into a prolonged period without Sun.
Update: With any scientific debate, there are details behind new research that may not be immediately apparent. As Ethan Siegel highlights in a recent ScienceBlogs article (What Wiped Out The Dinosaurs?, April 27th), the evidence for an asteroid impact wiping out the dinosaurs is overwhelming. Just because there appears to be a discrepancy in the location of impact sediment and K-T boundary does not mean the impact-extinction theory is wrong in any way. Keller’s research is an interesting investigation, worthy of further study, but this doesn’t change the fact that huge global damage would have been caused by the Chicxulub impact. This remains the prime candidate as to why the dinosaurs were suddenly made extinct 65 million years ago.
If Earthlings discovered a large asteroid heading towards our planet, how would we react? But more importantly would the space agencies and/or world governments be prepared for such an event? “Mankind is now technically able to predict, sometimes several decades in advance, the trajectory of Near Earth Objects (NEOs),” said Frans von der Dunk, professor of space law at the University of Nebraska-Lincoln. “Additionally, existing space technology could deflect the vast majority of threatening asteroids.” But even if a threatening object is discovered, von der Dunk said no mechanism exists for effective international decision-making on how to deal with a threat. To examine these issues, UNL hosted a conference on April 23 & 24, “Near-Earth Objects: Risks, Responses and Opportunities,” to look at the legal and institutional challenges of creating an international protocol of dealing with NEOs.
NEOs are an increasing area of concern among the world’s space scientists. Many experts believe that over the next 15 years, advances in technology will allow for the detection of more than 500,000 NEOs – and of those, several dozen will likely pose an uncomfortably high risk of striking Earth and inflicting local or regional damage.
Concept for a possible gravity tractor. Credit: JPL
Right now, if an Earth-bound asteroid were discovered, we have the technology today to send a spacecraft to an asteroid to act as a gravity tractor, or to impact the asteroid to alter the space rock’s trajectory. Other current options are to use a mass driver, rocket engines or a solar sail to push the asteroid on a different course.
But, von der Dunk told Universe Today, completely lacking is an official structure for preparation, planning and timely decision-making in the event of a potential collision, as well as what country or entity would have the authorization and responsibility to act, or take care of the financial implications.
Von der Dunk hopes the conference will shed more light on these issues.
“We hope to accomplish two things,” he said. “One is to generate more attention to this problem and make sure it will remain on people’s agenda, even though we recognize there are more immediate pressing global concerns, such as climate change or economic issues.” But even in terms of economic concerns von der Dunk said making decisions now about asteroid deflection is worthwhile because we can develop a proper process which could save millions or billions of dollars.
Instead of using scenarios like the movies “Deep Impact” or “Armageddon” – the typical Hollywood approach, von der Dunk said, we could take action early in the game. “Gravity tractors only require a couple of million dollars in cost.”
The other goal of the conference is to shed more insight into the protocols and legal issues of an Earth-bound asteroid. “What protocols should be followed to tackle the problem, what threshold would be sufficient to start taking action, who should take the action, who should pay for it, and who would be liable if something goes wrong? Those are the types of issues that we are putting on the table.”
While the actual capacities to take action against an NEO are still limited to a few space-fairing nations, von der Dunk said there’s also the possibility of global political fallout if there is a divergence between them. “One country may decide at a certain point not to bother about it, while another country with a greater chance of being hit, might want to take action,” he said. “The idea is to create a protocol and procedure of how we deal with these things to try to avoid the worst political fallout from happening, so if tomorrow, or ten years or hundred years from now and we know we have an asteroid heading in our direction we know we can actually do something about it and have a general legal understanding of how things will work.”
Von der Dunk specializes in space law and is a member of a panel created by the Association of Space Explorers, chaired by Apollo astronaut Rusty Schweickart. Von der Dunk has looked at what current protocols could be used in the event of an impending asteroid hit, but says nothing really exists. “I have looked at this issue and it quickly became clear to me that the current international treaty dealing with liability simply never foresaw the possibility of something going wrong in a case such as if the asteroid were deflected and then hit a different part of Earth than where it originally was going to hit,” he said. “And then a lawyer would be faced with taking some existing clauses which come closest and stretching them beyond what they were ever meant to be. We need to consider drafting a new international treaty agreement for this. At the conference we will discuss what such a treaty should look like, how should we phrase it, what particulars should be targeted.”
A number of members of Schweickart’s panel will be presenting at the conference, as well as “speakers from outside the community to broaden the issue,” von der Dunk said. “We will take stock of what is happening now, is it going in the right direction, discuss in more detail some of the legal issues such as liability, and add to that something in a more positive tone. Asteroids are not only about ‘deep impact,” but also about the possibilities of creating access to potentially very valuable minerals. If someone is going to mine an asteroid, we need the appropriate legal framework for that.”
Von der Dunk said attendees of the conference are lawyers, policy makers, members of think-tanks, and government representatives. Other speakers include former NASA astronaut Tom Jones, and Vice-Chairman of the United Nations Committee on the Peaceful Use of Outer Space (COPUOS), Ciro Arevalo.
The conference ends today with a simulation led by Dr. Eligar Sadeh of the Eisenhower Center For Space & Defense Studies of what actions and decisions would need to be made in the event of the discovery of an Earth-bound asteroid. “From this we may come to an understanding of why certain decisions have to be made at a certain point in time and how the consequential logic of a process like that flows,” von der Dunk said.
Von der Dunk is the leading academic expert on space law, and UNL’s College of Law is has the only master of laws program in space and telecommunications law offered in the United States.
Young asteroid tanning is big business in the Solar System (ESO)
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If you stay out in the Sun too long, you’ll eventually get a suntan (or sunburn); your skin will also get damaged and it will show signs of ageing faster. This might sound like a sunblock ad, but the same principal holds true for the small chunks of rock floating around in the Solar System. Yes, a young asteroid’s surface will age prematurely, but it’s not caused by the Sun’s ultraviolet rays, it’s caused by the solar wind…
Within a million years, an asteroid can turn from lunar grey to Martian red when left out in the solar wind. A million years is a tiny amount of time in relation to the Solar System’s lifetime. Why is this important? European Southern Observatory (ESO) researchers have realized that this finding will not only help astronomers relate an asteroid’s appearance with its history, but it can act as an indicator for after effects of impacts with other asteroids.
It turns out that the study of “space weathering” is fairly controversial, scientists have been mulling it over for a long time. Central to the problem is the fact that the appearance of the interior of meteorites found on Earth are remarkably different to the asteroids we see in space; asteroids are redder than their meteorite cousins. So what causes this redness?
“Asteroids seem to get a ‘sun tan’ very quickly,” says lead author Pierre Vernazza. “But not, as for people, from an overdose of the Sun’s ultraviolet radiation, but from the effects of its powerful wind.”
Although this is an interesting discovery, the speed at which the “tanning” occurs is astonishing. After an asteroid collision, fresh asteroid chunks are created with new surfaces. Within a million years these young asteroid surfaces will turn a dirty shade of red as the surface minerals are continuously battered by ionizing solar wind particles. “The charged, fast moving particles in the solar wind damage the asteroid’s surface at an amazing rate,” Vernazza added.
Naturally, a lot depends on the mineral composition of an asteroid’s surface, influencing how red its surface will become, but most of the tanning effect occurs in the first million years. Afterwards, the tanning continues, just at a slower rate.
Asteroid observations also reveal that the high proportion of “fresh surfaces” seen on near-Earth asteroid probably isn’t down to asteroid collisions. The frequency of collisions is far lower than the sun-tanning timescales, meaning that there shouldn’t be any “fresh surfaces” to be seen. It is far more likely that the upper layers of asteroids are renewed through planetary encounters, where the gravitational field of planets “shake off” the tanned dust.
It may not look like much, but that drawing could save a life someday — or 7 billion.
David French, a doctoral candidate in aerospace engineering at North Carolina State University is proposing a new tool for the anti-asteroid arsenal.
French said his PhD advisor Andre Mazzoleni, an associate professor of mechanical and aerospace engineering at the university, were not beholden to grant funds and “we just decided to go off on a direction that’s interesting and exciting.”
Mazzoleni has worked with tethers in other applications, and the two have now come up with a way to effectively divert asteroids and other threatening objects from impacting Earth by attaching a long tether and ballast to the incoming object.
By attaching the ballast, French explains, “you change the object’s center of mass, effectively changing the object’s orbit and allowing it to pass by the Earth, rather than impacting it.”
NASA’s Near Earth Object Program has identified more than 1,000 “potentially hazardous asteroids” and they are finding more all the time. “While none of these objects is currently projected to hit Earth in the near future, slight changes in the orbits of these bodies, which could be caused by the gravitational pull of other objects, push from the solar wind, or some other effect could cause an intersection,” French explains.
He said it’s hard to imagine the scale of both the problem and the potential solutions — but he points out that some asteroid impacts on Earth have been catastrophic.
“About 65 million years ago, a very large asteroid is thought to have hit the Earth in the southern Gulf of Mexico, wiping out the dinosaurs, and, in 1907, a very small airburst of a comet over Siberia flattened a forest over an area equal in size to New York City,” he said. “The scale of our solution is similarly hard to imagine.”
The idea is to use a tether somewhere in length between 1,000 kilometers (621 miles; roughly the distance from Raleigh to Miami) to 100,000 kilometers (62,137 miles; you could wrap this around the Earth two and a half times).
Other ideas that have emerged sound no less extreme, French notes. Those include painting the asteroids in order to alter how light may influence their orbit, a plan that would guide a second asteroid into the threatening one, and nuclear weapons.
“They probably all have their merits and drawbacks,” he said. “Nuclear weapons are already accessible; we’ve already made them. I can look at my own idea and say it’s long duration and very trackable.”
A tether effort could last in the ballpark of 20 to 50 years, he said, depending on the size and shape of the asteroid and its orbit, and the size of ballast.
French acknowledges there are “technical barriers that have to be surpassed.”
“First, you would have to mitigate the rotation of the asteroid,” he said, adding that the crescent-shaped piece connecting the poles on a globe might make a good conceptual model for a tether anchor, because it would allow for the asteroid’s rotation.
Another problem is the composition,” he added. “Some asteroids are just rubble piles.”
French said his idea was never to have all the kinks worked out on his model before presenting it; he just hoped to add another option to the asteroid-preparedness table.
“We’re opening up the concept, and we invite the broader scientific community to help us solve the issues,” he said.
Source: An NC State press release, via Eurekalert, and an interview with David French.
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Officials are now saying the bright fireball seen over Virginia in the US on Sunday was probably a natural meteor event and not part of a Russian rocket, a reversal from yesterday’s initial analysis. Space.com reported that an official from the U.S. Naval Observatory believed the loud boom and flash of light seen in the skies over Norfolk and Virginia Beach was likely the second stage of the Soyuz rocket that launched Expedition 19 to the International Space Station last Thursday. However, U.S. Strategic Command has since reported that the rocket re-entered Earth’s atmosphere near Taiwan, on the other side of the world, several hours after the reports of the fireball. So both its timing and entry location rule out the rocket as the explanation for the fireball. But the investigation is continuing to determine exactly what the object was.
The Joint Space Operations Center at Vandenberg Air Force Base in California also confirmed “the ‘bright light’ that was reported on the East Coast on Sunday, 29 March at 9:45 p.m. EST was not a result of any trackable manmade object on reentry,” according to Patricia Phillips at the Space News Examiner.
Space rocks the size of small cars plunge into Earth’s atmosphere several times a year, typically burning up before reaching the ground. “The atmosphere is very good at protecting us from falling rocks,” said Bill Cooke of NASA’s Marshall Space Flight Center. Ones that do reach the ground go unreported since they fall over uninhabited areas or in the ocean.
A few space rocks do occasionally make it to the surface though. In recent years, pieces of a bolide were found after a meteor event in western Canada, and fragments of a meteor that originated from an asteroid that blew up over the skies of Africa last October were also recovered in the Sudanese desert.
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Residents of Virginia in the US reported hearing booms and seeing flashes of light Sunday night, and originally, it was reported to be another possible meteor. But now officials from the U.S. Naval Observatory say it was likely the second stage of the Russian Soyuz rocket falling back to Earth. Parts of the rocket from last Thursday’s launch to the International Space Station would have fallen to Earth about that same time. “I’m pretty convinced that what these folks saw was the second stage of the Soyuz rocket that launched the crew up to the space station,” Space.com quoted Jeff Chester of the Naval Observatory in Washington, D.C.
Chester heard about the incident this morning and checked the listing for debris expected to enter the lower atmosphere during that time and found that second stage of the Soyuz rocket that launched last Thursday was re-enter Earth’s atmosphere during a window that started at 8 p.m. on March 29.
Chester ran a satellite tracking program that showed that the rocket debris should have come down exactly in the area where the fireball was spotted.
“This is just too much of a coincidence to be coincidence,” he said.
Chester said that U.S. Space Surveillance Network had not yet confirmed that this was the case, but said that he was “99 and four one-hundredths [percent] convinced that this is what it is.”
The descriptions of the boom and streak of light reported by local residents were “entirely consistent with re-entering space junk, especially something this big,” Chester said.
Space.com also reported that Delta airline pilot Bryce Debban reported seeing the streak of light on a flight from Boston to Raleigh-Durham when his plane was about 31,000 feet in the air.
The Soyuz rockets jettison their second stage after entering orbit in such a way that the second stage will slowly fall back to earth in a few days. But “you can control precisely where these things are going to come down,” Chester said.
It’s possible that some fragments of the rocket made it to the Earth’s surface, but they would likely have a couple of hundreds of miles east of Cape Hatteras, Chester said.
Map of the Nubian Desert of northern Sudan with the groundprojected approach path of the asteroid 2008 TC3 and the location of the recovered meteorites. Credit: P. Jenniskens, et. al
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Remember in October 2008 when Asteroid 2008 TC3 hit the scene – literally? This was the first asteroid that was predicted –and predicted correctly — to impact the Earth. Luckily, it wasn’t big enough to cause any problems, and its path was over a remote area in Africa. It streaked into the skies over northern Sudan in the early morning of October 7, 2008, and then exploded at a high 37 km above the Nubian Desert, before the atmosphere could slow it down. It was believed that the asteroid likely had completely disintegrated into dust. But meteor astronomer Peter Jenniskens thought there might be a chance to recover some of the remains of this truck-sized asteroid. And he was right.
Never before have meteorites been collected from such a high altitude explosion. Additionally, as it turns out, the assembled remnants are unlike anything in our meteorite collections, and may be an important clue in unraveling the early history of the solar system.
A meteor astronomer with the SETI Institute’s Carl Sagan Center, Jenniskens established a collaboration with Mauwia Shaddad of the Physics Department and Faculty of Sciences of the University of Khartoum. The two traveled to the Sudan. Various meteorites from 2008 TC3. Credit: P. Jenniskens, et. al. Click image for full description
Fifteen fresh-looking meteorites with a total mass of 563 g were recovered by 45 students and staff of the University of Khartoum during a field campaign on December 5-8, 2008. A second search on December 25-30 with 72 participants raised the total to 47 meteorites and 3.95 kg. Masses range from 1.5 g to 283 g, spread for 29km along the approach path in a manner expected for debris from 2008 TC3
“This was an extraordinary opportunity, for the first time, to bring into the lab actual pieces of an asteroid we had seen in space,” said Jenniskens, the lead author on a cover story article in the journal Nature that describes the recovery and analysis of 2008 TC3.
Click here for several images from NASA about the asteroid hit and the recovery of the meteorites.
Picked up by Arizona’s Catalina Sky Survey telescope on 6 October, 2008, Asteroid 2008 TC3 abruptly ended its 4.5 billion year solar-system odyssey only 20 hours after discovery, when it broke apart in the African skies. The incoming asteroid was tracked by several groups of astronomers, including a team at the La Palma Observatory in the Canary Islands that was able to measure sunlight reflected by the object.
Studying the reflected sunlight gives clues to the minerals at the surface of these objects. Astronomers group the asteroids into classes, and attempt to assign meteorite types to each class. But their ability to do this is often frustrated by layers of dust on the asteroid surfaces that scatter light in unpredictable ways.
Jenniskens teamed with planetary spectroscopist Janice Bishop of the SETI Institute to measure the reflection properties of the meteorite, and discovered that both the asteroid and its meteoritic remains reflected light in much the same way — similar to the known behavior of so-called F-class asteroids.
“F-class asteroids were long a mystery,” Bishop notes. “Astronomers have measured their unique spectral properties with telescopes, but prior to 2008 TC3 there was no corresponding meteorite class, no rocks we could look at in the lab.” Petrography of Almahata Sitta. Credit: Jenniskens, et. al. Click for full description
The good correspondence between telescopic and laboratory measurements for 2008 TC3 suggests that small asteroids don’t have the troublesome dust layers, and may therefore be more suitable objects for establishing the link between asteroid type and meteorite properties. That would allow us to characterize asteroids from afar.
Rocco Mancinelli, a microbial ecologist at the SETI Institute’s Carl Sagan Center, and a member of the research team, says that “2008 TC3 could serve as a Rosetta Stone, providing us with essential clues to the processes that built Earth and its planetary siblings.”
In the dim past, as the solar system was taking shape, small dust particles stuck together to form larger bodies, a process of accumulation that eventually produced the asteroids. Some of these bodies collided so violently that they melted throughout.
2008 TC3 turns out to be an intermediate case, having been only partially melted. The resulting material produced what’s called a polymict ureilite meteorite. The meteorites from 2008 TC3, now called “Almahata Sitta,” are anomalous ureilites: very dark, porous, and rich in highly cooked carbon. This new material may serve to rule out many theories about the origin of ureilites.
In addition, knowing the nature of F-class asteroids could conceivably pay off in protecting Earth from dangerous impactors. The explosion of 2008 TC3 at high altitude indicates that it was of highly fragile construction. Its estimated mass was about 80 tons, of which only some 5 kg has been recovered on the ground. If at some future time we discover an F-class asteroid that’s, say, several kilometers in size — one that could wipe out entire species — then we’ll know its composition and can devise appropriate strategies to ward it off.
As efforts such as the Pan-STARRS project uncover smaller near-Earth asteroids, Jenniskens expects more incidents similar to 2008 TC3. “I look forward to getting a call from the next person to spot one of these,” he says. “I would love to travel to the impact area in time to see the fireball in the sky, study its breakup and recover the pieces. If it’s big enough, we may well find other fragile materials not yet in our meteorite collections.”
Dusty debris around an old white dwarf star (NASA)
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What will happen to all the inner planets, dwarf planets, gas giants and asteroids in the Solar System when the Sun turns into a white dwarf? This question is currently being pondered by a NASA researcher who is building a model of how our Solar System might evolve as our Sun loses mass, violently turning into an electron-degenerate star. It turns out that Dr. John Debes work has some very interesting implications. As we use more precise techniques to observe existing white dwarf stars with the dusty remains of the rocky bodies that used to orbit them, the results of Debes’ model could be used as a comparison to see if any existing white dwarf stars resemble how our Sun might look in 4-5 billion years time…
A comparison of the Sun in its yellow dwarf phase and red giant phaseToday, our Sun is a healthy yellow dwarf star. If you want to be precise, it is a “G V star”. This yellow dwarf will happily burn 600 million tonnes of hydrogen per second in its core for 10 billion years, generating the light that is required to make our planet habitable. The Sun is approximately half-way through this hydrogen burning phase, so it’s OK, things aren’t going to change (for the Sun at least) for a long time yet.
But what happens then? What happens in 4-5 billion years when the supply of hydrogen runs out in the core? Although our Sun isn’t massive enough to entertain the thought of going out in a blaze of supernova glory, it will still go through an exciting, yet terrifying death. After evolving through the hydrogen-burning phase, the Sun will puff up into a huge red giant star as the hydrogen fuel becomes scarce, expanding 200 times the size it is now, probably swallowing the Earth. Helium, and then progressively heavier elements will be fused in and around the core. The Sun will never fuse carbon however, instead it will shed its outer layers forming a planetary nebula.
Once things calm down, a small sparkling jewel of a white dwarf star will remain. This tiny remnant will have a mass of around half that of our present Sun, but will be the size of the Earth. Needless to say, white dwarfs are very dense, intense gravitational pull countered not by fusion in the core (like all Main Sequence stars), but by electron degeneracy pressure.
Relative sizes of IK Pegasi A (left), IK Pegasi B (lower center; a white dwarf) and the Sun (NASA)When the Solar System reaches this phase in its evolution, what will it look like? What will become of the asteroids, gas giants, moons and rocky planets? I was very fortunate to chat with astrophysicist Dr John Debes, from NASA’s Goddard Space Flight Center, at January’s American Astronomical Society (AAS) conference in Long Beach (California) who is developing an n-body code simulating an evolving Solar System.
After the Sun has stopped hydrogen fusion in its core, it loses mass as it sheds its outer layers after the red giant phase and subsequent planetary nebula formation. It is estimated that the Sun will lose about 50% of its mass during this time, naturally affecting the Solar System as a whole. As the Sun loses mass, the outer planets (such as Jupiter) will drift outwards, increasing their orbital radii. In the simulation, Debes is very careful to ensure there is a gradual reduction in solar mass to ensure stability in the simulation.
What we are left with is an old Solar System, where little is left of the inner planets (it is likely that anything within the orbit of the Earth will have been swallowed by the Sun as it expanded through the red giant phase). Although the future white dwarf Solar System will seem very alien to present day, some things won’t change. Jupiter’s orbit might have receded with the drop in solar mass, it will remain a planetary heavyweight, causing disruption in asteroid orbits. Using known asteroid data, the motion of these chunks of rocks are allowed to evolve, and over millions of years, they may get thrown out of the Solar System, or more interestingly, pushed closer to the white dwarf. Once the whole system has settled down, resonances in the asteroid belt will become amplified; Kirkwood Gaps (caused by gravitational resonance with Jupiter) will widen, and according to Debes’ simulations, the edges of these gaps will become perturbed even more, making more asteroids available to be tidally disrupted and shredded to dust.
Artists concept of shredded asteroid around white dwarf (NASA/JPL-Caltech)The AAS conference was full of amazing research into white dwarf observations. The reason for this is that there are many white dwarf candidates out there with dusty metallic absorption lines. This means that there used to be rocky bodies orbiting these stars, but became pulverised (by tidal shear) for astronomers to analyse. These white dwarf systems can give us a clue as to what mechanisms could be supplying the white dwarfs with dusty material, even giving us a glimpse into the future of our Solar System.
“We have a physical picture for the link between planetary systems and dusty white dwarfs,” Debes said when describing his model in relation to the mysterious dusty white dwarf observations. “Dusty white dwarfs are truly a mystery! We think we know what might be going on, but we don’t have a smoking gun yet.”
However, Debes is getting close to finding a possible smoking gun, he’s basing his model on some of the key characteristics of these ancient dusty remnants to see what the Solar System could look like in billions of years time.
So, where does this dust come from? As the asteroid orbits are perturbed by Jupiter, they may get close enough to be tidally disrupted. Get too close and they will get shredded by the gravitational shear created by the steep tidal radius of the compact white dwarf. The asteroid dust then settles into the white dwarf. The presence of this dust has a very obvious signature in the absorption lines of spectroscopic data, allowing researchers to infer an accretion rate for metal-rich white dwarfs. In Debes’ model, he has set the upper limit to 1016 g/year and a lower limit to 1013 g/year, consistent with observed estimates.
Spectra of G29-38. Could this resemble the spectra of the Sun after turning into a white dwarf? (NASA/Spitzer)In his evolved Solar System model, Jupiter’s gravity controls this accretion rate, pushing asteroids toward the white dwarf and, by using a powerful supercomputer to track the perturbations and eventual shredding of known asteroids, there may be an opportunity to arrive at a profound conclusion. Debes is able to use his model to compare observations of known dusty white dwarfs with the simulated outcome of the Solar System. With reference to previous studies (in particularly Koester & Wilken, 2006 in the journal Astronomy & Astrophysics), Debes has found some similar white dwarf “Suns”.
“For G29-38, the canonical dusty white dwarf, they [Koester & Wilken] estimate a total mass of 0.55 solar masses–about what people believe the mass that our own sun will have remaining when it becomes a white dwarf,” Debes added. “But mass estimates are a bit uncertain–I’ve seen estimates ranging from 0.55-0.7 solar masses for this particular white dwarf.”
The Sun's future? The white dwarf G29-38 (NASA)“Another good candidate is a DAZ [a metal-rich white dwarf] called WD 1257+278, which does not show dust but is spot on with the mass expected for the Sun–0.54 MSun,” said Debes. “Its accretion rate is also consistent with my model predictions so far assuming an asteroid belt mass and characteristic perturbation timescale that I found in my simulations.”
Debes is continuing to make his model more and more sophisticated, but already the results are promising. Most exciting is that we may already be observing white dwarfs, like G29-38 or WD 1257+278, giving us a tantalizing glimpse of what our Solar System will look like when the Sun becomes a white dwarf star, ripping apart any remaining asteroids and planets as they stray too close to the Sun’s tidal shear. However, it also raises the question: if white dwarfs like G29-38 are being fed by the remains of tidally-blended asteroids, are there massive planets shepherding asteroids in these white dwarf systems too?
