Category: Asteroids

Scientists at the American Museum of Natural History and the University of Chicago have explained how a globe-encircling residue formed in the aftermath of the asteroid impact that triggered the extinction of the dinosaurs. The study, which will be published in the April issue of the journal Geology, draws the most detailed picture yet of the complicated chemistry of the fireball produced in the impact.

The residue consists of sand-sized droplets of hot liquid that condensed from the vapor cloud produced by an impacting asteroid 65 million years ago. Scientists have proposed three different origins for these droplets, which scientists call ?spherules.? Some researchers have theorized that atmospheric friction melted the droplets off the asteroid as it approached Earth?s surface. Still others suggested that the droplets splashed out of the Chicxulub impact crater off the coast of Mexico?s Yucatan Peninsula following the asteroid?s collision with Earth.

But analyses conducted by Denton Ebel, Assistant Curator of Meteorites at the American Museum of Natural History, and Lawrence Grossman, Professor in Geophysical Sciences at the University of Chicago, provide new evidence for the third proposal. According to their research, the droplets must have condensed from the cooling vapor cloud that girdled the Earth following the impact.

Ebel and Grossman base their conclusions on a study of spinel, a mineral rich in magnesium, iron and nickel contained within the droplets.

?Their paper is an important advance in understanding how these impact spherules form,? said Frank Kyte, adjunct associate professor of geochemistry at the University of California, Los Angeles. ?It shows that the spinels can form within the impact plume, which some researchers argued was not possible.?

When the asteroid struck approximately 65 million years ago, it rapidly released an enormous amount of energy, creating a fireball that rose far into the stratosphere. ?This giant impact not only crushes the rock and melts the rock, but a lot of the rock vaporizes,? Grossman said. ?That vapor is very hot and expands outward from the point of impact, cooling and expanding as it goes. As it cools the vapor condenses as little droplets and rains out over the whole Earth.?

This rain of molten droplets then settled to the ground, where water and time altered the glassy spherules into the clay layer that marks the boundary between the Cretaceous and Tertiary (now officially called the Paleogene) periods. This boundary marks the extinction of the dinosaurs and many other species.

The work that led to Ebel and Grossman?s Geology paper was triggered by a talk the latter attended at a scientific meeting approximately 10 years ago. At this talk, a scientist stated that spinels from the Cretaceous-Paleogene boundary layer could not have condensed from the impact vapor cloud because of their highly oxidized iron content. ?I thought that was a strange argument,? Grossman said. ?About half the atoms of just about any rock you can find are oxygen,? he said, providing an avenue for extensive oxidation.

Grossman?s laboratory, where Ebel worked at the time, specializes in analyzing meteorites that have accumulated minerals condensed from the gas cloud that formed the sun 4.5 billion years ago. Together they decided to apply their experience in performing computer simulations of the condensation of minerals from the gas cloud that formed the solar system to the problem of the Cretaceous-Paleogene spinels.

UCLA?s Kyte, who himself favored a fireball origin for the spinels, has measured the chemical composition of hundreds of spinel samples from around the world.

Ebel and Grossman built on on Kyte?s work and on previous calculations done by Jay Melosh at the University of Arizona and Elisabetta Pierazzo of the Planetary Science Institute in Tucson, Ariz., showing how the asteroid?s angle of impact would have affected the chemical composition of the fireball. Vertical impacts contribute more of the asteroid and deeper rocks to the vapor, while impacts at lower angles vaporize shallower rocks at the impact site.

Ebel and Grossman also drew upon the work of the University of Chicago?s Mark Ghiorso and the University of Washington?s Richard Sack, who have developed computer simulations that describe how minerals change under high temperatures.

The resulting computer simulations developed by Ebel and Grossman show how rock vaporized in the impact would condense as the fireball cooled from temperatures that reached tens of thousands of degrees. The simulations paint a picture of global skies filled with a bizarre rain of a calcium-rich, silicate liquid, reflecting the chemical content of the rocks around the Chicxulub impact crater.

Their calculations told them what the composition of the spinels should be, based on the composition of both the asteroid and the bedrock at the impact site in Mexico. The results closely matched the composition of spinels found at the Cretaceous-Paleogene boundary around the world that UCLA?s Kyte and his associates have measured.

Scientists had already known that the spinels found at the boundary layer in the Atlantic Ocean distinctly differed in composition from those found in the Pacific Ocean. ?The spinels that are found at the Cretaceous-Paleogene boundary in the Atlantic formed at a hotter, earlier stage than the ones in the Pacific, which formed at a later, cooler stage in this big cloud of material that circled the Earth,? Ebel said.

The event would have dwarfed the enormous volcanic eruptions of Krakatoa and Mount St. Helens, Ebel said. ?These kinds of things are just very difficult to imagine,? he said.

The results in this paper strengthen the link between the unique Chicxulub impact and the stratigraphic boundary marking the mass extinction 65 million years ago that ended the Age of Dinosaurs. The topic will be explored further in a new groundbreaking exhibition, ?Dinosaurs: Ancient Fossils, New Discoveries,? set to open at the American Museum of Natural History on May 14. After it closes in the New York, the exhibition will travel to the Houston Museum of Natural Science (March 3-July 30, 2006); the California Academy of Sciences, San Francisco (Sept. 15, 2006-Feb. 4, 2007); The Field Museum, Chicago (March 30-Sept. 3, 2007); and the North Carolina State Museum of Natural Sciences, Raleigh (Oct. 26, 2007-July 5, 2008).

Original Source: University of Chicago News Release

Scientists have discovered why there isn’t much impact-melted rock at Meteor Crater in northern Arizona.

The iron meteorite that blasted out Meteor Crater almost 50,000 years ago was traveling much slower than has been assumed, University of Arizona Regents’ Professor H. Jay Melosh and Gareth Collins of the Imperial College London report in Nature (March 10).

“Meteor Crater was the first terrestrial crater identified as a meteorite impact scar, and it’s probably the most studied impact crater on Earth,” Melosh said. “We were astonished to discover something entirely unexpected about how it formed.”

The meteorite smashed into the Colorado Plateau 40 miles east of where Flagstaff and 20 miles west of where Winslow have since been built, excavating a pit 570 feet deep and 4,100 feet across – enough room for 20 football fields.

Previous research supposed that the meteorite hit the surface at a velocity between about 34,000 mph and 44,000 mph (15 km/sec and 20 km/sec).

Melosh and Collins used their sophisticated mathematical models in analyzing how the meteorite would have broken up and decelerated as it plummeted down through the atmosphere.

About half of the original 300,000 ton, 130-foot-diameter (40-meter-diameter) space rock would have fractured into pieces before it hit the ground, Melosh said. The other half would have remained intact and hit at about 26,800 mph (12 km/sec), he said.

That velocity is almost four times faster than NASA’s experimental X-43A scramjet — the fastest aircraft flown — and ten times faster than a bullet fired from the highest-velocity rifle, a 0.220 Swift cartridge rifle.

But it’s too slow to have melted much of the white Coconino formation in northern Arizona, solving a mystery that’s stumped researchers for years.

Scientists have tried to explain why there’s not more melted rock at the crater by theorizing that water in the target rocks vaporized on impact, dispersing the melted rock into tiny droplets in the process. Or they’ve theorized that carbonates in the target rock exploded, vaporizing into carbon dioxide.

“If the consequences of atmospheric entry are properly taken into account, there is no melt discrepancy at all,” the authors wrote in Nature.

“Earth’s atmosphere is an effective but selective screen that prevents smaller meteoroids from hitting Earth’s surface,” Melosh said.

When a meteorite hits the atmosphere, the pressure is like hitting a wall. Even strong iron meteorites, not just weaker stony meteorites, are affected.

“Even though iron is very strong, the meteorite had probably been cracked from collisions in space,” Melosh said. “The weakened pieces began to come apart and shower down from about eight-and-a-half miles (14 km) high. And as they came apart, atmospheric drag slowed them down, increasing the forces that crushed them so that they crumbled and slowed more.”

Melosh noted that mining engineer Daniel M. Barringer (1860-1929), for whom Meteor Crater is named, mapped chunks of the iron space rock weighing between a pound and a thousand pounds in a 6-mile-diameter circle around the crater. Those treasures have long since been hauled off and stashed in museums or private collections. But Melosh has a copy of the obscure paper and map that Barringer presented to the National Academy of Sciences in 1909.

At about 3 miles (5 km) altitude, most of the mass of the meteorite was spread in a pancake shaped debris cloud roughly 650 feet (200 meters) across.

The fragments released a total 6.5 megatons of energy between 9 miles (15 km) altitude and the surface, Melosh said, most of it in an airblast near the surface, much like the tree-flattening airblast created by a meteorite at Tunguska, Siberia, in 1908.

The intact half of the Meteor Crater meteorite exploded with at least 2.5 megatons of energy on impact, or the equivalent of 2.5 million tons of TNT.

Elisabetta Pierazzo and Natasha Artemieva of the Planetary Science Institute in Tucson, Ariz., have independently modeled the Meteor Crater impact using Artemieva’s Separated Fragment model. They find impact velocities similar to that which Melosh and Collins propose.

Melosh and Collins began analyzing the Meteor Crater impact after running the numbers in their Web-based “impact effects” calculator, an online program they developed for the general public. The program tells users how an asteroid or comet collision will affect a particular location on Earth by calculating several environmental consequences of the impact.

Original Source: University of Arizona News Release

Although they’re both enormous asteroids, protoplanets really, and lie within the asteroid belt between Mars and Jupiter, Vesta and Ceres couldn’t be more different.

Vesta formed closer to the Sun, and probably shares many features of the inner planets. Scientists believe it formed in a hot, dry environment and will probably have layers of volcanic flows and a solid metallic core. But even the best photos from Hubble show a blurry gray world, bringing more questions than answers. It’s the brightest asteroid in the Solar System, measuring 530 km (329 miles) across. You can even see it with the unaided eye; in fact, it’s the only main belt asteroid you can see. Traveling to Vesta could be a little dangerous. “We know very little about Vesta’s internal structure,” explained Chief Engineer Dr. Marc Rayman, “it
has an unpredictable and possibly very irregular gravity field.”

Just a little further out – across an invisible line that separates the inner rocky planets from the outer planets – is Ceres; the largest asteroid in the Solar System, measuring 957 km (595 miles) across. Unlike Vesta, Ceres is believed to have formed in a cool, wet environment, and in the presence of water. This water is probably still there, in the form of ice caps, a thin water vapour atmosphere, or even as a liquid underneath the surface.

While most of the objects in the asteroid belt are pulverized chunks of rock, accumulations of material from different bodies, Vesta and Ceres remain largely unchanged from when they first formed 4.6 billion years ago. Revelations about the early history of the Solar System could be written on their surfaces.

The $370 million US spacecraft is scheduled for liftoff in June, 2006. After 4 or 5 years of travel time (depending on whether or not it’ll be making a flyby of Mars first) Dawn will arrive at Vesta in 2010 or 2011, studying it for almost a year before flying off to rendezvous with Ceres three years later. It has a suite of scientific instruments on board to study the two asteroids in great detail: their mass, volume, spin rate, chemical and elemental composition, and gravity. Oh, and it’ll be taking pretty pictures too.

Dawn will be the first spacecraft ever to orbit two separate objects in the solar system (and no, orbiting the Earth doesn’t count here). A feat that wouldn’t even be possible without its ion engine. A very similar engine helped Deep Space 1 set speed and duration records, and served as a model for Dawn’s development. It uses solar electricity to ionize xenon atoms and then hurl them out the back of the spacecraft. The thrust is tiny but fuel efficient, and the engine can keep running for months or even years providing a tremendous velocity.

And an ion engine gives controllers flexibility. “It gives us a very long launch window. We’re launching in June 2006 because that’s when the spacecraft will be ready. But we could still make it in November or even after that,” said Dr. Rayman. So far, though, the project is right on schedule. The completed spacecraft shipped this week from NASA’s Jet Propulsion Laboratory to Orbital Sciences for the next stage of assembly and testing.

If you’re interested in finding out more about this mission, stay tuned. Dr. Rayman is planning on keeping the world well informed, through the Internet. He learned how important this can be while working on Deep Space 1, taking the unusual step – at the time – of maintaining a web log to describe his experiences working with the spacecraft. “I was in the airport when I realized that we needed to get the word out. I dictated my first entry over the phone,” recalled Dr. Rayman. Rayman continued maintaining his popular DS1 blog, giving armchair mission controllers a unique insight into the day-to-day challenges and decisions that go into managing a spacecraft half a solar system away.

Expect more of the same with Dawn. “These missions belong to more than just NASA, or the United States. They’re humanity’s emissaries to the cosmos, and we want everyone to come along for the ride,” explained Rayman. But this time, he’ll get started earlier, bringing an Internet audience into the development stages as well as post-launch.

Official Dawn Mission Page

Written by Fraser Cain

1.) Which class of Earth-crossing asteroids do you find most interesting, Atens or Apollos? (Erimus)

Personally, I find the Atens more interesting, simply because their orbits keep them out of the opposition region for a larger fraction of the time than the Apollos, making them comparatively more difficult to find. Current population statistics are biased against Atens because of the emphasis on the opposition region by the surveys.

2.) Which particular NEO do you find most interesting? (Erimus)

Which day of the week is it? Let’s see, if it’s Friday, I’d take 2000 SG344. This object is interesting because of its low velocity relative to Earth, which argues against it having been perturbed out the main belt. We can pretty much rule out it being manmade, now that another better candidate for the Apollo 12 S-IVB has been found. So, I’m leaning toward it being a piece of lunar ejecta, which could well be unique among the known objects. Because of its relatively high impact probabilities, I’ve given the object higher priority for astrometric observation, getting it a year and a half ago when it was magnitude 26, probably the faintest NEO ever observed.

If it’s Saturday, I’d go with 2004 MN4. It’s hard to ignore an object that will pass less than 6 Earth radii from the Earth just about 24 years from now, becoming bright enough to be visible to the unaided eye.

If it’s Monday, I’d go with 2004 XZ130. With a record small semimajor axis of 0.617 AU and a record small aphelion distance of 0.898 AU, it’s the kind of asteroid I’ve been interested in finding for a long time. Because it never gets into the opposition region, it would never be found by an opposition survey. Not much is known about this population of asteroid, because of the observational bias. We’re taking the first steps toward reducing that bias.

Oh, it’s Thursday. Let’s see, choices, choices…

3) What is the impact to the Earth, if an asteroid were to hit the Earth? Will it changes the weather or give any chemical or other effects? have we prepared for that? Should ordinary people know and be aware of it? (Fari)

It all depends on the size of the object that hits. If it’s small like a meteorite, it would have no significant effect. Something larger, like the one that produced Meteor Crater in Arizona, won’t change the weather, but significant local damage would occur. An object of the kind that is believed to have wiped out the dinosaurs would indeed have a major effect on the weather. So much dust would be ejected into the Earth’s stratosphere, that sunlight would be blocked, halting photosynthesis and disrupting the food chain all the way up to humans. Some of us have personally witnessed the length of time it can take for small amounts of dust to settle out of the stratosphere, given the El Chicon and Mt. Pinatubo volcanic eruptions of the 1980s and 1990s. Imagine how long it would take for a large amount of dust to settle out of the statosphere.

Currently, humans are not prepared to deal with a major asteroid impact.

Ordinary people who wish to be scientifically literate should be aware of the situation, but it’s not something over which to lose sleep.

4) What do you believe the chances are that the earth will be hit by an asteriod/comet that could cause world wide devastation within our lifetime? (Guest_SeanO)

Very small, less than one in ten thousand. That’s based on an assumed human life span of approximately 100 years (a little long, but we’re dealing with just an order of magnitude estimate here) and an average of one such impact every million years.

5) Do you have any speculations you could share with us about the exploitation of these objects, especially the sort of required technologies and favoured strategies required? (eburacum45)

It is true that some asteroids became hot enough for a long enough time to melt internally and differentiate, with the heavy metals sinking to their cores. Once catastrophically disrupted by a collision with another asteroid, the cores have been exposed, with some of the fragments falling to Earth, producing our nickel-iron meteorites. Some entrepreneurs are very interested in these cores because of the rare metals that could be extracted from them, such as gold, silver, and platinum.

Meanwhile, others are interested in exploiting the material necessary to sustain human existence in outer space. The one item that is most essential to human life is water. That’s one reason why there is so much interest in looking for water in the permanently shaded regions of the lunar poles. But some near-Earth asteroids may be rich in hydrated minerals, so it might be possible to extract water from these objects. Now, water sounds a lot more mundane than gold, silver, or platinum, but when you consider the alternative of hauling water up from the bottom of the deep gravity well that is Earth, you begin to realize that a source of water in outer space would be worth its weight in gold.

In both cases, a source of energy would be needed for the extraction process, but the Sun provides ample amounts of that. We just need to find an efficient way of harnessing that power. Some people want to change the orbit of an asteroid and park it around the Earth, sort of like a second moon, so that it is easier to get to on a routine basis. Needless to say, most of this work is very speculative in nature. Some scientists are probably actively thinking about ways to do the job, but I’m not aware of any major development of infrastructure at this time. One challenge is to work in the weak gravity field of an asteroid. Many terrestrial approaches simply won’t work very well on an asteroid because of the weak gravity.

6) Why is the asteroid belt so far away, in relation to the rocky planets? Why for instance do we not have an asteroid belt between Earth and Venus? (Guest)

The rocky planets range from 0.4 to 1.5 AU from the Sun, and the main asteroid belt extends from roughly 2.1 to 3.2 AU. Practically next door neighbors considering the scale of the Solar System, which extends to roughly 50 AU when you think of the trans-Neptunian objects, and even farther when you think of the Oort cloud comets, like 50,000 AU. So the asteroid belt doesn’t seem all that far away to me.

Asteroids between Venus and Earth do not have particularly stable orbits, at least compared to the 2 to 3 AU region of the Solar System. Nevertheless, some asteroids are believe to inhabit this region of space. Because they never reach the opposition region, they are harder to find. Looking in the part of the sky close to the Sun has been a research interest of mine for over a decade, and we’re just now finding the first inhabitants of this region. The numbers are too small at this time to be thinking in terms of a “belt”, but who knows what we’ll find after an extended investigation?

7) Are two (or more) asteroids ever found in orbit around each other, or will objects that small inevitably drift apart? (gnosys)

There are approximately four dozen asteroids known to have satellites in orbit around them. In some cases, the primary is large and the secondary is small, as in the case of Dactyl orbiting Ida, as imaged by the Galileo spacecraft while en route to Jupiter. In other cases, the two components are more nearly equal in size, such as 90 Antiope. Satellites of asteroids have been found among the near-Earth population, the main belt between Mars and Jupiter, and among the trans-Neptunian objects.

As long as the satellite is in a bound orbit, a source of energy would be necessary to cause them to separate.

Over the past week, several independent efforts were made to search for pre-discovery observations of 2004 mn4. These efforts proved successful today when Jeff Larsen and Anne Descour of the Spacewatch Observatory near Tucson, Arizona, were able to detect and measure very faint images of asteroid 2004mn4 on archival images dating to 15 March 2004. These observations extended the observed time interval for this asteroid by three months allowing an improvement in its orbit so that an Earth impact on 13 April 2029 can now be ruled out.

As is often the case, the possibility of future Earth impacts for some near-Earth objects cannot be entirely ruled out until the uncertainties associated with their trajectories are reduced as a result of either future position observations, or in this case, heretofore unrecognized, pre-discovery observations. When these additional observations were used to update the orbit of 2004 MN4, the uncertainties associated with this object’s future positions in space were reduced to such an extent that none of the object’s possible trajectories can impact the Earth (or Moon) in 2029.

In the accompanying diagram, the most likely position of asteroid 2004 MN4 is shown at the end of the blue line near the Earth on 13 April 2029. However, since the asteroid’s position in space is not perfectly known at that time, the white dots at right angles to the blue line are possible alternate positions of the asteroid. Neither the nominal position of the asteroid, nor any of its possible alternative positions, touches the Earth, indicating that an Earth impact in 2029 is ruled out.

The passage of the asteroid by the Earth in 2029 alters its subsequent trajectory and expands the asteroid’s position uncertainty region (i.e., the line of white dots increases in extent) so the asteroid’s subsequent motion is less certain than it was prior to the 2029 close Earth approach. However, our current risk analysis for 2004 MN4 indicates that no subsequent Earth encounters in the 21st century are of any concern.

Original Source: NASA News Release

The probability that Asteroid 2004 MN4 will strike the Earth on April 13, 2029 has actually been upgraded to a 1-in-43 chance now that more observations have been made. The asteroid has reached an uprecedented 4 on the Torino scale. Of course, this still means that there’s a 98% chance that it’ll completely miss the Earth. The space rock is 400 metres (1,300 feet) across, so a direct impact with our planet would cause a significant amount of damage on a regional level. Update: as of Dec. 28th, the probability has been significantly downgraded thanks to further observations. It’ll definitely miss.

A recently rediscovered 400-meter Near-Earth Asteroid (NEA) is predicted to pass near the Earth on 13 April 2029. The flyby distance is uncertain and an Earth impact cannot yet be ruled out. The odds of impact, presently around 1 in 300, are unusual enough to merit special monitoring by astronomers, but should not be of public concern. These odds are likely to change on a day-to-day basis as new data are received. In all likelihood, the possibility of impact will eventually be eliminated as the asteroid continues to be tracked by astronomers around the world.

This object, 2004 mn4, is the first to reach a level 2 (out of 10) on the Torino Scale. According to the Torino Scale, a rating of 2 indicates “a discovery, which may become routine with expanded searches, of an object making a somewhat close but not highly unusual pass near the Earth. While meriting attention by astronomers, there is no cause for public attention or public concern as an actual collision is very unlikely. New telescopic observations very likely will lead to re-assignment to Level 0 [no hazard].” This asteroid should be easily observable throughout the coming months.

The brightness of 2003 qq47 suggests that its diameter is roughly 400 meters (1300 feet) and our current, but very uncertain, best estimate of the flyby distance in 2029 is about twice the distance of the moon, or about 780,000 km (480,000 miles). On average, an asteroid of this size would be expected to pass within 2 lunar distances of Earth every 5 years or so.

Most of this object’s orbit lies within the Earth’s orbit, and it approaches the sun almost as close as the orbit of Venus. 2004mn4’s orbital period about the sun is 323 days, placing it within the Aten class of NEAs, which have an orbital period less than one year. It has a low inclination with respect to the Earth’s orbit and the asteroid crosses near the Earth’s orbit twice on each of its passages about the sun.

2004 MN4 was discovered on 19 June 2004 by Roy Tucker, David Tholen and Fabrizio Bernardi of the NASA-funded University of Hawaii Asteroid Survey (UHAS), from Kitt Peak, Arizona, and observed over two nights. On 18 December, the object was rediscovered from Australia by Gordon Garradd of the Siding Spring Survey, another NASA-funded NEA survey. Further observations from around the globe over the next several days allowed the Minor Planet Center to confirm the connection to the June discovery, at which point the possibility of impact in 2029 was realized by the automatic SENTRY system of NASA’s Near-Earth Object Program Office. NEODyS, a similar automatic system at the University of Pisa and the University of Valladolid, Spain also detected the impact possibility and provided similar predictions.

Original Source: NASA News Release

Image credit: NASA/JPL
University of Arizona scientists have discovered why Eros, the largest near-Earth asteroid, has so few small craters.

When the Near Earth Asteroid Rendezvous (NEAR) mission orbited Eros from February 2000 to February 2001, it revealed an asteroid covered with regolith — a loose layer of rocks, gravel and dust — and embedded with numerous large boulders. The spacecraft also found places where the regolith apparently had slumped, or flowed downhill, exposing fresh surface underneath.

But what NEAR didn’t find were the many small craters that scientists expected would pock Eros’ landscape.

“Either the craters were being erased by something or there are fewer small asteroids than we thought,” James E. Richardson Jr. of UA’s planetary sciences department said.

Richardson concludes from modeling studies that seismic shaking has obliterated about 90 percent of the asteroid’s small impact craters, those less than 100 meters in diameter, or roughly the length of a football field. The seismic vibrations result when Eros collides with space debris.

Richardson, Regents’ Professor H. Jay Melosh and Professor Richard Greenberg, all with UA’s Lunar and Planetary Laboratory, report the analysis in the Nov. 26 issue of Science.

“Eros is only about the size of Lake Tahoe — 20 miles (33 kilometers) long by 8 miles (13 kilometers) wide,” Richardson said. “So it has a very small volume and a very low gravity. When a one-to-two-meter or larger object hits Eros, the impact will set off global seismic vibrations. Our analysis shows how these vibrations easily destabilize regolith overlaying the surface.”

A rock-and-dust layer creeps, rather than crashes, down shaking slopes because of Eros’ weak gravity. The regolith not only slides down horizontally, but also is launched ballistically from the surface and ‘hops’ downslope. Very slowly, over time, impact craters fill up and disappear, Richardson said.

If Eros were still in the main asteroid belt between Mars and Jupiter, a 200-meter crater would fill in about 30 million years. Because Eros is now outside the asteroid belt, that process takes a thousand times longer, he added.

Richardson’s research results match the NEAR spacecraft evidence. Instead of the expected 400 craters as small as 20 meters (about 70 feet) per square kilometer (three-fifths mile) on Eros’ surface, there are on average only about 40 such craters.

The modeling analysis also validates what scientists suspect of Eros’ internal structure.

“The NEAR mission showed Eros to most likely be a fractured monolith, a body that used to be one competent piece of material,” Richardson said. “But Eros has been fractured throughout by large impacts and is held together primarily by gravity. The evidence is seen in a series of grooves and ridges that run across the asteroid’s surface both globally and regionally.”

Large impacts fracture Eros to its core, but many smaller impacts fracture only the upper surface. This gradient of big fractures deep inside and numerous small fractures near the surface is analogous to fractures in the upper lunar crust, Richardson said. “And we understand the lunar crust — we’ve been there. We’ve put seismometers on the moon. We understand how seismic energy propagates through this kind of structure.”

The UA scientists’ analysis of how impact-induced seismic shaking has modified Eros’ surface has a couple of other important implications.

“If we eventually do send spacecraft to mine resources among the near-Earth asteroids or to deflect an asteroid from a potential collision with the Earth, knowing internal asteroid structure will help address some of the strategies we’ll need to use. In the nearer future, sample return missions will encounter successively less porous, more cohesive regolith as they dig farther down into asteroids like Eros, which has been compacted by seismic shaking,” Richardson noted.

“And it also tells us about the small asteroid environment that we’ll encounter when we do send a spacecraft out into the main asteroid belt, where Eros spent most of its lifetime. We know the small asteroids — those between the size of a beachball and a football stadium — are out there. It’s just that their ‘signature’ on asteroids such as Eros is being erased,” Richardson said.

This finding is important because the cratering record on large asteroids provides direct evidence for the size and population of small main-belt asteroids. Earth-based telescopic surveys have catalogued few main-belt asteroids that small. So scientists have to base population estimates for these objects primarily on visible cratering records and asteroid collisional history modeling, Richardson said.

Original Source: UA News Release

Today, September 29, 2004, is undisputedly the Day of Toutatis, the famous “doomsday” asteroid.

Not since the year 1353 did this impressive “space rock” pass so close by the Earth as it does today. Visible as a fast-moving faint point of light in the southern skies, it approaches the Earth to within 1,550,000 km, or just four times the distance of the Moon.

Closely watched by astronomers since its discovery in January 1989, this asteroid has been found to move in an orbit that brings it close to the Earth at regular intervals, about once every four years. This happened in 1992, 1996, 2000 and now again in 2004.

Radar observations during these passages have shown that Toutatis has an elongated shape, measuring about 4.6 x 2.4 x 1.9 km. It tumbles slowly through space, with a rotation period of 5.4 days.

The above images of Toutatis were taken with the ESO Very Large Telescope (during a technical test) in the evening of September 28. They were obtained just over 12 hours before the closest approach that happens today at about 15:40 hrs Central European Summer Time (CEST), or 13:40 hrs Universal Time (UT). At the time of these observations, Toutatis was about 1,640,000 km from the Earth, moving with a speed of about 11 km/sec relative to our planet.

They show the asteroid as a fast-moving object of magnitude 10, about 40 times fainter than what can be perceived with the unaided, dark-adapted eye. They also prove that Toutatis is right on track, following exactly the predicted trajectory in space and passing the Earth at a safe distance, as foreseen.

Detailed calculations, taking into account all available observations of this celestial body, have shown that although Toutatis passes regularly near the Earth, today’s passage is the closest one for quite some time, at least until the year 2562. The ESO observations, obtained at a moment when Toutatis was very close to the Earth, will help to further refine the orbital calculations.

The “parallax effect” demonstrated!
Simultaneous images obtained with telescopes at ESO’s two observatories at La Silla and Paranal demonstrate the closeness of Toutatis to the Earth. As can be seen on the unique ESO PR Photo 28e/04 that combines two of the exposures from the two observatories, the sighting angle to Toutatis from the two observatories, 513 km km apart, is quite different. Astronomers refer to this effect as the “parallax”. The closer the object is, the larger is the effect, i.e., the larger will be the shift of the line-of-sight.

Interestingly, the measured angular distance in the sky of the beginnings (or the ends) of the two trails (about 40 arcsec), together with the known distance between the two observatories and the position of Toutatis in the sky at the moment of the exposures fully define the triangle “Paranal-Toutatis-La Silla” and thus allow to calculate the exact distance to the asteroid.

It is found to be very close to that predicted from the asteroid’s position in its orbit and that of the Earth at the moment of this unique observation, 1,607,900 km. This exceptional, simultaneous set of observations thus provides an independent measurement of Toutatis’ distance in space and, like the measured positions, a confirmation of its computed orbit.

More information about Toutatis is available at the dedicated webpage by the French discoverers and also at the specialised Near-Earth Objects – Dynamic Site.

Original Source: ESO News Release

Early Wednesday morning, a 5,500 million pound asteroid measuring 5 kilometers in length will pass very close to Earth.

An asteroid two to three times that diameter is credited with causing the extinction of 85 percent of the world’s species, including the dinosaurs, when it hit our planet 65 million years ago.

Luckily for us, asteroid Toutatis is only a tourist, and doesn’t plan to stop here. It will come within 1.5 million kilometers (960,000 miles) of Earth, or four times the Earth-moon distance. Toutatis is the largest asteroid to come that close in more than a century.

Many smaller asteroids often pass well inside the moon’s orbit. The Earth is also hit continually with tiny meteors that often become “shooting stars” as they harmlessly burn up in the atmosphere.

But if a rock the size of Toutatis hit, the atmosphere would do little more than slow it down a bit before it slammed to Earth. The impact would create a vast crater, and toss so much dust and vaporized minerals into the air the skies would darken. Seismic waves created by the explosion would generate tsunamis and earthquakes, and red-hot rocks falling back to Earth would ignite forest fires.

Toutatis, also known as asteroid 4179, is 4.6 kilometers (2.9 miles) long and 2.4 kilometers (1.5 miles) wide. Although Toutatis looks like a large peanut, radar images revealed it is actually composed of two rocks that are in close contact. One of the rocks is approximately twice as large as the other.

Toutatis has a strange rotation –instead of the spinning on a single axis, like the planets and most other asteroids do, Toutatis tumbles so erratically that its orientation with respect to the solar system never repeats.

“The vast majority of asteroids, and all the planets, spin about a single axis, like a football thrown in a perfect spiral,” says Scott Hudson of Washington State University, “but Toutatis tumbles like a flubbed pass.”

Toutatis’s four-year orbit around the sun is also eccentric, extending from just inside the Earth’s orbit to the main asteroid belt between Mars and Jupiter.

Astronomer Christian Pollas discovered Toutatis on January 4, 1989. Pollas spotted the asteroid on photographic plates taken by Alain Maury and Derral Mulholland, who had taken the photos while observing Jupiter’s satellites.

Toutatis flew close by Earth in 1992 and 1996, but it hasn’t come this near to us since 1353. The next time it will pass this close again will be in the year 2562. The asteroid’s orbit around the sun is so eccentric that it can’t be predicted with much certainty for more than a few hundred years in the future. Since researchers can’t say Toutatis will never hit Earth, it is currently listed as a Potentially Hazardous Asteroid.

There is a rumor circulating on the Internet that the asteroid will strike Earth during this 2004 flyby. However, astronomers have been tracking the path of Toutatis ever since it was discovered, and they are certain it will pass safely by Earth.

Throughout history, several asteroids have hit Earth. The solar system was cluttered with asteroids while the Earth was young, and the face of the moon and other dead planetary bodies shows how frequent such impacts were. Impacts by large rocks are much less frequent today, but they can still occur.

There are thought to be more than 300,000 nearby small asteroids (asteroids about 100 meters across). Such asteroids should statistically hit Earth once every few thousand years. The most recent such asteroid strike occurred in 1908, when an asteroid measuring about 60 meters in diameter hit Russia. The “Tunguska” bolide exploded in the atmosphere and flattened about 700 square miles of Siberian forest.

Large (1 kilometer or greater) asteroids are far more rare and infrequent. There are only about 1,100 nearby large asteroids, and they are predicted to strike the Earth every half million years or so. But when these asteroids strike, they can cause catastrophic changes in the global climate. Asteroids that cause mass extinctions are thought to be 10 kilometers or greater in diameter.

The Spaceguard Survey was established to track large asteroids and comets that might pose a direct threat to Earth. So far, the Spaceguard Survey has found about half of these NEOs, and they expect to find the majority of them by 2008.

Although Toutatis will be in the far southern sky when it is closest to Earth, the asteroid is expected to brighten a few days prior to a 10th magnitude point of light visible from the Northern Hemisphere. Sky-watchers should look for it near the bright star Delta Capricorni.

Toutatis won’t be visible to the naked eye, but binoculars should suffice for spotting it in the night sky. A telescope would provide the best viewing, because it would allow the viewer to detect the slow motion of Toutatis against the background stars.

Original Source: Astrobiology Magazine Article