Camera Network Spies Anomalous Meteorite

A network of time-lapse cameras set up in the Nullarbor Plain desert of Western Australia has allowed researchers to track a fallen meteorite to the ground, and enabled them to determine its original orbit and parent body. The meteorite has a composition different than that of other meteors, leading researchers to believe that it originates from a different parent body than most meteorites that impact Earth. The Desert Fireball Network, a project coordinated by the Imperial College of London, was able to track the meteor when it entered the atmosphere, giving researchers an impact location and information on where it originated.

The Bunburra Rockhole meteorite – so named for the location where it was discovered – fell to the Earth on July 20th, 2007. The Desert Fireball Network cameras recorded the fireball produced when the meteor passed through the Earth’s atmosphere, and by studying the entry angle of the meteor, researchers from the Imperial College were able to locate it on the ground. It was found within 100 meters (300 feet) of where they had predicted it to be.

This meteorite weighs 324 grams (12 oz), and is composed of a rare type of basalt igneous rock. More specific information on the meteorite itself can be found on the Meteorological Society’s index. Most meteorites of this composition come from one parent body, the asteroid 4 Vesta. However, the Bunburra Rockhole meteorite likely came from a different asteroid with a different orbit, which means that the formation process for the asteroid happened in a different place in the Solar System than for 4 Vesta.

The researchers determined that the Bunburra Rockhole originated from an asteroid located in the innermost main asteroid belt between Mars and Jupiter. Because the Desert Fireball Network captured images on multiple cameras of how it entered the Earth’s atmosphere, the researchers were able to triangulate the position of the rock, and model its orbit backwards in time to determine its origins.

A fireball streaks across the sky over the Australian desert. When recorde by three different cameras, the origin of the meteorite can be deterimined. Image Credit: Phil Bland, Imperial College of London

Dr Gretchen Benedix of the Natural History Museum – where the largest fragment of the meteorite is located – analyzed the mineral content of the meteorite. She said in a press release:

“It’s vital to have a meteorite with information about where it comes from in the solar system…. We’ve known for a long time that most meteorites are from the asteroid belt, but we don’t know exactly where. This kind of information helps us fit one more piece in the puzzle of how the solar system formed and evolved. The fact that this meteorite is compositionally unusual increases it’s value even more. It helps us to uncover more information about the conditions of the early solar system.”

The Desert Fireball Network monitors the Nullarbor desert in Western Australia, and has tracked a total of 7 meteorites, three of which have been recovered. The desert is an excellent location for this type of project, as observing conditions are clear many nights out of the year, and the sparse vegetation and monotone landscape make finding the meteorites easier than in other locations.

The results of the meteorite mineral and orbital study are published in Science, and two previous papers about the Bunburra Rockhole are available on the Desert Fireball Network site.

Source: Natural History Museum, Imperial College of London

Asteroid Explosion over Indonesia

This has taken awhile to filter into the Western press, but an asteroid exploded over the town of Bone,Indonesia on October 8th at around 11am local time. Initially, locals called the police to report that a plane had crashed, or that an earthquake shook the ground, as reported in the Jakarta Globe. The Jakarta Post quoted Thomas Djamaluddin, head of the Lapan Center for Climate and Atmosphere Science Implementation as saying that the explosion was due to a meteorite or bit of space junk that had entered the Earth’s atmosphere. As it turns out after further analysis, the explosion was due to an asteroid about 5-10 meters (15-30 feet) in diameter exploding in the air between 15 and 20 km (nine to 12 miles) above sea level. Nobody was injured as a result of the explosion, but it evidently caused quite a scare with the local population!

In a press release from the Near Earth Object (NEO) program, the explosion was detected by many International Monitoring System (IMS) infrasound stations, five of them 10,000 km (6200 miles) away, and one 18,000 km (11,100 miles) from the blast.  These stations monitor seismic waves, infrasound (low frequency soundwaves), hydroacoustic, and radionuclide emissions as part of the Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO). They are well equipped to monitor explosions of nuclear weapons, but also detect other events such as meteorite impacts and asteroid explosions, tsunamis and earthquakes.

When analyzed, the amount and intensity of low frequency sound waves created by the explosion allowed researchers Elizabeth Silber and Peter Brown of the Meteor Infrasound Group at the Univ. of Western Ontario to determine that the explosion caused by the asteroid was on the scale of 30 – 50 kilotons of TNT. To give you an idea of how powerful of an explosion this is, the bombs dropped over Hiroshima and Nagasaki in World War II exploded with the force of 20 kilotons of TNT.

The fireball – also called a bolide – created a dusty tail upon entering the atmosphere of the Earth. It is estimated that the asteroid was traveling around 72,000 km/hour (45,000 miles/hour) when it hit the atmosphere. As an asteroid enters the thick Earth atmosphere, it slows down abruptly and heats up due to the process of ablation. If this asteroid were made of metal instead of rock, it would likely have impacted the ground causing a lot of damage. Fortunately for the residents of Bone and the surrounding area, the rock broke up in a large fireball instead. There haven’t been any reports of pieces that have touched down as of yet.

Asteroids of this size are predicted to impact the Earth about every 2-12 years, and the last one of this magnitude was a bolide over the Marshall Islands on February 1, 1994. That impactor was estimated to be between 4.4 and 13.5 meters. A full analysis of that event is available on the SAO/NASA Astrophysics Data System.

Of course, events like this always raise the question of why the object wasn’t detected before it even entered the atmosphere. The NEO program has cataloged over 600 objects in the size of 10 meters, but there are many, many more out there. The cost of a monitoring and cataloging all of the Near Earth Objects would be in the hundreds of millions of dollars, but more events like this may spur the political will and capital to further efforts at protecting human lives from the potential damage of meteorite impacts.

Source: Night Sky, Spaceweather.com, JPL Press Release

Giant Impact Near India — Not Mexico — May Have Killed Dinosaurs

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A huge, mysterious basin off the coast of India could be the largest, multi-ringed impact crater ever found on Earth. And if a new study is right, this impact may supercede the one that created the Chicxulub crater off Mexico’s Yucatán Peninsula as what may have been responsible for killing the dinosaurs 65 million years ago. Sankar Chatterjee of Texas Tech University and a team of researchers have been studying a 500-kilometer-wide (300-mile-wide) depression on the Indian Ocean seafloor which was likely created by a bolide perhaps 40 kilometers (25 miles) in diameter. Such an event would have triggered worldwide climate changes, including intensified volcanism, that led to mass extinction.

Since the 1990’s the leading candidate for what killed the dinosaurs was a ten-kilometer-wide (six-mile-wide) asteroid thought to have carved out the Chicxulub crater. This impact may have done the job, but if not, 300,000 later the impact that created the Shiva basin surely would have finished off large life on Earth.

The massive Shiva basin, a submerged depression west of India that is intensely mined for its oil and gas resources. Some complex craters are among the most productive hydrocarbon sites on the planet.

“If we are right, this is the largest crater known on our planet,” Chatterjee said. “A bolide of this size, creates its own tectonics.”

However, some geologists have disputed whether the Shiva depression was created by an impact, or if it is just a hole in Earth’s crust, possibly created by volcanism. Christian Koeberl, a geochemist at the University of Vienna in Austria, has been adamant in the past that Shiva is not an impact crater. He said not only is there no evidence of impact in the case of Shiva, there is no crater structure. He calls Shiva, “a figment of imagination.”

“There’s not even ambiguous evidence, or inconclusive evidence,” says Koeberl. “There are a couple of people that keep pushing for some crater in the Indian Ocean, but this is inconsistent not only with the regional geology and geophysics, but also with anything we know about impact cratering.”

But Chatterjee feels sure that Shiva is an impact crater and said the geological evidence is dramatic. Shiva’s outer rim forms a rough, faulted ring some 500 kilometers in diameter, encircling the central peak, known as the Bombay High, which would be 3 miles tall from the ocean floor (about the height of Mount McKinley). Most of the crater lies submerged on India’s continental shelf, but where it does come ashore it is marked by tall cliffs, active faults and hot springs. The impact appears to have sheared or destroyed much of the 30-mile-thick granite layer in the western coast of India.

If the huge depression was created by an impact, Earth’s crust at the point of collision would have been vaporized, leaving nothing but ultra-hot mantle material to well up in its place. It is likely that the impact enhanced the nearby Deccan Traps volcanic eruptions that covered much of western India. What’s more, the impact broke the Seychelles islands off of the Indian tectonic plate, and sent them drifting toward Africa.

The team hopes to go India later this year to examine rocks drill from the center of the putative crater for clues that would prove the strange basin was formed by a gigantic impact.

“Rocks from the bottom of the crater will tell us the telltale sign of the impact event from shattered and melted target rocks. And we want to see if there are breccias, shocked quartz, and an iridium anomaly,” Chatterjee said. Asteroids are rich in iridium, and such anomalies are thought of as the fingerprint of an impact.

Read the Abstract

Source: Geological Society of America

Asteroid Pallas is Also a Protoplanet

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Some objects in the solar system are in a “gray area,” and can be classified under more than one heading. Add the asteroid Pallas to that group. New close-up images of Pallas from the Hubble Space Telescope reveal that the second largest asteroid in the solar system appears to be a protoplanet, as well.

Britney E. Schmidt, a UCLA doctoral student, led a team of researchers to create a 3D model of the 600km-wide rock which lies within the main asteroid belt between the orbits of Jupiter and Mars.

With the Hubble images, Schmidt and her colleagues were able to take new measurements of Pallas’ size and shape. What they found showed that Pallas wasn’t just a big rock made of hydrated silicate and ice.

An artist’s conception of an impact event on Pallas. This artwork was created using the three-dimensional shape model published by Britney Schmidt, et al. in Science. Credit: Image courtesy of B. E. Schmidt and S. C. Radcliffe
An artist’s conception of an impact event on Pallas. This artwork was created using the three-dimensional shape model published by Britney Schmidt, et al. in Science. Credit: Image courtesy of B. E. Schmidt and S. C. Radcliffe

“It was incredibly exciting to have this new perspective on an object that is really interesting and hadn’t been observed by Hubble at high resolution,” Schmidt said of the first high-resolution images of Pallas, which is believed to have been intact since its formation, most likely within a few million years of the birth of our solar system.

“We were trying to understand not only the object, but how the solar system formed,” Schmidt said. “We think of these large asteroids not only as the building blocks of planets but as a chance to look at planet formation frozen in time.”

Visible in the Hubble images were areas of dark and light on Pallus’ surface, indicating that the water-rich body might have undergone an internal change in the same way planets do.

“That’s what makes it more like a planet — the color variation and the round shape are very important as far as understanding, is this a dynamic object or has it been exactly the same since it’s been formed?” Schmidt said. “We think it’s probably a dynamic object.”

For the first time, a large depression was also seen on Pallas. They were unable to determine if it was a crater, but the depression did suggest something else important: that it could have led to Pallas’ small family of asteroids orbiting in space.

“It’s interesting, because there are very few large, intact asteroids left,” Schmidt said. “There were probably many more. Most have been broken up completely. It’s an interesting chance to almost look into the object, at the layer underneath. It’s helping to unravel one of the big questions that we have about Pallas, why does it have this family?”

The massive body is unique, she said, partly because “its orbit is so much different from other asteroids. It’s highly inclined.”

“It was incredibly exciting to have this new perspective on an object that is really interesting and hadn’t been observed by Hubble at high resolution,” said Schmidt.

“When people think of asteroids, they think of ‘Star Wars’ or of tiny little rocks floating through space,” Schmidt said. “But some of these have been really physically dynamic. Around 5 million years after the formation of the solar system, Pallas was probably doing something kind of interesting.”

Source: PhysOrg

More Water ‘Out There:’ Ice Found on Asteroid

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For the first time, astronomers have confirmed that an asteroid contains frozen water on its surface. Analysis of asteroid 24 Themis shows evidence of water ice along with organic compounds widespread across the surface. The scientists say these new findings support the theory that asteroids brought both water and organic compounds to the early Earth, helping lay the foundation for life on the planet.

Humberto Campins of the University of Central Florida in Orlando and colleagues recorded spectra of 24 Themis over a seven-hour period, and were able to study 84 percent of the rotational period of the spinning rock, Rob Cowen reported in Science News. Using NASA’s Infrared Telescope Facility on Hawaii’s Mauna Kea, the spectra revealed the consistent presence of frozen water as different parts of the asteroid’s surface came into view.

Analyses of the sunlight reflected off the asteroid also show that organic compounds are widespread on the surface, he added, including polycyclic aromatic hydrocarbons, CH2 and CH3.

The new finding corroborates earlier observations of the same asteroid by astronomers Andrew S. Rivkin and Joshua Emery who also used the Infrared Telescope Facility. Over several years, Rivkin and Emery had found evidence of frozen water in single spots on 24 Themis but had not studied the asteroid as it made one entire rotation. Together, the two teams’ findings reveal that the asteroid’s entire surface is coated with frozen water, Campins says.

The 160-kilometer wide asteroid averages a distance from the sun of about 3.2 times that of Earth’s. At that range, frozen water on the surface would readily vaporize, Campins said. That means the ice must be continually replenished, possibly by a reservoir of frozen water within the rock.

One possibility is that ice lies buried several meters below the surface of 24 Themis, and when hit by space debris, the ice makes its way to the surface. If this is the case, it could confirm that some asteroids resemble comets, becoming active suddenly and venting material into space when pockets of ice vaporize, Campins said.

Another option is that an action similar to the recent findings of water on the Moon, where solar wind interacts with a rocky body without an atmosphere to create H2O and OH molecules. Without an atmosphere, the body is exposed to solar wind, which includes hydrogen ions. The hydrogen is able to interact with oxygen in surface of the asteroid to create water molecules.

Campins shared his findings at the annual meeting of the American Astronomical Society’s Division for Planetary Sciences.

Source: Science News

Apophis’ Odds of Earth Impact Downgraded

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NASA scientists have recalculated the path of the large asteroid Apophis, significantly downgrading the odds of it hitting Earth. Using new information, the refined path indicates a 1 in 250,000 chance of impact in 2036, reduced from the 1 in 45,000 odds calculated earlier. The asteroid is expected to make a record-setting — but harmless — close approach to Earth on Friday, April 13, 2029, when it comes no closer than 18,300 miles above Earth’s surface.

The new information provided a more accurate glimpse of 2036 Apophis’ orbit well into the latter part of this century. Among the findings is another close encounter by the asteroid with Earth in 2068 with chance of impact currently at approximately 1 in 333,000. As with earlier orbital estimates where Earth impacts in 2029 and 2036 could not initially be ruled out due to the need for additional data, it is expected that the 2068 encounter will diminish in probability as more information about 2029 Apophis is acquired.

Initially, Apophis was thought to have a 2.7 percent chance of impacting Earth in 2029. Additional observations of the asteriod ruled out any possibility of an impact in 2029.

The Apophis asteroid is approximately the size of two-and-a-half football fields.

“The refined orbital determination further reinforces that Apophis is an asteroid we can look to as an opportunity for exciting science and not something that should be feared,” said Don Yeomans, manager of the Near-Earth Object Program Office at JPL. “The public can follow along as we continue to study Apophis and other near-Earth objects by visiting us on our AsteroidWatch Web site and by following us on the @AsteroidWatch Twitter feed.”

The new data were documented by near-Earth object scientists Steve Chesley and Paul Chodas at NASA’s Jet Propulsion Laboratory. A majority of the data that enabled the updated orbit of Apophis came from observations made by Dave Tholen and collaborators at the University of Hawaii’s Institute for Astronomy in Manoa. Tholen pored over hundreds of previously unreleased images of the night sky made with the University of Hawaii’s 88-inch telescope, located near the summit of Mauna Kea.

Tholen made improved measurements of the asteroid’s position in the images, enabling him to provide Chesley and Chodas with new data sets more precise than previous measures for Apophis. Measurements from the Steward Observatory’s 90-inch Bok telescope on Kitt Peak in Arizona and the Arecibo Observatory on the island of Puerto Rico also were used in Chesley’s calculations.

“Apophis has been one of those celestial bodies that has captured the public’s interest since it was discovered in 2004,” said Chesley. “Updated computational techniques and newly available data indicate the probability of an Earth encounter on April 13, 2036, for Apophis has dropped from one-in-45,000 to about four-in-a million.”

The science of predicting asteroid orbits is based on a physical model of the solar system which includes the gravitational influence of the sun, moon, other planets and the three largest asteroids.

NASA detects and tracks asteroids and comets passing close to Earth using both ground and space-based telescopes. The Near Earth-Object Observations Program, commonly called “Spaceguard,” discovers these objects, characterizes a subset of them and plots their orbits to determine if any could be potentially hazardous to our planet.

Source: NASA

Understanding 2008 TC3 a Year After Impact

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The first asteroid to have been spotted before hitting Earth, 2008 TC3, crashed in northern Sudan one year ago on October 6. Several astronomers have been trying to piece together a profile of this asteroid, pulling together information from meteorites found at the impact site and the images captured of the object in the hours before it crashed to Earth.

“We have a gigantic jigsaw puzzle on our hands, from which we try to create a picture of the asteroid and its origins,” said SETI Institute astronomer Peter Jenniskens, who worked at the crash site, “and now we have with a composite sketch of the culprit, cleverly using the eyewitness accounts of astronomers that saw the asteroid sneak up on us.” Their description? 2008 TC3 looked like a loaf of walnut-raisin bread.

“The asteroid now has a face,” said Jenniskens, chair of the special session at the fall meeting for the Division for Planetary Sciences of the American Astronomical Society. Last December, Jenniskens and Sudan astronomer Muawia Shaddad went to the crash site and recovered 300 fragments in the Nubian Desert. Like detectives, students from the University of Khartoum helped sweep the desert to look for remains of the asteroid. They found many different-looking meteorites close to, but a little south, of the calculated impact trajectory.

The team has also been able to recreate the shape of the asteroid from looking at images captured by Astronomers Marek Kozubal and Ron Dantowitz of the Clay Center Observatory in Brookline, Massachusetts, who tracked the asteroid with a telescope and captured the flicker of light during a two hour period just before impact.

An irregular shape and rapid tumbling caused asteroid 2008 TC3 to flicker when it reflected sunlight on approach to Earth.

Peter Scheirich and colleagues at Ondrejov Observatory and Charles University in the Czech Republic combined all the various observations to work out the shape and orientation of the asteroid.

Watch a video recreation of 2008 TC3 tumbling in space.

Larger version. (1.32 MB Mpeg 4 file)

Video of 2008 TC3 as seen through a telescope (large file, 7.63 MB)

Other forensic evidence based on analysis of the recovered meteorites at the Almahata Sitta site showed the asteroid was an unusual “polymict ureilite” type. Jason S. Herrin of NASA’s Johnson Space Center confirmed that the meteorites still carry traces of being heated to 1150-1300 degrees C, before rapidly cooling down at a rate of tens of degrees C per hour, during which carbon in the asteroid turned part of the olivine mineral iron into metallic iron. Hence, asteroid 2008 TC3 is the remains of a minor planet that endured massive collisions billions of years ago, melting some of the minerals, but not all, before a final collision shattered the planet into asteroids.

Mike Zolensky of NASA’s Johnson Space Center first pointed out that, as far as ureilites are concerned, his meteorite is unusually rich in pores, with pore walls coated by crystals of the mineral olivine. He now reports, from X-ray tomography work with Jon Friedrich of Fordham University in New York, that those pores appear to outline grains that have been incompletely welded together and that the pore linings appear to be vapor phase deposits. According to Zolensky, “Almahata Sitta may represent an agglomeration of coarse- to fine-grained, incompletely reduced pellets formed during impact, and subsequently welded together at high temperature.”

The carbon in the recovered meteorites is among the most cooked of all known meteorites. Carbon crystals of graphite and nanodiamonds have been detected. Still, it turns out that some of the organic matter in the original material survived the heating. Amy Morrow, Hassan Sabbah, and Richard Zare of Stanford University have found polycyclic aromatic hydrocarbons in high abundances. Amazingly, Michael Callahan and colleagues of NASA’s Goddard Space Flight Center now report that even some amino acids have survived.

Jenniskens and Shaddad plan to revisit the scene of the crash in the Nubian Desert. They reported their findings at the Division for Planetary Sciences of the American Astronomical Society meeting in Puerto Rico.

Listen to Oct. 6th’s 365 Days of Astronomy podcast by Emily Lakdawalla about 2008 TC3.

Source: AAS Planetary Science Division

NASA Tests New Robotic Lander for Future Moon, Asteroid Missions

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The best way to study the new-found water on the Moon would be with in-situ instruments. Since humans won’t be making any lunar landings for at least a decade, the next best option is robotic spacecraft. NASA’s Marshall Space Flight Center is developing and testing a new robotic lander to explore not only the Moon, but also asteroids and Mars. This design is definitely next generation: it’s bigger than any lander yet and MSFC is currently testing the all-important final of reaching the destination: landing.

“Specifically, what we are doing at Marshall is identifying the terminal – or the final – phase of landing, and designing a robotic lander to meet those needs,” said Brian Mulac, a test engineer at Marshall, quoted in an article in the Huntsville Times. “That last part is the highest risk of setting down on the moon.”

Of course, parachutes can’t be used for landing on the Moon or asteroids, since neither destination has an atmosphere, so thrusters are key for landing.

Large, oval-shaped tanks on the craft are used to store fuel for thrusters. Thrusters guide the lander, controlling the vehicle’s altitude and speed for landing. An additional thruster on this test vehicle, above, offsets the effect of Earth’s gravity so that the other thrusters can operate as they would in a lunar environment.

Just in case the tests don’t go as planned, a huge net is place under the lander to catch the vehicle and avoid damaging it.

As the saying goes, it’s not the fall that’s dangerous, but the sudden stop.

Landing on Mars requires a different architecture, such as the Mars Science Laboratory’s sky-crane, because of the pesky, thin atmosphere on the Red Planet. Read our previous article with Rob Manning of JPL about the issues of landing large payloads on Mars.

Sources: Huntsville Times, Gizmodo

K-T Boundary

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What killed the dinosaurs? That’s a question that has puzzled paleontologists since dinosaurs were first discovered. Maybe the global climate changed, maybe they were killed by disease, volcanoes, or the rise of mammals. But in the last few decades, a new theory has arisen; an asteroid strike millions of years ago drastically changed the Earth’s environment. It was this event that pushed the dinosaurs over the edge into extinction. What’s the evidence for this asteroid impact? A thin dark line found in layers of sediment around the world; evidence that something devastating happened to the planet 65 million years ago. This line is known as the K-T boundary.

What is the K-T boundary? K is actually the traditional abbreviation for the Cretaceous period, and T is the abbreviation for the Tertiary period. So the K-T boundary is the point in between the Cretaceous and Tertiary periods. Geologists have dated this period to about 65.5 million years ago.

When physicist Luis Alvarez and geologist Walter Alvarez studied the K-T boundary around the world, they found that it had a much higher concentration of iridium than normal – between 30-130 times the amount of iridium you would expect. Iridium is rare on Earth because it sank down into the center of the planet as it formed, but iridium can still be found in large concentrations in asteroids. When they compared the concentrations of iridium in the K-T boundary, they found it matched the levels found in meteorites.

The researchers were even able to estimate what kind of asteroid must have impacted the Earth 65.5 million years ago to throw up such a consistent layer of debris around the entire planet. They estimated that the impactor must have been about 10 km in diameter, and release the energy equivalent of 100 trillion tons of TNT.

When that asteroid struck the Earth 65.5 million years ago, it destroyed a region thousands of kilometers across, but also threw up a dust cloud that obscured sunlight for years. That blocked photosynthesis in plants – the base of the food chain – and eventually starved out the dinosaurs.

Researchers now think that the asteroid strike that created the K-T boundary was probably the Chicxulub Crater. This is a massive impact crater buried under Chicxulub on the coast of Yucatan, Mexico. The crater measures 180 kilometers across, and occurred about 65 million years ago.

Geologists aren’t completely in agreement about the connection between the Chicxulub impact and the extinction of the dinosaurs. Some believe that other catastrophic events might have helped push the dinosaurs over the edge, such as massive volcanism, or a series of impact events.

We have written many articles about the K-T boundary for Universe Today. Here’s an article about how the dinosaurs probably weren’t wiped out by a single asteroid, and here’s an article about how asteroids and volcanoes might have done the trick.

Here’s more information from the USGS, and an article from NASA.

We have recorded an episode of Astronomy Cast all about asteroid impacts. Listen to it here: Episode 29: Asteroids Make Bad Neighbors.

Reference:
USGS

What are Planetoids?

Planetoid is another term for asteroids, which are also called minor planets. Planetoids are small celestial bodies that orbit the Sun. Planets are simply defined as asteroids, but the term asteroid is not well defined either. In 2006, The International Astronomical Union (IAU) defined it as  a “small Solar System body” (SSSB), which does not really tell us anything either. Webster’s Dictionary defines an asteroid as, “any of the thousands of small planets ranging from 1,000 km (621 mi) to less than one km (0.62 mi) in diameter, with orbits usually between those of Mars and Jupiter; minor planet; planetoid.”

Asteroids – planetoids – were first discovered in 1801, and many more have been discovered since then. Up until 1977, almost all the asteroids discovered were near Jupiter. However, then astronomers began to discover planetoids even farther out and started calling them centaurs and trans-Neptunian objects (TNOs). When a region of space in the outer Solar System filled with celestial bodies was discovered, it was called the Kuiper Belt and the objects in it were called Kuiper Belt Objects (KBOs). The large number of synonyms for planetoids is one reason why keeping these terms straight is so difficult.

Some of the largest planetoids are spherical and look like tiny versions of planets. The smaller ones are irregular in shape though. The objects range in size from around ten meters to hundreds of kilometers in diameter. Objects smaller than ten meters are called meteoroids. Unfortunately, astronomers do not know that much about the materials that make up planetoids. They are believed to be composed of various materials including ice, rock, and different metals.

Most planetoids are in a region called the asteroid belt, which is situated between Mars and Jupiter. There are millions of planetoids in this region. Despite the millions of objects, all of them combined are believed to have a mass of only about 4% of the Moon’s mass. After being discovered, the planetoids are given a temporary designation. If they are officially recognized, they are given a number and maybe a name. The first few planetoids were given symbols just like the planets. All except one of the first fifteen asteroids were given  extremely complex symbols. For example, one symbol was a star with a plant growing out of it. However, that soon ended when astronomers realized that there were many more planetoids. Planetoids, and other celestial bodies, are a subject of study by astronomers who hope to learn more about how the universe was formed from these ancient rocks.

Universe Today has articles on minor planets and planetesimals.

Check out articles on asteroids and planetoids beyond Pluto.

Astronomy Cast has an episode on asteroids.

References:
NASA StarChild: The Asteroid Belt
Planet-Like Body Discovered at Fringes of Our Solar System