Potentially Hazardous Asteroid 2010 GU21

From The Astronomer's Telegram on 2010 GU21...

Broadband Photometry of the Potentially Hazardous Asteroid 2010 GU21

ATel #2592; M. Hicks (JPL/Caltech), J. Somers (Moorpark), J. Foster (CSULA), A. McAuley (CSULA)
on 30 Apr 2010; 21:51 UT
Password Certification: Michael D. Hicks (Michael.Hicks@jpl.nasa.gov)

Subjects: Optical, Asteroids, Planets, Planets (minor), Solar System Objects

The Near-Earth Asteroid (NEA) 2010 GU21 was discovered by the Catalina Sky Survey on April 5 2010 (MPEC 2010-G55) and has been designated as a Potentially Hazardous Asteroid (PHA) by the Minor Planet Center. We obtained Bessel BVRI photometry over the course of three nights at the JPL Table Mountain 0.6-m telescope (TMO), as illustrated in [1][2][3] and summarized in Table 1. Though the nights were clear and photometric to the 1-3% level, the high humidity, nearly full Moon, and low lunar elongation hampered our observations. 2010 GU21 will pass within approximately 8 lunar distances on May 05.25 2010 UT. This object can be considered a potential low delta-V spacecraft rendezvous target (dV=6.18 km/s).

The rotationally averaged colors (B-R=1.114+/-0.028 mag; V-R=0.402+/-0.020 mag; R-I=0.376+/-0.019 mag) of 2010 GU21 were found most compatible with an Xc-type spectral classification, an association obtained through a comparison of our colors with the 1341 asteroid spectra in the SMASS II database (Bus & Binzel 2002) [Figure 4 and Table 2]. X-type asteroids include high albedo E-types, moderate albedo M-types, and low-albedo P-type asteroids (Barucci & Tholen 1989). Moderate resolution spectroscopy, thermal flux measurements, and/or solar phase curves would be very useful in resolving this ambiguity. 2010 GU21 remains brighter than V=18 (our nominal cut-off for photometry at TMO) and at moderate declinations until May 10 2010 UT. We welcome collaborations with other observers.

Our light-time corrected photometry was converted to reduced magnitude assuming a phase parameter G=0.05, consistent with a low-albedo asteroid. The BVI data were registered to R using our nightly measured colors. After converting the photometry from magnitude to flux units, we performed a rotational period search using standard Fourier techniques. Figure 5 plots chi-squared 5th and 6th-order Fourier model misfit as a function of assumed rotation period. Assuming a double-peaked lightcurve, we found a best-fit synodic period P_syn = 4.326+/-0.005 hr, as shown in Figure 6 . Our photometry yields an absolute magnitude H_v=20.78+/-0.02 mag, implying an effective diameter D~200m for an albedo rho=0.05.

Copyright 2010. All rights reserved. The research described in this telegram was carried out at the Jet Propulsion Laboratory, under a contract with the National Aeronautics and Space Administration. The student participation was supported by the National Science Foundation under REU grant 0852088 to Cal State LA.

Link: Astronomer's Telegram (Broadband Photometry of the Potentially Hazardous Asteroid 2010 GU21)

Radar Observations of Near-Earth Asteroid 2005 YU55

Radar image of asteroid 2005 YU55. Image credit: NASA/Cornell/Arecibo 

From the JPL Asteroid Watch news story...

Near-Earth asteroid 2005 YU55 was "imaged" by the Arecibo Radar Telescope in Puerto Rico on April 19. Data collected during Arecibo's observation of 2005 YU55 allowed the Near-Earth Object Program Office at NASA's Jet Propulsion Laboratory to refine the space rock's orbit, allowing scientists to rule out any possibility of an Earth impact for the next 100 years.

The space rock was about 2.3 million kilometers (1.5 million miles) from Earth at the time this image of the radar echo was generated. The ghostly image has a resolution of 7.5 meters (25 feet) per pixel. It reveals 2005 YU55 as a spherical object about 400 meters (1,300 feet) in size.

Not only can the radar provide data on an asteroid's dimensions, but also on its exact location in space. Using Arecibo's high-precision radar astrometry capability, scientists were able to reduce orbit uncertainties for YU55 by 50 percent.

"At one time we had classified 2005 YU55 as a potential threat," said Steve Chesley, a scientist at JPL's Near-Earth Object Program Office. Prior to the Arecibo radar passes on April 19 thru 21, we had eliminated almost all upcoming Earth flybys as possibilities of impact. But there were a few that had a low remaining probability of impact. After incorporating the data from Arecibo, we were able to rule impacts out entirely for the next 100 years."

With more observations in the coming years, scientists may be able to accurately plot 2005 YU55's orbit even further out.

NASA detects, tracks and characterizes 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.

Link: JPL Story ("Radar Clicks Asteroid's Pic")

Water Ice, Organic Molecules on Asteroid 24 Themis

From the article and in Nature (Editor's Summary)...

In the April 29, 2010 issue of the journal Nature, two independent groups of astronomers announced the discovery of water ice and organic molecules on the surface of the 123 mile diameter asteroid, 24 Themis.

Andrew S. Rivkin and Joshua P. Emery observed 24 Themis seven different times between 2002 and 2008 with the NASA Infrared Telescope Facility 3 meter telescope. An independent group of nine astronomers led by Humberto Campins observed 24 Themis over 84% of the asteroid's 8.37 hour rotational period on the night of January 23, 2008 using the same NASA telescope. Observations over most of the rotational period show that the water ice covers most of the asteroid's surface.

Both groups observed infrared spectra of the asteroid to determine the composition of its surface. There is a spectral feature at an infrared wavelength of 3.1 micrometers (A micrometer is a millionth of a meter.) that indicates the presence of water ice. Both groups of astronomers found strong absorption at this wavelength and concluded that 24 Themis has water ice covering the surface. The fact that two independent groups found the same results lends credence to the results.

After subtracting the effects of water ice in the spectrum, the astronomers found the residual spectral signature of organic molecules including hydrocarbons and aromatics. These organic molecules do not imply that there is life on the asteroid, however they do add to the considerable evidence that organic molecules, which are necessary for life, form easily.

24 Themis is about 479 million kilometers from the Sun in the main asteroid belt between Mars and Jupiter. At this distance from the Sun, water ice on the surface of the asteroid would sublimate into space. This fact suggests to astronomers that 24 Themis has a good supply of water ice beneath its surface to resupply the surface ice.

Many small bodies beyond the orbit of Jupiter have large amounts of ice in their composition, but this discovery is the first time that a rocky asteroid in the main asteroid belt has been found to contain ice. What are the implications of this discovery?

One theory for the origin of Earth's oceans is that impacts from asteroids or comets containing ice supplied the water. A competing theory is that the water originated in Earth's interior and was outgassed by volcanoes. If more asteroids have water ice and if a mission to asteroids shows that the water on the asteroids has the same ratio of hydrogen isotopes as Earth's water, then the theory that Earth's water originated from asteroid collisions would gain credence.

Link: Article

Letter
Nature 464, 1320-1321 (29 April 2010) | doi:10.1038/nature09029; Received 21 September 2009; Accepted 24 February 2010

Water ice and organics on the surface of the asteroid 24 Themis

Humberto Campins1, Kelsey Hargrove1, Noemi Pinilla-Alonso2, Ellen S. Howell3, Michael S. Kelley4, Javier Licandro5,6, T. Mothé-Diniz7, Y. Fernández1 & Julie Ziffer8

1. University of Central Florida, PO Box 162385, Orlando, Florida 32816-2385, USA
2. NASA-Ames Research Center, Moffett Field, California 94035, USA
3. NAIC-Arecibo Observatory, Arecibo, Puerto Rico 00612
4. University of Maryland, College Park, Maryland 20742, USA
5. Instituto de Astrofísica de Canarias, Calle Vía Láctea s/n, E-38200 La Laguna, Spain
6. Department of Astrophysics, University of La Laguna, E-38205 La Laguna, Spain
7. Universidade Federal Do Rio De Janeiro, RJ 20080-090, Brazil
8. University of Southern Maine, Department of Physics, Portland, Maine 04104, USA

Correspondence to: Humberto Campins1 Email: campins@physics.ucf.edu.

It has been suggested1, 2, 3 that Earth’s current supply of water was delivered by asteroids, some time after the collision that produced the Moon (which would have vaporized any of the pre-existing water). So far, no measurements of water ice on asteroids4, 5 have been made, but its presence has been inferred from the comet-like activity of several small asteroids, including two members of the Themis dynamical family6. Here we report infrared spectra of the asteroid 24 Themis which show that ice and organic compounds are not only present on its surface but also prevalent. Infrared spectral differences between it and other asteroids make 24 Themis unique so far, and our identification of ice and organics agrees with independent results7 that rule out other compounds as possible sources of the observed spectral structure. The widespread presence of surface ice on 24 Themis is somewhat unexpected because of the relatively short lifetime of exposed ice at this distance (~3.2 au) from the Sun. Nevertheless, there are several plausible sources, such as a subsurface reservoir that brings water to the surface through ‘impact gardening’ and/or sublimation.

Link: Nature Article

Link: Supplemental Material (PDF)


Letter
Nature 464, 1322-1323 (29 April 2010) | doi:10.1038/nature09028; Received 22 September 2009; Accepted 24 February 2010

Detection of ice and organics on an asteroidal surface

Andrew S. Rivkin1 & Joshua P. Emery2

1. Johns Hopkins University Applied Physics Laboratory, Laurel, Maryland 20723, USA
2. Earth and Planetary Science Department, University of Tennessee, Knoxville, Tennessee 37996, USA

Correspondence to: Andrew S. Rivkin1 Email: andy.rivkin@jhuapl.edu.

Recent observations, including the discovery1 in typical asteroidal orbits of objects with cometary characteristics (main-belt comets, or MBCs), have blurred the line between comets and asteroids, although so far neither ice nor organic material has been detected on the surface of an asteroid or directly proven to be an asteroidal constituent. Here we report the spectroscopic detection of water ice and organic material on the asteroid 24 Themis, a detection that has been independently confirmed2. 24 Themis belongs to the same dynamical family as three of the five known MBCs, and the presence of ice on 24 Themis is strong evidence that it also is present in the MBCs. We conclude that water ice is more common on asteroids than was previously thought and may be widespread in asteroidal interiors at much smaller heliocentric distances than was previously expected.

Link: Letter in Nature

Link: Supplemental Material (PDF)

Link: Nature Article - Editor's Summary

Scheeres Paper: Scaling forces to asteroid surfaces: The role of cohesion

Paper from Scheeres (et. al) on cohesion in asteroids. From the article...

Asteroids are strange celestial objects, particularly the smaller spinning variety, like the little potato-shaped asteroid named Itokawa. Scientists have been puzzling over what holds these smaller asteroids together, since they are too small for gravity to overcome the centripetal forces and hold the various bits of rubble as one to make an asteroid. And it turns out that Itokawa and its siblings have something in common with the gecko.

The puzzle is based on 2005 data collected by Japan's Hayabusa mission, which showed that at the rate at which Itokawa is spinning, the centripetal forces should overcome gravity and the pieces should fly off into space. But instead, they remain intact, giving Itokawa its unique spud-like shape. What is making the difference? Suggestions have included pressure arising from radiation from the sun, and friction and electrostatic forces occurring in the ionized dust.

Now a new analysis by a team of University of Colorado scientists suggests that the real culprit might be van der Waals forces. Named after Dutch scientist Johannes van der Waals, this is the force that causes attraction or repulsion between molecules in chemistry, that cannot be attributed to the usual chemical bonds or electrostatic interactions of ions. It's not a force that gets bandied about much these days in astrophysics, but here on Earth, it's the force behind geckos' ability to "adhere" to smooth surfaces even at bizarre angles. Apparently it's the creature's body orientation, and resulting change in gravity acting upon it, that triggers the gecko grip.

Those same van der Waals forces kick in in the latter stages of small asteroid evolution, according to the Colorado scientists, after the spinning objects have thrown off larger rocks that would incur larger gravitational effects. Eventually all that is left is the smaller bits of rubble, which act much like molecules and form bonds via van der Waals.

It's an intriguing notion, and Daniel Scheeres, who heads the Colorado team, thinks it might also be able to shed some light on how Saturn's rings may have formed -- those rings, after all, are made up almost entirely of dust and tiny bits of rubble that also seem to be strangely attracted to one another. So the humble gecko could hold the secret not just to a possible dry glue here on Earth, but to how certain celestial objects form as well.

Scaling forces to asteroid surfaces: The role of cohesion

D.J. Scheeres, C.M. Hartzell, P. Sanchez, M. Swift
(Submitted on 12 Feb 2010)

Abstract:

The scaling of physical forces to the extremely low ambient gravitational acceleration regimes found on the surfaces of small asteroids is performed. Resulting from this, it is found that van der Waals cohesive forces between regolith grains on asteroid surfaces should be a dominant force and compete with particle weights and be greater, in general, than electrostatic and solar radiation pressure forces. Based on this scaling, we interpret previous experiments performed on cohesive powders in the terrestrial environment as being relevant for the understanding of processes on asteroid surfaces. The implications of these terrestrial experiments for interpreting observations of asteroid surfaces and macro-porosity are considered, and yield interpretations that differ from previously assumed processes for these environments. Based on this understanding, we propose a new model for the end state of small, rapidly rotating asteroids which allows them to be comprised of relatively fine regolith grains held together by van der Waals cohesive forces.

Comments: 54 pages, 7 figures
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:1002.2478v1 [astro-ph.EP]

Link: Paper - "Scaling forces to asteroid surfaces: The role of cohesion" (PDF)

Link: Earth and Planetary Astrophysics

Link: Article - Discovery News ("Strange Attractors: How are Asteroids Like Geckos?")

Meteor Over Midwestern United States (14 April 2010)

From the AP Article...

Alarge meteor streaked across the Midwestern sky momentarily turning night into day, rattling houses and causing trees and the ground to shake, authorities said Thursday. There were no immediate reports of injuries. Witnesses say the meteor lit up the sky Wednesday about 10:10 p.m. National Weather Service offices across the Midwest say it was visible from southwestern Wisconsin and northern Iowa to central Missouri. Radar information suggests the meteor landed in the southwest corner of Wisconsin, either Grant or Lafayette counties, said Ashley Sears, a meteorologist with the National Weather Service's Milwaukee office. Officials in both counties said no one has reported seeing a meteor or crater.

Link: CNN Article

Link: AP Article

Link: YouTube Video (Meteor/Fireball - Howard County Sheriff)

Article on White Sands Missile Range Solar Furnace

The Solar furnace uses mirrors to focus the sun onto a small point. Normally used to simulate the heat from a nuclear explosion, the furnace was used to test a theory that a space based version of the furnace could be used to deflect a meteor headed for Earth. 

From the article on the White Sands Missile Range Solar Furnace...

White Sands Missile Range test facilities are being used by scientists to develop methods for saving the world.

While the White Sands Missile Range Solar Furnace was originally built to test the survivability of Army systems and materials against the nuclear threat of the cold war, the facility is serving other purposes as well.

While initially designed for research and testing relating to nuclear survivability in July 2009, the furnace was used to test a theory on saving the world. A threat that has quietly hung above earth for millions of years in that of a meteorite large enough that its impact with earth would result in a disaster of a magnitude that it would cause the extinction of humanity.

The theory, presented by Purdue University Geophysicist Jay Melosh, was that should a meteor be spotted approaching earth then a spacecraft equipped with a system similar to that of the solar furnace could be launched to intercept the meteor.

Once it arrives at the meteor the spacecraft would deploy its system and use the power of the sun to superheat a spot on the meteor. The heat would cause that spot on the meteor to melt and fracture, throwing off small bits of debris; the force of which would act like a small rocket slowly pushing the meteor and changing its course.

"In theory it should work. If you put something in its space, its reverse force should push it in the opposite direction," said Randy Brady, chief of WSMR's Nuclear Effects Division.

To simulate this action, WSMR test officers placed slabs of green marble, a material Melosh selected for its similarity to meteors, into the solar furnace.

When exposed to the beam generated by the furnace, the marble reacted violently, melting and throwing off small chunks of rock, proving the theory and surprising some of the WSMR staff that worked on the project. "I didn't think you could melt rock," Brady said.

The facility operates on the simple principle of focusing the light and heat of the sun onto a four-inch diameter point, much the same way that a magnifying glass can.

The furnace uses two primary sets of mirrors, each several stories tall. One large flat set can pivot around to catch the rays of the sun and direct them though a shutter system into the second set of mirrors, which focus the light and heat onto the target.

In the target area, the temperature can get around 6,000 degrees Fahrenheit, though for test purposes the furnaces output uses a different measurement system to account for the amount of energy over time. "We don't really use temperature, we use calories per centimeter squared," Brady said.

Originally built in 1952 the furnace was moved from Natick, Mass., to White Sands in 1976. White Sands' remote location and typically clear skies help both ensure the secrecy of many of the furnace tests and helps keep the furnace operating efficiently. "The good thing here is that the sun is really easy to track and there aren't many clouds," Brady said. During a normal test the subject is placed in the target area and exposed to the heat of the system for only a few seconds at most, simulating the sudden heat put out by a nuclear explosion.

While the furnace still sees regular use, conducting around 10 tests a year, Brady hopes to see more scientific research conducted as the facility is extremely affordable to operate, costing only around $1,700 per day.

Solar furnace could save Earth
14 April 2010
Drew Hamilton, WSMR, N.M.

Link: Article

President Obama's Space Exploration Speech (15 April 2010) and Human Asteroid Mission Mention

President Obama's speech on space exploration in Florida talked about human asteroid missions, if briefly. Also I believe former NASA Astronaut and current Google employee (and gravity tractor proponent) Ed Lu was in the audience as well as Louis Friedman from The Planetary Society. Excerpts from the speech below.

Selections from the President's speech on 15 April 2010:

"Early in the next decade, a set of crewed flights will test and prove the systems required for exploration beyond low Earth orbit. (Applause.) And by 2025, we expect new spacecraft designed for long journeys to allow us to begin the first-ever crewed missions beyond the Moon into deep space. (Applause.) So we’ll start -- we’ll start by sending astronauts to an asteroid for the first time in history. (Applause.) By the mid-2030s, I believe we can send humans to orbit Mars and return them safely to Earth. And a landing on Mars will follow. And I expect to be around to see it. (Applause.)"

Earlier in the speech the President mentioned an image he has in his private office, an image I believe of the Hubble Space Telescope (HST) image depicting the 2009 Jupiter Impact Event (as mentioned in the NASA Advisory Council meeting minutes from 18-19 February 2010).

Selections from the President's speech on 15 April 2010:

"Here at the Kennedy Space Center we are surrounded by monuments and milestones of those contributions. It was from here that NASA launched the missions of Mercury and Gemini and Apollo. It was from here that Space Shuttle Discovery, piloted by Charlie Bolden, carried the Hubble Telescope into orbit, allowing us to plumb the deepest recesses of our galaxy. And I should point out, by the way, that in my private office just off the Oval, I’ve got the picture of Jupiter from the Hubble. So thank you, Charlie, for helping to decorate my office."

From the NASA Advisory Council meeting minutes from 18-19 February 2010 referring to the image in the President's office:

"Mr. Bolden turned his attention to HSF (Human Space Flight). He emphasized the importance of a destination and noted that the President had spoken yesterday with astronauts and students at the White House. The President was enthusiastic about HSF and talked about going to Mars and returning to the Moon. The President understands that we must have a destination. For Mr. Bolden, the ultimate destination is Mars. Over the coming years we will try to figure out how to best get there, what the limiting capabilities are, what capabilities we don't have today, and then build a plan. There are intermediate destinations, e.g., the Moon and Near Earth objects (NEOs). Mr. Bolden explained that one of the jobs that he shares with Secretary of Defense Gates is defense of the planet against things, not people. At a recent Cabinet meeting, Dr. John Holdren, the President's Science Advisor, showed the President an HST image depicting a large asteroid impact on Jupiter that no one had predicted. This got his attention, which generally happens when the talk is about NASA."

Link: Remarks by the President on Space Exploration in the 21st Century, John F. Kennedy Space Center, Merritt Island, Florida, 15 April 2010

Link: NASA Advisory Council Public Meeting (18-19 February 2010): Meeting Minutes

JAXA's Asteroid Explorer Hayabusa Team: Tweeting Upcoming Return of Hayabusa to Earth

Follow the return of the Japanese spacecraft Hayabusa on the JAXA twitter feed.

Link: Twitter Feed (Hayabusa_JAXA)

Link: JAXA Hayabusa Page (in Japanese)

NASA Advisory Council Ad-Hoc Task Force on Planetary Defense Meeting 15-16 April 2010

An upcoming meeting for an Ad-Hoc Task Force on Planetary Defense of the NASA Advisory Council (NAC). Here is information for the upcoming meeting. Clark Chapman is one of the members of this Planetary Defense Task Force of NASA's Advisory Council. One wonders if this is coming out of the recent restructuring announcement of the NASA Advisory Council. Here is more information from the NAC meeting minutes in February 2010:

Dr. Ford [Dr. Kenneth Ford, Chair, NASA Advisory Council] stated that the NAC is in the process of formulating a new Ad-Hoc Task Force on Planetary Defense. In response to a request to clarify the term, he stated that :planetary defense" it is not the same as "planetary protection" against biological contamination. This Task Force's Terms of Reference are directed at looking at objects colliding with or striking Earth. It is focused on NEOs, but does not include the orbital space debris problem. This Task Force should start to meet in the April or May 2010 time frame. Its purpose will be to provide advice to Mr. Bolden, who shares responsibilities with the Department of Defense (DoD) in this area. In response to a question, Dr. Ford stated that the Ad-Hoc Task Force would meet as often as necessary and would be decoupled from the NAC meeting schedule. In response to a question, Dr. Ford clarified that a Council member can serve on more than one Committee.

Federal Register: March 30, 2010 (Volume 75, Number 60)] [Notices] [Page 15742-15743] From the Federal Register Online via GPO Access [wais.access.gpo.gov] [DOCID:fr30mr10-116]

NATIONAL AERONAUTICS AND SPACE ADMINISTRATION
[Notice: (10-035)]
NASA Advisory Council; Ad-Hoc Task Force on Planetary Defense; Meeting
AGENCY: National Aeronautics and Space Administration.
ACTION: Notice of meeting.
SUMMARY: In accordance with the Federal Advisory Committee Act, Public Law 92-463, as amended, the National Aeronautics and Space Administration announces a meeting of the Ad-Hoc Task Force on Planetary Defense of the NASA Advisory Council.
DATES: Thursday, April 15, 2010, 11:30 a.m.-5:30 p.m., and Friday, April 16, 2010, 9 a.m.-1 p.m. (times are EDT).
ADDRESSES: Boston Marriott Cambridge Hotel; Two Cambridge Center, 50 Broadway; Cambridge, Massachusetts 02142; (617) 494-6600.

FOR FURTHER INFORMATION CONTACT: Ms. Jane Parham, Exploration Systems Mission Directorate, National Aeronautics and Space Administration Headquarters, Washington, DC 20546, 202/358-1715; jane.parham@nasa.gov.

SUPPLEMENTARY INFORMATION: The agenda topics for the meeting will include:

* Ad-Hoc Task Force on Planetary Defense Terms of Reference.
* NASA Near Earth Object (NEO) Program Status.
* Viewpoints of various scientific organizations on NEO activities.
* Ad-Hoc Task Force Planning.

The meeting will be open to the public up to the seating capacity of the room. It is imperative that the meeting be held on this date to accommodate the scheduling priorities of the key participants. Visitors will need to sign in and show a valid government-issued picture identification such as driver's license or passport. For questions, please call Ms. Jane Parham, at (202) 358-1715.

Dated: March 25, 2010. P. Diane Rausch, Advisory Committee Management Officer, National Aeronautics and Space Administration. [FR Doc. 2010-7124 Filed 3-29-10; 8:45 am] BILLING CODE 7510-13-P

Link: SpaceRef notice

Link: Federal Register Notice

Link: NASA Advisory Council Restructuring Announcement (23 Feb. 2010

Link: NASA Advisory Council Public Meeting (18-19 February 2010) Agenda (and presentations)

Link: NASA Advisory Council Public Meeting (18-19 February 2010): Meeting Minutes

Cardiff Uniov. Prof. Bill Napier's Paper on Potential for Debris of Comet as Cause of Extinctions Around 13,000 Years Ago

Selections from article on Prof. Napier's theory about debris from a comet impacting the Earth 13,000 years ago...

An hour-long hailstorm from space bombarded the Earth 13,000 years ago - plunging the planet into a mini-ice age, scientists claimed today.

The catastrophe was caused by a disintegrating comet and saw the planet sprayed by thousands of frozen boulders made of ice and dust.

The collisions wiped out huge numbers of animal species all over the world, disrupted the lives of our stone age ancestors and triggered a freeze that lasted more than 1,000 years.

The theory is the brainchild of Professor Bill Napier, from Cardiff University, who says it explains the mysterious period of extinction around 11,000 BC.

Scientists have long been puzzled by what caused a sudden cooling of up to 8C (14F) just as the Earth was warming up at the end of the last ice age.

The change in climate caused retreating glaciers to advance once again, and coincided with the extinction of 35 families of North American mammals.

[From another article...]

Napier points to a flurry of near-Earth objects (NEOs) called the Taurid Complex, which he suggests came from a comet 50-100 kilometers (31-62 miles) in diameter -- a truly giant hunk of rock and ice -- that entered the inner solar system between 20,000 and 30,000 years ago. The Taurid Complex contains some of the largest NEOs we know about.

"Palaeolithic extinctions and the Taurid Complex"
W.M. Napier
03 Mar 2010
Monthly Notices of the Royal Astronomical Society

Abstract:

Intersection with the debris of a large (50-100 km) short-period comet during the Upper Palaeolithic provides a satisfactory explanation for the catastrophe of celestial origin which has been postulated to have occurred around 12900 BP, and which presaged a return to ice age conditions of duration ~1300 years. The Taurid Complex appears to be the debris of this erstwhile comet; it includes at least 19 of the brightest near-Earth objects. Sub-kilometre bodies in meteor streams may present the greatest regional impact hazard on timescales of human concern.

Link: Paper: arXiv:1003.0744v1

Link: Discovery.com article

Link: Daily Mail Article