• Earth gets a rush of weekend asteroid visitors

    Reuters/NASA/JPL-Caltech

    The passage of asteroid 2012 DA14 through the Earth-moon system, is depicted in this handout image from NASA.

    By Irene Klotz, Reuters

    CAPE CANAVERAL, Fla. - An asteroid as big as a city block shot relatively close by the Earth on Saturday, the latest in a series of visiting celestial objects including an asteroid the size of a bus that exploded over Russia last month, injuring 1,500. 

    Discovered just six days ago, the 460-foot long (140-meter) Asteroid 2013 ET passed about 600,000 miles from Earth at 3:30 p.m. EST. That's about 2-1/2 times as far as the moon, fairly close on a cosmic yardstick. 

    "The scary part of this one is that it's something we didn't even know about," Patrick Paolucci, president of Slooh Space Camera, said during a webcast featuring live images of the asteroid from a telescope in the Canary Islands. 

    Moving at a speed of about 26,000 miles per hour, the asteroid could have wiped out a large city if it had impacted the Earth, added Slooh telescope engineer Paul Cox. 

    Asteroid 2013 ET is nearly eight times larger than the bus-sized asteroid that exploded over Chelyabinsk, Russia, on February 15. The force of the explosion, equivalent to about 440 kilotons of dynamite, created a shock wave that shattered windows and damaged buildings, injuring more than 1,500 people. 

    Later that day, another small asteroid, known as DA14, passed about 17,200 miles from Earth, closer than the orbiting networks of communications and weather satellites. 

    "One of the reasons why we're finding more of these objects is that there are more people looking," Cox said. 

    Two other small asteroids, both about the size of the Russian meteor, will also be in Earth's neighborhood this weekend. Asteroid 2013 EC 20 passed just 93,000 miles away on Saturday - "a stone's thrown," said Cox. 

    On Sunday, Asteroid 2013 EN 20 will fly about 279,000 miles from Earth. Both were discovered just three days ago. "We know that the solar system is a busy place," said Cox. 

    "We're not sitting here on our pale, blue dot on our own in nice safety ... This should be a wakeup call to governments." 

    NASA has been tasked by the U.S. Congress to find and track all near-Earth objects 0.62 miles or larger in diameter, and estimates about 95 percent have been identified. 

    However, only about 10 percent of smaller asteroids have been discovered, NASA scientists have said. 

    The effort is intended to give scientists and engineers as much time as possible to learn if an asteroid or comet is on a collision course with Earth, in hopes of sending up a spacecraft or taking other measures to avert catastrophe. 

    About 100 tons of material from space hit Earth every day. Astronomers currently expect an object about the size of what hit Russia to strike the planet about every 100 years.

  • Here we go again: Big asteroid set to buzz Earth

    Gianluca Masi / Virtual Telescope Project

    This image of the asteroid 2013 ET was obtained on Monday by the Italy-based Virtual Telescope Project. The asteroid is visible as a faint dot in the center of the frame.

    By Mike Wall
    Space.com

    A newly discovered asteroid the size of a football field will cruise through Earth's neighborhood this weekend, just days after another space rock made an even closer approach to our planet.

    The 330-foot-wide (100 meters) asteroid 2013 ET will miss Earth by 600,000 miles (960,000 kilometers) when it zips by on Saturday. The space rock flyby will come just days after the 33-foot (10 m) asteroid 2013 EC approached within 230,000 miles (370,000 km) of us early Monday.

    When asteroid 2013 ET passes Earth, it will be at a range equivalent to 2.5 times the distance between the planet and the moon, making it too faint and far away for most stargazers to spot in the night sky. But the Virtual Telescope Project in Italy, run by astrophysicist Gianluca Masi, will webcast a live telescope view of the space rock's flyby on Friday, beginning at 2 p.m. EST. You can access the free broadcast here.

    There is no danger that 2013 ET will hit Earth, researchers say, just as 2013 EC posed no threat. But their flybys are slightly unsettling nonetheless, since both asteroids were discovered mere days ago.

    Indeed, many space rocks are hurtling undetected through Earth's neck of the cosmic woods. Astronomers estimate that the number of near-Earth asteroids tops 1 million, but just 9,700 have been discovered to date.

    Undetected objects can strike Earth without warning, as the surprise meteor explosion over Russia last month illustrated. The 55-foot (17 m) asteroid that caused the Feb. 15 Russian fireball detonated in the atmosphere before astronomers even knew it existed.

    While many scientists stress the urgent need for expanded and improved asteroid-detection efforts, there is some good news: Humanity is unlikely to go the way of the dinosaurs anytime soon.

    NASA researchers have identified and mapped the orbits of 95 percent of the 980 or so near-Earth asteroids at least 0.6 miles (1 km) wide, which could threaten human civilization if they hit us. None of these behemoths are on a collision course with Earth in the foreseeable future.

    For comparison, the asteroid believed to have wiped out the dinosaurs 65 million years ago likely measured about 6 miles (10 km) across, scientists say.

    Follow Mike Wall on Twitter @michaeldwall. Follow us @Spacedotcom, Facebook or Google+. This article was first published on Space.com.

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  • Don't blame a comet for Clovis culture demise, scientists say

    Dan Durda

    A 130-foot-meteor created the mile-wide Meteor Crater in Arizona. The comet proposed to have impacted life in North America was significantly larger, but no crater indicating its collision has been found.

    By Nola Taylor Redd
    LiveScience

    A comet crashing into the Earth some 13,000 years ago was thought to have spelled doom to a group of early North American people, and possibly the extinction of ice age beasts in the region.

    But the space rock was wrongly accused, according to a group of 16 scientists in fields ranging from archaeology to crystallography to physics, who have offered counterevidence to the existence of such a collision.

    "Despite more than four years of trying by many qualified researchers, no unambiguous evidence has been found (of such an event)," Mark Boslough, a physicist at Sandia National Laboratories in New Mexico, told LiveScience.

    "That lack of evidence is therefore evidence of absence."

    Changing times
    Almost 13,000 years ago, a prehistoric Paleo-Indian group known as the Clovis culture suffered its demise at the same time the region underwent significant climate cooling known as the Younger Dryas. Animals such as ground sloths, camels and mammoths were wiped out in North America around the same period. [Wipe Out: The 10 Most Mysterious Extinctions]

    In 2007, a team of scientists led by Richard Firestone of the Lawrence Berkeley National Laboratory in California suggested these changes were the result of a collision or explosion of an enormous comet or asteroid, pointing to a carbon-rich black layer at a number of sites across North America. The theory has remained controversial, with no sign of a crater that would have resulted from such an impact.

    "If a four-kilometer (2.5-mile) comet had broken up over North America only 12.9 thousand years ago, it is certain that it would have left an unambiguous impact crater or craters, as well as unambiguous shocked materials," Boslough said.

    Boslough, who has spent decades studying the effects of comet and asteroid collisions, was part of a team that predicted the visibility of plumes from the impact of the 1994 Shoemaker-Levy 9 comet with Jupiter.

    "Comet impacts may be low enough in density not to leave craters," Firestone told LiveScience by email.

    He also points to independent research by William Napier at the University of Cardiff in the United Kingdom that indicates such explosions could have come from a debris trail created by Comet Encke, which also would not have left a crater.

    A large rock plunging into the Earth's atmosphere may detonate in the air without coming into contact with the ground. Such an explosion occurred in Siberia in the early 20th century; the explosive energy of the so-called Tunguska event was more than 1,000 times more powerful than the atomic bomb dropped on Hiroshima.

    "No crater was formed at Tunguska, or the recent Russian impact," Firestone said.

    But Boslough said this math doesn't add up. The object responsible for the Tunguska event was very small, about 130 to 160 feet (40 to 50 meters) wide, while the recent explosion over Russia was smaller, about 56 feet (17 meters). The proposed North American space rock linked with the Clovis demise is estimated to have been closer to 2.5 miles (4 kilometers) across.

    "The physics doesn't support the idea of something that big exploding in the air," he said, noting that the original research team doesn't provide any explanation or models for how such a breakup might occur. [The 10 Greatest Explosions Ever]

    If such a large object crashed into the Earth, the resulting crater would be too large to miss, particularly when it was only a few thousand years old, Boslough said. He pointed to Meteor Crater in Arizona, which is three times as old and formed by an object "a million times smaller in terms of explosive energy."

    "Meteor Crater is an unambiguous impact crater with unambiguous shocked minerals," Boslough said. If a 2.5-mile comet had broken into pieces, it could have made a million Meteor Craters, he added.

    Firestone argued that water or ice could have absorbed the impact, possibly leaving behind no crater.

    Boslough disagreed. Even if the comet had plunged into the ice sheet covering much of NorthAmerica, the crater formed beneath it would still be sizable. "We wouldn't be able to miss that right now — it would be obvious," Boslough said.

    The arguments and evidence against the impact were published in the December 2012 American Geophysical Union monograph.

    "Extraordinary claims require extraordinary evidence"
    Powerful impacts are Boslough's field, but the other 15 scientists working on the paper offered up other sources of counterevidence for the existence of a collision.

    "We all independently came to the conclusion that the evidence doesn't support a Younger Dryas impact," Boslough said. [Asteroid Basics: A Space Rock Quiz]

    "We all came to this based on our own very narrow piece of the puzzle."

    For instance, the initial team studying the event announced the discovery of a carbon-rich black layer, colloquially known as a "black mat," at a number of sites in North America. Containing charcoal, soot and nanodiamonds, such material could be formed by a violent collision.

    But this isn't the only possible source.

    "The things they call impact markers are not necessarily indicators of high-pressure shocks," Boslough said. "There are other processes that potentially could have formed them."

    Speaking of the black mat found in central Mexico, Firestone said, "Boslough is correct that there are other black mats, but these are dated to 12,900 years ago at the time of impact." He points to independent research published this fall that located hundreds to thousands of samples.

    However, radiocarbon dating of one of the sites in Gainey, Mich., suggested its samples were contaminated.

    Melted rock formations and microscopic diamonds found in a lake in Central Mexico last year were also suggested as evidence for the collision, but Boslough'steam disagrees with the age of the sediment layer in the region.

    Boslough said the standard for indicating a strong shock occurred is pretty high in the impact community, and the findings by the original team don't meet them. Nor do they offer up any physical models that propose how an impact or airburst would have occurred — and the ones Boslough has run just don't pan out.

    "It's really a stretch to claim that there was this large impact event with no crater and no unambiguous shock material, because large impacts are such rare events," Boslough said.

    "When somebody is making a claim that something extraordinary happened, something out of the ordinary and with a very low probability, and they have ambiguous evidence, then the default is that it didn't happen," he continued.

    "Extraordinary claims require extraordinary evidence."

    Firestone stands firm.

    "All the evidence has now been confirmed by others," he said.

    "Boslough has no data supporting his arguments, and ignores the counter arguments of Bill Napier."

    Follow LiveScience on Twitter @livescience. We're also on Facebook and Google+.

    Copyright 2013 LiveScience, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

  • Fireball over N. California causes stir

    Traveling at 33,000 mph, a massive meteor hit the Earth's atmosphere creating a giant shockwave that blew out windows of glass, injuring nearly 1,000 people and creating panic. On the same day, an asteroid half the size of a football field came within 17,000 miles from Earth. NBC's Tom Costello reports.

    By Gil Aegerter, NBC News

    A fireball streaking across the Northern California sky Friday night brought a flood of witness reports -- the same day that a meteor exploded over Russia and an asteroid made a near-Earth fly-by.

    The fireball was seen around 7:45 p.m., by witnesses as far north as Fairfield and as far south as Gilroy, NBCBayArea.com reported. It was also reportedly seen in Sacramento and Walnut Creek. NBC station KSBW of Monterey said the object was visible along California's Central Coast, too.

    NBCBayArea.com said Candice Guruwaiya gave this account on Facebook of seeing it in San Jose: "I was leaving Safeway on Branham and Snell when I saw it. ... It was a bright green when it first appeared, then it went to a bright yellow. It was awesome!"

    The fireball was seen about 24 hours after a meteor exploded over Russia's Chelyabinsk region and a 150-foot-wide asteroid came within 17,200 miles of Earth.

    An astronomer at the Chabot Space and Science Center in Oakland told NBCBayArea.com that Friday night's event wasn't related to the asteroid's passing:

    Gerald McKeegan, an astronomer at Chabot, said he did not see it, but based on accounts he thinks it was a "sporadic meteor," which can happen several times a day but  most of the time happens over the ocean, away from human eyes. Sporadic meteors bring as much as 15,000 tons of space debris to Earth each year, according to McKeegan. He explained that meteors, which are hunks of rock and metal from space that fall to Earth, burn up as they go through Earth's atmosphere, which is what apparently  caused tonight's bright flash of light.

    He said it was likely smaller than another meteor that landed in the Bay Area in October, which caused a loud sonic boom as it fell, breaking apart and spreading rocks, called meteorites, in the North Bay.

    More about the cosmic hits (and near-misses):

    Astrophysicist Neil deGrasse Tyson, director of the Hayden Planetarium, speaks to NBC's Lester Holt about the meteor and asteroid that approached Earth on Friday.

  • Meteor warning system in the works — but not ready yet

    Yekaterina Pustynnikova / AP

    In this photo provided by Chelyabinsk.ru, a meteor contrail is seen over Chelyabinsk on Friday, Feb. 15, 2013.

    By Suzanne Choney

    There aren't yet any advance warning systems that could give Earthlings a heads-up before an untracked space rock hits. But a telescope project in Hawaii aims to change that, and potentially provide a chance for those in threatened areas to evacuate. A meteor alert might have made a difference to Russia's Chelyabinsk region on Friday.

    Read: Nuclear-like in its intensity, Russia meteor blast is largest since 1908

    "There are excellent ongoing surveys for asteroids that are capable of seeing such a rock with one to two days' warning, but they do not cover the whole sky each night, so there's a good chance that any given rock can slip by them for days to weeks. This one obviously did," astronomer John Tonry of the Institute for Astronomy at the University of Hawaii told NBC News Friday.

    Tonry is one of the key players in a NASA-backed effort to build ATLAS (Asteroid Terrestrial-impact Last Alert System), two observatories in Hawaii that can simultaneously scan the entire visible sky twice a night.

    "If ATLAS were up and running we might very well have seen" the meteor that hit Russia, he said, and "could have provided one to two days' warning."

    However, he adds, the success of detection "depends on a couple of assumptions." One is that it's not cloudy. Another is that the asteroid doesn't go over the South Pole, "where ATLAS cannot see."

    Telescopes, Tony said, "can only see the sky above the horizon, obviously. A telescope that's sited in the northern hemisphere (which ATLAS will be) cannot see all the way to the South Pole of the sky." And, "if the asteroid were coming from that direction, there's a good chance that it would never rise above the horizon for a northern telescope before it hits."

    While it would "easy to build multiple copies of ATLAS and put some in the south, and spread them out so they see different weather patterns ... that's for the future," he said.

    Dozens were hospitalized and nearly 1,000 residents suffered minor injuries from fallen debris and the impact of the meteor's powerful landing. NBC's Tom Costello reports.

    The ATLAS telescopes are "just now" being built, Tonry said; ATLAS should "start running around the end of 2014 and be fully operational by the end of 2015." NASA has provided $5 million in funding for ATLAS.

    At one time, NASA considered launching an asteroid-hunting probe, but that didn't go forward because of the cost, estimated at $500 million almost a decade ago.

    Other private efforts are in the works, too.

    Last year, leaders of the nonprofit B612 Foundation, including Apollo astronaut Rusty Schweickart, started a campaign to fund and launch a space telescope that will hunt for potential killer asteroids over the course of five and a half years.

    Another venture, from a group called Planetary Resources, ultimately wants to do asteroid mining, but says its first step is to "launch an orbital fleet of 'personal space telescopes' capable of looking out into the heavens or back down on Earth," wrote Alan Boyle, NBC News.com's Science editor last year.

    More about cosmic hits (and near misses):

    Suzanne Choney is a contributing writer for NBC News.com. You can follow her on Twitter.

    NASA looks at the flyby of asteroid 2012 DA14 from several amateur observatories across Australia.

     

  • Russia gets rude welcome to 'fireball month'

    EUMETSAT

    The meteor that exploded over the Urals of central Russia was seen by Meteosat-9, at the edge of the satellite view. This image was taken on Friday.

    By Tanya Lewis
    Space.com

    It's fireball season on Earth, and it is starkly clear for residents in eastern Russia where a bright fireball exploded in the atmosphere early Friday.

    For reasons scientists don't quite understand, there appears to be an increase in the number of bright meteors visible blazing through the night sky during the month of February. The notion hit home today when a meteor exploded over Russia's Ural Mountains, injuring more than 900 people and damaging thousands of buildings, according to news reports. (Another space rock, the asteroid 2012 DA14, is on course to pass very close to Earth Friday evening, but will not hit the planet.)

    On a typical night of the year, stargazers might see about 10 "sporadic" fireballs, created by fragments of asteroids or comets that burn up in the Earth's atmosphere. But that number spikes by 10 to 30 percent during the spring. The fireballs come from the asteroid belt, but their exact source remains unknown.

    Meteor enthusiasts have known about the "Fireballs of February" since the 1960s and 70s, when amateur astronomers noticed many more bright, sound-producing streaks than usual. Follow-up studies during the 1980s failed to show any increase in the rate of fireballs, but a 1990 study suggested otherwise.

    Astronomer Ian Holliday studied photographic records of roughly a thousand fireballs from the 1970s and 80s, finding what looked like a fireball stream crossing Earth's orbit during February, late summer and fall. Halliday's results are somewhat controversial, but the phenomenon appears real.

    The February fireball increase is somewhat puzzling because the number of ordinary meteors normally peaks during autumn in the Northern Hemisphere, according to researchers at NASA's Meteoroid Environment Center. The two most visible meteor showers most years, the Perseid meteor shower and the Leonid meteor shower, peak in mid-August and mid-November.

    Scientists are probing the mystery of the spring surge in fireballs using NASA's All-Sky Fireball Network, a slew of cameras designed to observe meteors brighter than the planet Venus. The data collected could reveal where these fireballs come from, scientists say. Modeling the meteoroid environment will also benefit spacecraft designers.

    Follow Space.com on Twitter @Spacedotcom. We're also on Facebook  and Google+.

    Copyright 2013 Space.com, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.

  • Another doomsday threat dies out: Asteroid Apophis won't hit us in 2036

    Apophis, nicknamed the "Doomsday Asteroid," was once considered a potential threat, but now scientists realize the chance of the asteroid colliding with Earth is negligible. NBC's Brian Williams reports.

    By Alan Boyle, Science Editor, NBC News


    Radar observations made during this week's close encounter with the asteroid Apophis have ruled out the risk of a catastrophic cosmic collision in 2036, NASA says. Experts say it'll be much farther away at that time than it is right now.

    The crucial readings came on Wednesday when the space rock, which is thought to measure at least 885 feet (270 meters wide), approached within 9 million miles (14.5 million kilometers) of Earth. NASA is monitoring Apophis with its 230-foot (70-meter) Goldstone radio dish in California. Optical readings also have come in from the Magdalena Ridge Observatory in New Mexico and the Pan-STARRS observatory in Hawaii.

    The bottom line? "We have effectively ruled out the possibility of an Earth impact by Apophis in 2036," Don Yeomans, manager of NASA's Near-Earth Object Program Office at the Jet Propulsion Laboratory, said today in the all-clear news release. "The impact odds as they stand now are less than one in a million, which makes us comfortable saying we can effectively rule out an Earth impact in 2036. Our interest in asteroid Apophis will essentially be for its scientific interest for the foreseeable future."

    Jon Giorgini, who developed JPL's online Horizons database to keep track of solar system objects, would go even further. He says that according to calculations based on the Goldstone data, Apophis will probably pass by Earth at a distance of 36 million miles (58 million kilometers, or 0.39 AU), and absolutely no closer than 14 million miles (22 million kilometers, or 0.15 AU). "That is a very extreme minimum," he told NBC News. "Nothing else plausible can get you closer."

    Apophis, a.k.a. 2004 MN4, created a huge splash when it was discovered in 2004 because the initial assessment of its orbit gave a 1-in-40 chance of Earth impact in 2029. That would be catastrophic: The space rock is big enough to wipe out a city if it struck land, or create killer tsunami waves if it splashed into the ocean.

    Additional orbital data quickly eliminated the risk for 2029, but showed that it would pass within 20,000 miles (32,000 kilometers) of our planet at that time. That's so close that Earth's gravitational field will perturb Apophis' orbit. The experts worried that if the asteroid passed through a particular half-mile-wide zone in space, known as a "keyhole," its orbit would be perturbed just enough to set up a smash-up during the 2036 encounter. Fortunately, the latest observations indicate that Apophis will miss the keyhole by a long shot.

    Did I just hear a cosmic sigh of relief?

    UH / IA

    The asteroid Apophis, highlighted here by a white circle, was discovered in June 2004.

    There are still a few uncertainties surrounding Apophis: Astronomers don't yet have enough data to determine how the asteroid is spinning or how solar radiation is affecting its orbital path — a phenomenon known as the Yarkovsky effect. Giorgini said that even under the worst-case scenario, the effect won't push Apophis into a collision in 2036. But there could conceivably be other risky encounters in the decades or centuries ahead.

    "There's a non-linear amplification that can really move it around more," Giorgini said.

    Also, there are questions about Apophis' exact size. Just this week, readings from the European Space Agency's Herschel space telescope suggested that the asteroid may be nearly 20 percent bigger than previously thought. But that larger size estimate is based on the assumption that Apophis is a spheroid, and astronomers already know that it's elongated.

    "We're not seeing that larger size in the radar data," Giorgini said.

    By the end of next month, continued radar observations from Goldstone as well as the Arecibo Observatory in Puerto Rico should give astronomers a much better fix on Apophis' spin and its size. When those factors are fully accounted for, the Jet Propulsion Observatory will update its official risk assessment for Apophis — and could take this bad boy off the hit list for good.

    Update for 6:30 p.m. ET: Clark Chapman, senior scientist at the Southwest Research Institute, weighed in on the current state of the asteroid hunt in an email:

    "One thing you should be aware of, and might mention, is that the next Planetary Defense Conference, an every-two-year international meeting, will be held April 15-19 in Flagstaff, Arizona. ... Some presentations are already listed in the program, which should be finalized a week from now, which is the due date for abstracts.

    "An interesting tie-in with the new observations of Apophis is that a similar thing happened with 2011 AG5 a few weeks ago, when observations with the huge Gemini telescope in Hawaii showed that it would, in 2023, miss the roughly 350-km-wide 'keyhole' and, therefore, not strike the Earth in 2040.  Prior to these critical observations, the chance of a 2040 impact was unusually high (though still low in everyday terms) at 1 in 500.

    "A point to be realized is that while the chances of impact in these cases are very low by ordinary standards, they aren't zero, and the consequences of an impact could be very terrible, so it is important to plan and prepare for the possibility of impact until it is ruled out.

    "It was important to get these observations of AG5 in the autumn of 2012, because if it had turned out that AG5 was actually on an impact trajectory, it would have given us an additional year to mount a deflection mission and succeed in deflecting it from the 2023 keyhole. Without making a major observational effort with a very large telescope this autumn, the next routine observational opportunity wasn't until this coming autumn."

    Update for 8:30 p.m. ET: One of NASA's experts on the asteroid threat and two former NASA astronauts have weighed in on the report about Apophis. David Morrison of NASA's Ames Research Center sent these comments via email:

    "One possible angle is the recent proposal from [NASA Administrator] Charlie Bolden, based on a Keck study, that we retrieve a 7-meter carbonaceous near-Earth asteroid and bring it into lunar orbit. There are many questions about this idea, but the one I have in mind is our assumed ability, without Sentinel, to find 7-meter C-type asteroids in Earthlike orbits. If you can't find them, you can’t protect against them, or do anything with them as potential resources." 

    Now here's an email from Ed Lu, a veteran of two space shuttle missions and an extended stay on the International Space Station. Lu now serves as chairman and CEO of the B612 Foundation, which is planning to launch the Sentinel space telescope to track half a million near-Earth asteroids:

    "While it is great that Apophis is much better understood, and we know it won't hit us in 2036, the greatest danger from an asteroid strike is from the ones we haven't yet found.  Of asteroids larger than the one that struck Tunguska in 1908, we know of less than 1 percent of them.  And as David Morrison points out, we can't protect ourselves from the unknown asteroids (or make use of them either). The B612 Foundation Sentinel Space Telescope is going to work on finding and tracking these asteroids."

    And here are some comments from Apollo 9 astronaut Rusty Schweickart, who has played a key role in raising awareness about the threats and opportunities presented by near-Earth objects. It was Schweickart who warned in the wake of Hurricane Katrina that asteroids like Apophis could spark a much more devastating "cosmic Katrina":    

    "I'm hoping that you don’t follow the bad (surprisingly wide) precedent of stating that [the risk from] Apophis has been eliminated.  Please look on the JPL risk page  and especially the more detailed info and note that 1) The 2036 impact possibility is, while significantly reduced, still possible, and 2) that the 2068 impact possibility is now elevated ... to a level that exceeds what the 2036 impact was prior to this apparition.

    "There’s certainly good news re the 2036 impact decreasing in probability ... but frankly it was 1 in 234,000 prior to the new observations ... not exactly an impact probability to worry one. (There are many NEOs with higher impact probability ... but no one pays attention to them ... they aren't the 'poster child' that Apophis is.) My personal reaction was one of surprise that the new 2036 impact was not zero!

    "But/And ... there are more radar observations to integrate in ... as well as optical tracking both now and for the next several years.  Apophis isn't going away ... the impact possibilities are simply shifting around a bit with refinement of the tracking data. 2036 is now less probable; 2068 is now more probable (but still very low).

    "Until JPL and the other guys get more data (enough to really define the Yarkovsky effect), we really won’t be able to get definitive data for longer time scales that we can rely on."

    JPL's Giorgini said the risk assessment that Schweickart mentioned won't be full updated until after Goldstone and Arecibo finish their observational campaign in mid-February — so there may still be a non-zero risk listed until then. But Giorgini is confident that the 2036 risk will disappear when all the observations are factored in. (As of this writing, the estimated risk of collision is listed at 1 chance out of 10,989,000.) But you're right, Rusty: In order to eliminate the risk completely, astronomers will have to get more data about Apophis' physical characteristics. And then there are all those other unknown killer asteroids that might be out to get us...

    More doomsday worries addressed:

    Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.