Tunguska Revision , and a Possible NEA Impact on Mars Back to ArchiveArticle Posted : December 21 , 2007 A proposed downsizing of the energy of the 1908 Tunguska airburst implies an increase in the expected frequency of such impacts . In addition , a possible Mars - impactor has been discovered by the Spaceguard Survey . 퉁 그 스카 1908 년 퉁 그 스카 충 돌체 의 크기 가 적 다는 이론 즉 이것 은 그만한 충돌 이 더 빈번 하 는 의미 화 성 에서 이러 한 가능 성 있 는 충 돌체 발견 됨 ( 1 ) TUNGUSKA IMPACTOR SIZE REVISIONThe 1908 Tunguska airburst from a small asteroid has generally been estimated to have had an energy of 10 - 15 megatons . The corresponding size for a rocky impactor is roughly 60 meters in diameter . Mark Boslough of Sandia Laboratory , however , has generated new supercomputer simulations that suggest a smaller Tunguska explosion . In part his models require less energy in the explosion because he includes the substantial downward momentum of the rocky impactor , rather then modeling it as a stationary explosion . If this revision ( down to an estimated energy of 3 - 5 megatons , and a corresponding diameter perhaps as low as 40 m ) is correct , the expected frequency of such impacts changes , from once in a couple of millennia to once in a few hundred years . If smaller impactors can do the damage previously associated with larger ones , of course , the total hazard from such impacts is increased . Below is a press release from Sandia and a newspaper article discussing this new work . David Morrison 과거 퉁 그 스카 충 돌체 는 크기 가 60 미터 정도 폭발력 은 10 - 15 메가톤 으로 추정 그러나 마크 보슬 롭 은 슈퍼 컴퓨터 시물 레이 션 을 통해 이 보다 작 은 40 미터 급 이 며 폭발력 도 3 - 5 메가톤 으로 추정 그렇 다면 이것 은 충돌 확률 이 수백만 년 에서 수백 년 간격 으로 좁혀 지 는 것 즉 충돌 이 더 빈번 할 수 있 다는 얘기 - - - - - ------------------------- ( 1 a ) SANDIA PRESS RELEASE : NEW ESTIMATE OF TUNGUSKA IMPACTOR SIZEDecember 17 , 2007 ALBUQUERQUE , N . M . — The stunning amount of forest devastation at Tunguska a century ago in Siberia may have been caused by an asteroid only a fraction as large as previously published estimates , Sandia National Laboratories supercomputer simulations suggest . " The asteroid that caused the extensive damage was much smaller than we had thought , " says Sandia principal investigator Mark Boslough of the impact that occurred June 30 , 1908 . " That such a small object can do this kind of destruction suggests that smaller asteroids are something to consider . Their smaller size indicates such collisions are not as improbable as we had believed . " Because smaller asteroids approach Earth statistically more frequently than larger ones , he says , " We should be making more efforts at detecting the smaller ones than we have till now . " The new simulation - which more closely matches the widely known facts of destruction than earlier models - shows that the center of mass of an asteroid exploding above the ground is transported downward at speeds faster than sound . It takes the form of a high - temperature jet of expanding gas called a fireball . This causes stronger blast waves and thermal radiation pulses at the surface than would be predicted by an explosion limited to the height at which the blast was initiated . " Our understanding was oversimplified , " says Boslough , " We no longer have to make the same simplifying assumptions , because present - day supercomputers allow us to do things with high resolution in 3 - D . Everything gets clearer as you look at things with more refined tools . " The new interpretation also accounts for the fact that winds were amplified above ridgelines where trees tended to be blown down , and that the forest at the time of the explosion , according to foresters , was not healthy . Thus previous scientific estimates had overstated the devastation caused by the asteroid , since topographic and ecologic factors contributing to the result had not been taken into account . " There ' s actually less devastation than previously thought , " says Boslough , “ but it was caused by a far smaller asteroid . Unfortunately , it ’ s not a complete wash in terms of the potential hazard , because there are more smaller asteroids than larger ones . ” Boslough and colleagues achieved fame more than a decade ago by accurately predicting that that the fireball caused by the intersection of the comet Shoemaker - Levy 9 with Jupiter would be observable from Earth . Simulations show that the material of an incoming asteroid is compressed by the increasing resistance of Earth ’ s atmosphere . As it penetrates deeper , the more and more resistant atmospheric wall causes it to explode as an airburst that precipitates the downward flow of heated gas . Because of the additional energy transported toward the surface by the fireball , what scientists had thought to be an explosion between 10 and 20 megatons was more likely only three to five megatons . The physical size of the asteroid , says Boslough , depends upon its speed and whether it is porous or nonporous , icy or waterless , and other material characteristics . " Any strategy for defense or deflection should take into consideration this revised understanding of the mechanism of explosion , " says Boslough . One of most prominent papers in estimating frequency of impact was published five years ago in Nature by Sandia researcher Dick Spalding and his colleagues , from satellite data on explosions in atmosphere . " They can count those events and estimate frequencies of arrival through probabilistic arguments , " says Boslough . The work was presented at the American Geophysical Union meeting in San Francisco on Dec . 11 . A paper on the phenomenon , co - authored by Sandia researcher Dave Crawford and entitled " Low – altitude airbursts and the impact threat " has been accepted for publication in the International Journal of Impact Engineering . The research was paid for by Sandia ' s Laboratory - Directed Research and Development office . - - - - - - - - - - - - - - - - - - - ------------------------- ( 1 b ) SMALL ASTEROIDS POSE BIG NEW THREATBy Charles Q . Choi , SPACE . com 19 December 2007 The infamous Tunguska explosion , which mysteriously leveled an area of Siberian forest nearly the size of Tokyo a century ago , might have been caused by an impacting asteroid far smaller than previously thought . The fact that a relatively small asteroid could still cause such a massive explosion suggests " we should be making more efforts at detecting the smaller ones than we have till now , " said researcher Mark Boslough , a physicist at Sandia National Laboratory in Albuquerque , N . M . The explosion near the Podkamennaya Tunguska River on June 30 , 1908 , flattened some 500 , 000 acres ( 2 , 000 square kilometers ) of Siberian forest . Scientists calculated the Tunguska explosion could have been roughly as strong as 10 to 20 megatons of TNT — 1 , 000 times more powerful than the atom bomb dropped on Hiroshima . Wild theories have been bandied about for a century regarding what caused the Tunguska explosion , including a UFO crash , antimatter , a black hole and famed inventor Nikola Tesla ' s " death ray . " In the last decade , researchers have conjectured the event was triggered by an asteroid exploding in Earth ' s atmosphere that was roughly 100 feet wide ( 30 meters ) and 560 , 000 metric tons in mass — more than 10 times that of the Titanic . The space rock is thought to have blown up above the surface , only fragments possibly striking the ground . Now new supercomputer simulations suggest " the asteroid that caused the extensive damage was much smaller than we had thought , " Boslough said . Specifically , he and his colleagues say it would have been a factor of three or four smaller in mass and perhaps 65 feet ( 20 meters ) in diameter . The simulations run on Sandia ' s Red Storm supercomputer — the third fastest in the world — detail how an asteroid that explodes as it runs into Earth ' s atmosphere will generate a supersonic jet of expanding superheated gas . This fireball would have caused blast waves that were stronger at the surface than previously thought . At the same time , previous estimates seem to have overstated the devastation the event caused . The forest back then was not healthy , according to foresters , " and it doesn ' t take as much energy to blow down a diseased tree than a healthy tree , " Boslough said . In addition , the winds from the explosion would naturally get amplified above ridgelines , making the explosion seem more powerful than it actually was . What scientists had thought to be an explosion between 10 and 20 megatons was more likely only three to five megatons , he explained . All in all , the researchers suggest that smaller asteroids may pose a greater danger than previously believed . Moreover , " there are a lot more objects that size , " Boslough told SPACE . com . NASA Ames Research Center planetary scientist and astrobiologist David Morrison , who did not participate in this study , said , " If he ' s right , we can expect more Tunguska - sized explosions — perhaps every couple of centuries instead of every millennia or two . " He added , " It raises the bar in the long term — ultimately , we ' d like to have a survey system that can detect things this small . " Boslough and his colleagues detailed their findings at the American Geophysical Union meeting in San Francisco on Dec . 11 . A paper on the phenomenon has been accepted for publication in the International Journal of Impact Engineering . = = = = = = = = = ========================= ( 2 ) POSSIBLE " TUNGUSKA - SIZE " IMPACT on MARSThe Spaceguard Survey has discovered a small NEA that might hit Mars , not Earth , Orbit calculations from JPL indicate that Asteroid 2007 WD 5 is on a trajectory that will bring it close to Mars on January 30 , with a chance of hitting ( based on the current uncertainty in the orbit ) of better than 1 %. only once ( 3 years ago ) has the orbital analysis of a NEA indicated a higher probability of impact , and the target then was Earth . For about two days around Christmas , 2004 , the calculated impact probability for NEA Apophis was better than 1 %, reaching a maximum of slightly greater than 1 in 50 . Of course , additional orbital data then showed that while Apophis would come close in April 2029 , it would not hit . The same thing will probably happen with the orbital analysis of 2007 WD 5 . The present orbital estimate has considerable uncertainty , and not all computer simulations make exactly the same prediction . An impact is quite unlikely , but those of us in the business of surveying for possible impacts must always consider the possibility of a hit , even if the odds are against it . Should 2007 WD 5 actually hit Mars , it would have an impact energy similar to that of the 1908 Tunguska impact on Earth , making a roughly 1 - km - diameter crater . This crater , with its freshly exposed ejecta , would be extremely interesting to study from several spacecraft now in orbit around Mars . We can hope that this might happen , providing us a new window into the martian subsurface -- but the most likely ending for this story will be a miss , with 2007 WD 5 quickly fading from memory . Below are the NASA / JPL press release and two newspaper stories that discuss this possible Mars impact . David Morrison - - - - - - - - - - - - - - - - - - - - - - - ------------------------- ( 2 a ) NASA / JPL NEWS RELEASE : ASTRONOMERS MONITOR ASTEROID TO PASS NEAR MARS December 21 , 2007 WASHINGTON - Astronomers funded by NASA are monitoring the trajectory of an asteroid estimated to be 50 meters wide that is expected to cross Mars ' orbital path early next year . Observations provided by the astronomers and analyzed by NASA ' s Near - Earth Object Office at the Jet Propulsion Laboratory in Pasadena , Calif . , indicate the object may pass within 30 , 000 miles of Mars at about 6 a . m . EST ( 3 a . m . PST ) on Jan . 30 , 2008 . " Right now asteroid 2007 WD 5 is about half - way between Earth and Mars and closing the distance at a speed of about 27 , 900 miles per hour , " said Don Yeomans , manager of the Near Earth Object Office at JPL . " Over the next five weeks , we hope to gather more information from observatories so we can further refine the asteroid ' s trajectory . " NASA detects and tracks asteroids and comets passing close to Earth . The Near Earth Object Observation Program , commonly called " Spaceguard , " plots the orbits of these objects to determine if any could be potentially hazardous to our planet . Asteroid 2007 WD 5 was first discovered on Nov . 20 , 2007 , by the NASA - funded Catalina Sky Survey and put on a " watch list " because its orbit passes near Earth . Further observations from both the NASA - funded Spacewatch at Kitt Peak , Ariz . , and the Magdalena Ridge Observatory in New Mexico gave scientists enough data to determine that the asteroid was not a danger to Earth , but could potentially impact Mars . This makes it a member of an interesting class of small objects that are both near Earth objects and " Mars crossers . " Because of current uncertainties about the asteroid ' s exact orbit , there is a 1 - in - 75 chance of 2007 WD 5 impacting Mars . If this unlikely event were to occur , it would be somewhere within a broad swath across the planet north of where the Opportunity rover is located . " We estimate such impacts occur on Mars every thousand years or so , " said Steve Chesley , a scientist at JPL . " If 2007 WD 5 were to thump Mars on Jan . 30 , we calculate it would hit at about 30 , 000 miles per hour and might create a crater more than half - a - mile wide . " The Mars Rover Opportunity is exploring a crater approximately this size right now . Such a collision could release about three megatons of energy . Scientists believe an event of comparable magnitude occurred here on Earth in 1908 in Tunguska , Siberia , but no crater was created . The object was disintegrated by Earth ' s thicker atmosphere before it hit the ground , although the air blast devastated a large area of unpopulated forest . - - - - - - - - - - - - - - - - - - - - ------------------------- ( 2 b ) ASTEROID on TRACK FOR POSSIBLE MARS HITBy John Johnson Jr . , Los Angeles Times Staff WriterDecember 21 , 2007 Talk about your cosmic pileups . An asteroid similar to the one that flattened forests in Siberia in 1908 could plow into Mars next month , scientists said Thursday . Researchers attached to NASA ' s Near - Earth Object Program , who sometimes jokingly call themselves the Solar System Defense Team , have been tracking the asteroid since its discovery in late November . The scientists , at the Jet Propulsion Laboratory in La Cañada Flintridge , put the chances that it will hit the Red Planet on Jan . 30 at about 1 in 75 . A 1 - in - 75 shot is " wildly unusual , " said Steve Chesley , an astronomer with the Near - Earth Object office , which routinely tracks about 5 , 000 objects in Earth ' s neighborhood . " We ' re used to dealing with odds like one - in - a - million , " Chesley said . " Something with a one - in - a - hundred chance makes us sit up straight in our chairs . " The asteroid , designated 2007 WD 5 , is about 160 feet across , which puts it in the range of the space rock that exploded over Siberia . That explosion , the largest impact event in recent history , felled 80 million trees over 830 square miles . The Tunguska object broke up in midair , but the Martian atmosphere is so thin that an asteroid would probably plummet to the surface , digging a crater half a mile wide , Chesley said . The impact would probably send dust high into the atmosphere , scientists said . Depending on where the asteroid hit , such a plume might be visible through telescopes on Earth , Chesley said . The Mars Reconnaissance Orbiter , which is mapping the planet , would have a front - row seat . And NASA ' s two JPL - built rovers , Opportunity and Spirit , might be able to take pictures from the ground . Because scientists have never observed an asteroid impact -- the closest thing being the 1994 collision of comet Shoemaker - Levy with Jupiter -- such a collision on Mars would produce a " scientific bonanza , " Chesley said . The asteroid is now behind the moon , he said , so it will be almost two weeks before observers can plot its course more accurately . The possibility of an impact has the Solar System Defense Team excited . " Normally , we ' re rooting against the asteroid , " when it has Earth in its cross hairs , Chesley said . " This time we ' re rooting for the asteroid to hit . " - - - - - - ------------------------- ( 2 c ) ASTEROID MAY HIT MARS IN NEXT MONTHBy Alicia Chang ( MSNBC & AP ) LOS ANGELES ( AP ) — Mars could be in for an asteroid hit . A newly discovered hunk of space rock has a 1 in 75 chance of slamming into the Red Planet on Jan . 30 , scientists said Thursday . " These odds are extremely unusual . We frequently work with really long odds when we track . .. threatening asteroids , " said Steve Chesley , an astronomer with the Near Earth Object Program at NASA ' s Jet Propulsion Laboratory . The asteroid , known as 2007 WD 5 , was discovered in late November and is similar in size to an object that hit remote central Siberia in 1908 , unleashing energy equivalent to a 15 - megaton nuclear bomb and wiping out 60 million trees . Scientists tracking the asteroid , currently halfway between Earth and Mars , initially put the odds of impact at 1 in 350 but increased the chances this week . Scientists expect the odds to diminish again early next month after getting new observations of the asteroid ' s orbit , Chesley said . " We know that it ' s going to fly by Mars and most likely going to miss , but there ' s a possibility of an impact , " he said . If the asteroid does smash into Mars , it will probably hit near the equator close to where the rover Opportunity has been exploring the Martian plains since 2004 . The robot is not in danger because it lies outside the impact zone . Speeding at 8 miles a second , a collision would carve a hole the size of the famed Meteor Crater in Arizona . In 1994 , fragments of the comet Shoemaker - Levy 9 smacked into Jupiter , creating a series of overlapping fireballs in space . Astronomers have yet to witness an asteroid impact with another planet . " Unlike an Earth impact , we ' re not afraid , but we ' re excited , " Chesley said .