Afireball over Utah overnight Tuesday was the talk of the state. And an asteroidthat could have delivered nearly half the energy of the Hiroshima atom bombwhizzed past Earth earlier this month, NASA reported recently. Meanwhile, ameteor shower dazzled skywatchers around the globe early Tuesday morning.
What'sgoing on? Nothing all that unusual, astronomers say.
Thefireball over Utah ? said to have turnednight briefly into day ? was the sort of event that happens more than mostpeople realize. Space rocks disintegrate as they crash through Earth'satmosphere. Those the size of a softball up to perhaps a small car can createdramatic fireballs. We don't see most of these fireballs because they occurover unpopulated land areas or over the oceans. Earth is two-thirds ocean. Afireball during daytime could also go largely unnoticed.
Thefireball was notable for occurring near the peak of the annual Leonidsmeteor shower, but early reports suggest the events were unrelated. TheLeonid "shooting stars" are created by tiny bits of comet debris,mostly no larger than a pea. This year's shower was better than average,skywatchers reported.
Meanwhile,asteroid 2009 VA flew past Earth Nov. 6 at about 4:30 p.m. EST. It didn'tdisintegrate, but boy was it a close call. The space rock was discovered by theCatalina Sky Survey only some 15 hours before it approached us. It passed just8,700 miles (14,000 km) from our planet's surface ? slightly less than Earth'sdiameter.
Theasteroid was just about 23 feet wide (7 meters). Still, had its path collidedwith Earth, it would have packed the energy of "about six kilotons ofTNT," explained astronomer Donald Yeomans, manager of NASA's Near-EarthObject program office at the Jet Propulsion Laboratory in Pasadena, Calif.
On average,objects the size of 2009 VA pass this close about twice per year and impactEarth about once every five years, NASA added. However,most asteroids burn up in the atmosphere at about 80 feet (25 meters) in diameter and smaller,probably for an impressive fireball event.
This nearmiss makes is the third-closest known asteroid to squeak past Earth. The otherclose approaches include the yard-wide (1 meter) asteroid 2008 TS26, which camewithin 3,820 miles (6,150 km) of the Earth's surface on October 9, 2008, andthe 23-feet-wide (7 meter) asteroid 2004 FU162 that passed within 4,060 miles(6,535 km) on March 31, 2004.
Sofar 795 near-Earth asteroids more than a half-mile wide (1 km) have been foundto date, Yeomans said, and another 84 near-Earth comets of probably similardiameters. About two near-Earth objects (NEOs) that size are found every month,explained Lindley Johnson, program executive for the NEO Observation Program.
Scientistshave ruled out the chances of an Earth impact for such large objects for thenext couple of centuries, Yeomans added ? on average, a NEO that size hits theEarth roughly every 500,000 years. Still, researchers estimate more than 150NEOs that large remain to be found.
Lesserobjects alsopose a risk. The object scientists currently know of with the largestprobability of impact, 2007 VK184, has about a 1 in 3,000 chance. If thisroughly 425-foot-wide (130 meters) asteroid hit our planet, it would strikewith an energy of roughly 150 million tons of TNT.
"Fouror five objects in the several tens of meters to 100 meters in size have agreater than a 1 in a million chance of hitting us ? the odds are in our favor,but people win the lottery every day, too," Johnson said. "We'recertainly trying to keep an eye on those."
State of our defenses
NASAhas spent about $40 million on the NEO Observation Program since 1998 toidentify 90percent of the total population of roughly 1,050 potential hazards one kilometer or sowide.
"We'reprobably within a year of achieving that goal ? we're at about 85 percent rightnow," Johnson said. The program has a current annual budget of about $4.2 million.
Congresshas asked NASA to extend the search to objects "that are 140 meters in size (460feet) or larger," Yeomans said. Altogether, there are roughly 100,000 ofthese, Johnson explained, of which 20,000 might potentially be hazardous. Sofar scientists have detected about 5 percent of these 100,000 objects, "including about2,250 objects from 300 meters (985 feet) to one kilometer in size and 1,500 to1,600 objects 100 meters (330 feet) to 300 meters in size," Johnson added.
However,the necessary funds to extend the search down to such objects have not yet beenmade available.
"We'retrying to do what we can with the same telescopes used for large NEO search ?we find them all the time, but we don't have the capability need to achievethat 90 percent goal for 140-meter-sized objects by 2020," Johnson said.
Thereis an ongoing National Research Council panel that should report by the end of the yearon options to meet the goal regarding these smaller NEOs, he added.
"It'snot something that we should worry about on a daily basis, but it is a naturalhazard that can cause levels of devastation greater than anything we know aboutor have experienced, more than any earthquake or hurricane," Johnson said."So it's certainly worth the effort to at least find out if there is anynear-term hazard."
"AndNEOs represent more than just a hazard to us," he noted. "They'repotentially a source of resources and destinations as well, as we explore thesolar system."
SPACE.comStaff contributed to this report.
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