A fireball over Utah overnight Tuesday was the talk of the state. And an asteroid that could have delivered nearly half the energy of the Hiroshima atom bomb whizzed past Earth earlier this month, NASA reported recently. Meanwhile, a meteor shower dazzled skywatchers around the globe early Tuesday morning.
What's going on? Nothing all that unusual, astronomers say.
The fireball over Utah said to have turned night briefly into day was the sort of event that happens more than most people realize. Space rocks disintegrate as they crash through Earth's atmosphere. Those the size of a softball up to perhaps a small car can create dramatic fireballs. We don't see most of these fireballs because they occur over unpopulated land areas or over the oceans. Earth is two-thirds ocean. A fireball during daytime could also go largely unnoticed.
The fireball was notable for occurring near the peak of the annual Leonids meteor shower, but early reports suggest the events were unrelated. The Leonid "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't disintegrate, but boy was it a close call. The space rock was discovered by the Catalina Sky Survey only some 15 hours before it approached us. It passed just 8,700 miles (14,000 km) from our planet's surface slightly less than Earth's diameter.
The asteroid was just about 23 feet wide (7 meters). Still, had its path collided with Earth, it would have packed the energy of "about six kilotons of TNT," explained astronomer Donald Yeomans, manager of NASA's Near-Earth Object 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 impact Earth 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 near miss makes is the third-closest known asteroid to squeak past Earth. The other close approaches include the yard-wide (1 meter) asteroid 2008 TS26, which came within 3,820 miles (6,150 km) of the Earth's surface on October 9, 2008, and the 23-feet-wide (7 meter) asteroid 2004 FU162 that passed within 4,060 miles (6,535 km) on March 31, 2004.
So far 795 near-Earth asteroids more than a half-mile wide (1 km) have been found to date, Yeomans said, and another 84 near-Earth comets of probably similar diameters. About two near-Earth objects (NEOs) that size are found every month, explained Lindley Johnson, program executive for the NEO Observation Program.
Scientists have ruled out the chances of an Earth impact for such large objects for the next couple of centuries, Yeomans added on average, a NEO that size hits the Earth roughly every 500,000 years. Still, researchers estimate more than 150 NEOs that large remain to be found.
Lesser objects also pose a risk. The object scientists currently know of with the largest probability of impact, 2007 VK184, has about a 1 in 3,000 chance. If this roughly 425-foot-wide (130 meters) asteroid hit our planet, it would strike with an energy of roughly 150 million tons of TNT.
"Four or five objects in the several tens of meters to 100 meters in size have a greater 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're certainly trying to keep an eye on those."
State of our defenses
NASA has spent about $40 million on the NEO Observation Program since 1998 to identify 90 percent of the total population of roughly 1,050 potential hazards one kilometer or so wide.
"We're probably within a year of achieving that goal we're at about 85 percent right now," Johnson said. The program has a current annual budget of about $4.2 million.
Congress has asked NASA to extend the search to objects "that are 140 meters in size (460 feet) or larger," Yeomans said. Altogether, there are roughly 100,000 of these, Johnson explained, of which 20,000 might potentially be hazardous. So far scientists have detected about 5 percent of these 100,000 objects, "including about 2,250 objects from 300 meters (985 feet) to one kilometer in size and 1,500 to 1,600 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 been made available.
"We're trying 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 achieve that 90 percent goal for 140-meter-sized objects by 2020," Johnson said.
There is an ongoing National Research Council panel that should report by the end of the year on options to meet the goal regarding these smaller NEOs, he added.
"It's not something that we should worry about on a daily basis, but it is a natural hazard that can cause levels of devastation greater than anything we know about or 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 any near-term hazard."
"And NEOs represent more than just a hazard to us," he noted. "They're potentially a source of resources and destinations as well, as we explore the solar system."