Story URL: http://news.medill.northwestern.edu/chicago/news.aspx?id=216471
Story Retrieval Date: 4/17/2015 11:38:48 AM CST
“It was not seen before it hit the atmosphere,” said Bill Cooke, head of NASA’s Meteoroid Environment Office at the Marshall Space Flight Center in Huntsville, Ala.
And that was too late as the meteorite exploded and hit approximately 900 miles southeast of Moscow last Friday in Chelyabinsk, a city of more than 1 million people.
NASA initially increased the estimated size of the meteorite from 14.9 meters to 16.7 across [49 feet to 55] 7,000 tons to 10,000. The meteorite entered the atmosphere around 9:30 a.m. local time –or 9:30 p.m. Central Standard Time on Feb. 14 – and released 500 kilotons of energy in the process. It was the largest reported meteor landing since 1908, when one struck Tunguska, Siberia.
The event may be worrying many around the world, but Cooke noted that an “event of the magnitude over Russia occurs once every 50 years or so.” An event similar to the one over Tunguska occurs once every few centuries, he added.
“I sleep very well at night,” Cooke said. “I’m not building a bunker or anything like that.”
“NASA has had a program to look for the really big ‘oh my goodness civilization is going to end’ rocks for a long time,” Cooke added. These larger rocks, typically asteroids, don’t appear to be on a collision course with us any time soon, but he said we do need to get better at identifying the smaller rocks that are coming at us out of the Sun.
Initial reports noted that the meteorite was traveling from north to south. Cooke explained that it became clear the object was in fact approaching from the east.
“When the first reports come in, it’s often hard to make clear what is going on,” Cooke said. NASA is currently going through the initial crude estimates from the event. These figures haven’t changed much, but estimates of the object’s size have now increased slightly to 18 or 19 meters and from 10,000 tons to about 11,000.
“Right now we’re just refining those initial estimates,” he said.
Although scientists don’t possess the ability to deflect or alter the course of incoming asteroids, NASA has been surveying the skies for them.
“You’ve got to give NASA some significant praise,” said Martin Elvis, senior astrophysicist at the Smithsonian Astrophysical Observatory. “Because they have identified 95 percent of the ones that could destroy us as a species, the things equivalent to dinosaur killers. It turns out, we are lucky that this didn’t seem to be one of those. We’d like to turn this 95 percent into 100 percent.”
Elvis offered some rough quantification of the destructive power of potential future strikes. “Even things that are 10 times the diameter of this rock that just hit us in Chelyabinsk, 10 times bigger doesn’t sound much worse but it’s actually 1,000 times the mass,” he said, noting the formula for the mass of spherical objects is their radius cubed.
The Sentinel mission, run by the B612 Foundation, will look for these so called “civilization destroyers,” Elvis said. According to the B612 website, the California-based foundation plans to “build and operate the first privately funded, launched, and operated interplanetary mission – an infrared space telescope to be placed in orbit around the Sun to discover, map, and track asteroids whose orbits approach Earth and threaten humanity.”
However, finding rocks similar to the one that hit Chelyabinsk is more difficult because they are smaller and fainter, and couldn’t be detected using the telescope planned for the Sentinel mission, Elvis said. Finding the smaller would require telescopes that are either closer to the asteroids themselves or a much bigger telescope, which would be very expensive. According to Elvis, a suitable telescope, on the scale of the James Webb Telescope [the successor of the Hubble] could cost as much as $10 billion.
Renee French, a Northwestern University graduate student in the Department of Earth and Planetary Sciences, volunteers at the Adler Planetarium. She answered questions about the Russian meteor hit via social media and about the asteroid, clearly visible to millions as it zoomed across the San Francisco Bay area.
“This meteorite is actually not related to the asteroid that came close to us. They were on completely different trajectories,” she said. “There’s no way that they could be related,” she added, referring to 2012 DA14, the asteroid that buzzed by Earth last Friday.
DA14 was approximately 150 feet in diameter and passed just over 17,000 miles above Earth’s surface last Friday afternoon, according to NASA.
DA14’s orbit was predicted and known well in advance, however, unlike that of the meteor that exploded over Chelyabinsk.
French said that, based on collected samples, the meteorite is classified as an “ordinary chondrite,” a type of stony meteorite – the most common type—which makes up 80 percent of all found meteorites.
If it had been of the solid, iron variety, the impact would have been “much, much worse,” Elvis said. Even so, if this stony meteorite had come in a little slower and a little lower, the impact could have been much greater regardless of the meteorite’s composition.
“The biggest event that happened was not when it landed, but [when] it exploded in the air before it hit the ground,” said Susan van der Lee, a seismologist and associate professor in the Department of Earth and Planetary Sciences at Northwestern University.
“There was a big explosion in the atmosphere before it could even hit the ground,” she added. “That then caused a shock wave to propagate through the atmosphere that was so strong that it blew out windows in a large area.” She said that atmospheric shock waves, coupled with the impact with the ground, created seismic waves that were recorded on seismograms.
Much work needs to be done, especially when it comes to tracking smaller objects (such as last Friday’s meteorite) and asteroids “coming out of the Sun” (while we look at them from the ground), according to Elvis. He said NEOCam, or Near Earth Object Camera, proposed out of NASA’s Jet Propulsion Laboratory by Amy Mainzer, would be positioned near the Sun for that purpose.
“It’s time to do some serious modeling,” Elvis said. “If we take a statistical approach and take a model of population in the computer and see how many will come by in a given time, that all has to be done now. I think it’s urgent, now.”
“That’s how we can find them all,” Elvis added. “But doesn’t have to do with how we can move them around.”
As they say in both baseball and boxing, you can’t hit what you can’t see.