News reports of incoming asteroids are a dime a dozen. Most headlines shout about these incoming marauding space rocks, but buried in the text is the big reveal that those asteroids will fly safely by, missing Earth by a large margin. Though the tabloid press may dull our fears of these scary interplanetary vagabonds, our planet getting hit by an asteroid is a credible threat. We've been hit before, and we will get hit again.
To prepare ourselves for a possible future impact, there are several astronomical campaigns (based in space and on the ground) specifically designed to find and track asteroids that come too close to Earth for comfort. These types of asteroids are known as "Near-Earth Asteroids," or NEAs, and NASA has announced that 15,000 of these space rocks have been identified and cataloged.
NASA's Near Earth Object (NEO) Observations Program, which tracks both asteroids and comets, has discovered 95% of these asteroids, boosting the number of known NEAs from 10,000 in August 2013 to 15,000 in October this year. Though impressive, there's a lot more out there and we are currently finding new NEAs at a rate of 30 every week.
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"The rising rate of discovery is due to dedicated NEO surveys and upgraded telescopes coming online in recent years," said Kelly Fast, the manager of NASA's NEO Observations Program, in a statement. "But while we're making great progress, we still have a long way to go."
And this is the scary thing (and I don't mean tabloid headline "scary"); it's thought that only 27% of NEAs measuring over 140 meters (460 feet) in diameter have been detected so far. The US Congress mandated NASA to detect at least 90% of these objects by 2020 so they can be tracked and their impact hazard worked out.
But why asteroids over 140 meters wide?
A good comparison is the Chelyabinsk meteor that exploded above the Russian city on Feb. 15, 2013. This powerful "superbolide" delivered an energy equivalent of 500 kilotons of TNT into the atmosphere (29 times more energetic than the atomic bomb that was dropped on Hiroshima in 1945), creating an incredibly bright explosion and shock wave that caused structural damage to hundreds of buildings and over a thousand injuries. Pieces of the meteor made it to the ground as meteorites. It was quite the show and, considering this was the biggest chunk of space rock to hit Earth since the remote 1908 Tunguska event (in Siberia), Chelyabinsk was incredibly unfortunate to be in the cross hairs.
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It's perhaps a little sobering to remember that before it hit our atmosphere, the asteroid that became the Chelyabinsk meteoroid was estimated to be around 17 meters (56 feet) wide.
17 meters wide.
This meteor was definitely a localized event, but its impact was felt throughout a populated city and its entry could be seen over 100 miles away. A 140 meter-wide asteroid, if it hit Earth, would have a regional impact, delivering the equivalent of 300 megatons of energy - that's 600 times more energetic than the Chelyabinsk impact, or 17,400 times bigger than the Hiroshima bomb. It's thought that Earth gets hit by something this size once every 20,000 years on average. And there's even bigger rocks out there, some that have, most likely, not yet been discovered.
So yes, there is a threat, but we have no idea when the next big asteroid will hit us (so there's no need to panic) and that's why great effort is being made to track these objects and make sure they'll miss Earth in future orbits around the sun.
Another recent breakthrough also announced by NASA is the early detection and characterization of asteroid 2016 UR36 on the night of Oct. 25-26 . This asteroid is small and sailed safely past Earth on Oct. 31 at a distance of 310,000 miles (around 1.3 times the Earth-moon distance). But this detection was different; it was found through an automated system called Scout that sped up the detection process, providing astronomers with a fast estimation of its orbital trajectory around the sun. Though it was quickly ruled out as an Earth impactor, it can be easy to see how critical such a system would be if a space rock was found barreling right at us.
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"When a telescope first finds a moving object, all you know is it's just a dot, moving on the sky," astronomer Paul Chodas of NASA's Jet Propulsion Laboratory, which runs the Scout program, told NPR. "You have no information about how far away it is. The more telescopes you get pointed at an object, the more data you get, and the more you're sure you are how big it is and which way it's headed. But sometimes you don't have a lot of time to make those observations."
The Scout detection of this particular space rock, which is estimated to be between 5-25 meters wide, provided astronomers with a warning and estimate of its flyby distance five days before the asteroid buzzed Earth. This is an incredible achievement considering how tiny the asteroid is and how far away it was at detection.
So while we are getting better at detecting, tracking and cataloging the biggest asteroids, we do have a lot of work to do to detect the smaller rocks that can cause significant damage on a local scale, like Chelyabinsk. So this is certainly an area where automated systems like Scout can help.
"While no known NEO currently poses a risk of impact with Earth over the next 100 years," said NASA Planetary Defense Officer Lindley Johnson, "we've found mostly the larger asteroids, and we have a lot more of the smaller but still potentially hazardous ones to find."
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