New analyses suggest that an asteroidal fragment's collision with Earth on February 15, 2013, might not be the once-per-century event that researchers thought. Instead, these potent wallops might occur more frequently — and with more destructive power — than previously thought.
Anyone living in Chelyabinsk, Russia, and who looked skyward at precisely 9:20:32 a.m. last February 15th, saw the incredible spectacle of a massive meteoric fireball brighter than the early-morning Sun. It broke apart violently 20 to 30 miles (30 to 45 km) up. Then, 88 seconds later, powerful shock waves knocked some residents off their feet, injured more than 1,000 (including flash-induced sunburns), and shattered windows in nearly half of the city's apartment buildings.
Scary close-call aside, scientists count themselves fortunate that the cosmic blast occurred over this urban center. That's because the city's security-obsessed residents captured the event in a myriad of building-mounted video cameras and on dashcams mounted inside their cars. All these looks, combined with defense satellites looking down on Earth from space and a worldwide infrasound network maintained by Comprehensive Nuclear-Test-Ban Treaty Organization (CTBTO), have allowed researchers to dissect the event with unprecedented precision. Yesterday they made their findings public, in two articles published in Nature and a third in Science.
The object itself was a wayward interloper from the asteroid belt. Careful trajectory reconstruction by Jiří Borovička (Astronomical Institute, Czech Academy of Sciences) and others shows that the object came in on a highly elliptical, low-inclination orbit that's a close match to that of the unnamed asteroid 86039. Mostly likely, the two bodies were once part of a single object that sent fragments flying across Earth's path after a violent collision with another asteroid some time in the past.
Researchers have also pulled together estimates of the impact's energy, gauged as the kinetic-energy equivalent of exploding TNT. A research team led by Peter Brown (University of Western Ontario), visible-light emission alone implies a blast of at least 470,000 tons (470 kilotons) of TNT. But the powerful seismic shock yields a rather uncertain "best estimate" of 430 kilotons, while sensors on military satellites suggest 530 kilotons. Finally, records from the CTBTO's infrasound network imply a somewhat higher yield of 600 kilotons.
Uncertainties aside, the Chelyabinsk blast represents the most energetic impact on Earth since the iconic blast over the Tunguska region of Siberia in 1908. The rocky object had a diameter close to 62 feet (19 m) and a mass of roughly 12,000 metric tons (nearly twice the initial estimate).
The ground-level damage might have been much greater, researchers conclude, save for two fortunate circumstances. First, the body came in at a very shallow angle, just 17° from horizontal. As it broke up, the resulting shock wave expanded with a cylindrical shape, rather than from a single explosive point, which tended to spread the shock energy over a wider, less concentrated area. Had the flight path been more vertical, as was the case with the Tunguska blast, the damage would likely have been far more extensive.
Second, recovered fragments show that the incoming object was a single cohesive body — but only barely so, according to analysis by the "Chelyabinsk Airburst Consortium," led by Olga Popova (Institute for Dynamics of Geospheres, Russian Academy of Sciences).
The teams reports that the Chelyabinsk parent body must have endured an abrupt thermal or collisional event 4.45 billion years ago, 115 million years after the solar system formed, that left its interior crisscrossed by a network of fractures filled with metal-rich glass. These preexisting fractures, explains consortium member Peter Jenniskens (SETI Institute), "left the body weaker, and it broke apart along those veins."
But researchers are hardly feeling complacent about all this. For example, Brown and his team went on to compile a worldwide catalog of all such airbursts over the past two decades. (Chelyabinsk might have been the most powerful, but it wasn't the only one recorded.) They find that the impacts from objects a few tens of meters across must be occurring 7 to 10 times more frequent than estimates based only on telescopic surveys, though there's a lot of uncertainty because the events are so sparse. Still, Brown's census suggests that a Chelyabinsk-type blast should happen not just once per century on average, as had been thought, but instead every few decades.
Another concern is researchers didn't believe objects in this mass range would disintegrate so low in the atmosphere. Now the thinking is that these blasts are driven deeper down by their own momentum, an idea first put forward six years ago by Mark Boslough (Sandia National Laboratories) to explain Tunguska's 800 square miles of devastation. Moreover, Boslough points out, impacts appear to be "more damaging than nuclear explosions of the same yield" because up to half the energy in a nuclear blast escapes as radiation rather than as shock and heat.
It's a set of results both exciting and sobering. Millions of Earth-threatening objects with masses comparable to Chelyabinsk's have yet to be discovered. To make matters worse, February's rogue impactor approached from Earth's sunlit side, making it impossible to detect beforehand.
So what's the "action plan" to defend Earth from objects once considered too small to do any real damage? Ideas are being kicked around. A team at the University of Hawaii hopes to operate a Asteroid Terrestrial-impact Last Alert System (ATLAS) by 2015. The privately funded B612 Foundation has proposed its Sentinel spacecraft, which would scan the infrared sky from a location well inside Earth's orbit. Conceptually similar is NEOCam, first proposed in 2005.
Meanwhile, NASA managers have launched a "Grand Challenge" to bring innovative concepts to light, and the United Nations is trying to establish an "International Asteroid Warning Group".
These are useful first steps toward a more comprehensive global strategy. But some researchers wonder whether February's eye-opener, combined with the realization that relatively small asteroids can do serious damage, should spur a more rapid buildup of detection and defense systems. One has to wonder how different the response would have been had that errant space rock come in over Chicago, instead of Chelyabinsk.