© Youtube
A meteorite causes panic as it falls in the Ural Mountains region of Russia, Feb. 2013.
A new video tells the tale of an unlikely bit of astronomical forensics.

Is there anything sneakier than an asteroid? They run silent, run deep and run very, very fast - hurtling toward Earth from any point in the vast bowl of the sky at speeds that can exceed 40,000 mph (64,000 k/h). They typically whiz right past us or plunge harmlessly and spectacularly into the atmosphere, burning up before they hit the ground - unless, of course, they explode in the sky or collide with the surface, leaving a massive footprint of destruction for miles around.

Asteroids - or, in their atmospheric incarnations, meteors or meteorites - don't do that kind of damage very often anymore, perhaps once every 70 to 100 years on average.

Comment: This may change very soon, as Earth is facing an imminent cosmic threat.

But when they do, they can spell big trouble. You could ask the folks living in the Tunguska region of Russia in 1908, where a 330 ft. (100 m) rock exploded in the sky one morning, flattening trees in an 830 sq. mi. (2,150 sq. km) radius. You could ask the dinosaurs - if they weren't all dead of course, thanks to a 6 mi. (10 km) rock that struck off the Yucatan 65 million years ago, throwing up a sky darkening debris screen that made the planet too cold for them to survive.

You could also ask the folks of Chelyabinsk, just to the east of Russia's Ural mountains, who on Feb. 15 of this year, experienced the damage that a 66 ft. (20 m) meteor can do when it explodes in the sky. The air-burst, which injured 1,491 people and damaged 7,200 buildings, fortunately claimed no lives and the damage it did was fixable, meaning that in the long history of potshots from space the Earth has endured in its 4.5 billion year life, this was a comparatively small and forgettable one.

But nothing's fully forgettable in the modern, wired era, and in this case, that's turning out to be a very good thing. Within a day of the strike, the Internet was inundated with footage of the event taken by security cameras, cellphones and the ubiquitous dashboard videocameras the Russians use to record accidents or carjacking attempts to be used later as evidence. That made the stuff of great YouTube watching, but as Meteor Strike, a new DVD released by PBS shows, it also made for great science.

Even in the modern era, when observatories around the world comb the skies for near-Earth asteroids and catalogue them in a growing census that allows us to track and anticipate their flight paths, there is plenty of ordnance that slips through by. The Chelyabinsk meteor was one of them, but the massive trove of forensic evidence stored by the eyewitnesses has allowed scientists from around the world to analyze the meteor and its path to destruction, providing a better understanding of the mechanics of these events, and in turn help us better prepare to defend ourselves against them.

Before the scientists investigating the meteor incident even began their analysis of the cellphone videos, they turned to a bigger and more institutional monitoring system: a global web of 46 atmospheric sensors used to detect evidence of nuclear test blasts. A nuke test - even one below ground - sets off a great wavefront of what's known as infrasound, or extremely low frequency sounds inaudible to the human ear. An asteroid bursting in the sky gives off a decidedly audible bang, but there's an infrasound component too.

When Peter Brown, an astrophysicist at Western Ontario University, checked the records from the bomb blast sensors on the day of the meteor strike, he found that the infrasound given off not only radiated across Chelyabinsk and much of Russia, but around the world several times. "The Earth was really ringing," he says.

That gave some sense of the mass of the rock (big) and the force of the blast (bigger), but not much more. The next step required a detailed analysis of those more-serendipitous videos. The video that showed the asteroid earliest in its plunge actually captured it at the moment it began to glow. That would have been at an altitude of about 43 mi. (70 km), since above that point the air is not thick enough to produce rock-burning friction. The angle of flight was about 20 degrees, and by triangulating from the various perspectives from which the scene was recorded, the scientists could determine that the meteor broke up no higher than 18 mi. (30 km) above ground. It took about 10 seconds from entry to breakup, which crude arithmetic means a speed of 40,000 mph (64,000 k/h). And when you know the physics of speed, angle and atmospheric heating - which the folks who study asteroids do - you can calculate the mass of the meteor. In this case it was about 7,000 metric tons, or 15 million lbs. (7 million kg). That's the mass equivalent of the Eiffel Tower plunging toward Chelyabinsk.

Those, at least, were the back of the envelope calculations. To refine them further, the investigators had to travel to Chelyabinsk and find the precise spots from which the videos were recorded. That involved a simple but ultimately reliable business of standing in the general vicinity of each recording and comparing the view from different position until the perspectives matched. Other videos taken from known locations - such as fixed security cameras - required a lot less calibration. And one, shot from a traffic camera atop a building, allowed investigators to analyze the high-speed, sun dial-like shifting of shadows from light posts, as the flaring meteor sped by. That added another level of granularity to the data.

Finally, the meteor hunters turned paleontologist, tromping out into the still snow-covered fields around Chelyabinsk, hunting for bits of the meteor that were scattered across the terrain as it blew up. What they were actually looking for were small, pellet-sized holes in the otherwise pristine snow, telltale wounds from the shotgun spray the meteor gave off. When they found one, they excavated in a large cone surrounding the hole, sifting the snow until, on occasion, they hit meteor paydirt. They shipped those samples to a lab in Yekaterinburg, where the makeup of the meteor was analyzed. The findings: it was about 10% metal and 90% rock.

That composition, plus the shallow angle of the approach helped prevent Chelyabinsk's scary day from being its last day. An iron meteor approaching at a sharper angle would surely have stuck the ground. At the right speed, the blast released would have wiped out the town.

There will, surely, be more meteor hits to come. NASA's Near Earth Object Program Office coordinates the world's detection effort, and is developing ways to deflect or destroy meteors. And an old NASA hand, Apollo 9 astronaut Rusty Schweickart, is similarly leading the private sector's efforts. Last year, Schweickart and his partners established the Sentinel Foundation, a nonprofit grouping working to build and launch an infrared satellite that would look for asteroids from the high ground of solar orbit.

"Sentinel will discover 50% of the city-buster asteroids," Schweickart says on the PBS video, using a simultaneously new and spooky term. That might seem to leave a huge margin for error, but it's a lot more protection than we've got so far. "Right now, we're driving around the solar system without any insurance," Schweickart says. He and his fellow skywatchers are working to provide us a little.