Feb. 15, 2013 at 5:29 PM ET
What exactly fell on Russia's Chelyabinsk region on Friday? Was it an asteroid, meteor, meteoroid, meteorite or fireball? You could make a case for "any of the above," depending on your definitions and the precise part of the phenomenon you're trying to describe.
The Chelyabinsk incident is the biggest known cosmic impact since another Russian blast that occurred a century ago, the Tunguska incident of 1908. There's good reason for that notoreity: Hundreds of injuries were reported. NASA estimated that the energy released by the Chelyabinsk impact amounted to 300 kilotons of TNT, which suggests the blast was more than 10 times as powerful as the atom bombs that were dropped on Japan at the end of World War II.
NASA's assessment put the Chelyabinsk object's width at 15 meters (50 feet), and its mass at 7,000 tons. Much of that mass burned up during the object's atmospheric entry at a velocity of 40,000 mph (18 kilometers per second). "The fireball was brighter than the sun," the space agency said in a statement.
Astronomers use different terms to describe cosmic objects of different sizes: When the rock is no wider than a meter (3.3 feet), it's known as a meteoroid. But once you start getting into the 1- to 10-meter range, the term "asteroid" applies. Earlier estimates suggested the Chelyabinsk object was a meteoroid, but the latest assessment would put it in the class of a small asteroid.
Bill Cooke, who heads the Meteoroid Environment Office at NASA's Marshall Space Center, said the object was "a small asteroid or a large meteoroid, depending on how you want to define it."
When pieces of the meteoroid (or asteroid) survive their fiery fall through the atmosphere and hit the surface, those pieces are called meteorites. Russian authorities say a hole in the ice on Chebarkul Lake, near Chelyabinsk, marks a spot where at least one meteorite left its mark. There are already reports of Chelyabinsk meteorites turning up on online auction sites, but those are more likely to be "meteor-wrongs" — rocks wrongly assumed to be meteorites.
The term "meteor" refers to the fiery aerial display created by a falling meteoroid or asteroid. Meteors are called fireballs if they shine brighter than the planets in the night sky (magnitude -4), and bolides if the blast is even brighter (around magnitude -14). There's no question that the Chelyabinsk meteor qualifies as a bolide.
Some asteroids are made of iron and nickel, and survive their fall more easily. However, the fact that the Chelyabinsk object appeared to break up into pieces while it was still miles high indicates that it was made of less dense stuff. The stresses of atmospheric entry caused the rock to break apart explosively, creating the midair flash and generating a shock wave. The shock wave produced the loud "bang" that set off car alarms, blew out windows and apparently collapsed the roof of a zinc factory warehouse. Flying glass from all those broken windows caused many of the injuries that were reported.
What about the asteroid flyby?
The Chelyabinsk object streaked through Russian skies just hours before a 150-foot-wide (45-meter-wide) asteroid known as 2012 DA14 was due to make a remarkably close approach, coming within 17,200 miles of Earth's surface. However, the two objects were in dramatically different orbits, and that's one of the factors that led NASA to conclude that the two cosmic events were "not related."
"It's clearly coincidental, but it's a pretty amazing coincidence," said former Apollo astronaut Rusty Schweickart, a co-founder of the B612 Foundation. For years, Schweickart and his colleagues have been trying to raise awareness about the hazards posed by asteroids, and Friday's double dose of cosmic reality certainly serves as a consciousness-raiser.
"It's a torpedo across the bow," Schweickart told NBC News, "and it serves as an indication that these things really do happen."
Objects as small as the Chelyabinsk asteroid are difficult to detect — but the feat is not impossible, given the right circumstances. In 2008, a 2- to 5-meter-wide asteroid known as 2008 TC3 was spotted using the Catalina Sky Survey 1.5-meter telescope in Arizona, 20 hours before its impact in the Sudanese desert. The Chelyabinsk object would have been particularly hard to spot because it came in from the blind spot on Earth's sunlit side.
The Chelyabinsk object is no more, but there are still lots of other space rocks to be found. In 2011, NASA estimated that there are a million potentially hazardous near-Earth asteroids smaller than 100 meters (330 feet). Several organizations — including NASA, the B612 Foundation and Planetary Resources — are working on plans to detect and track more of the threats that are out there. To learn more about those efforts, click on the links below:
Alan Boyle is NBCNews.com's science editor. Connect with the Cosmic Log community by "liking" the log's Facebook page, following @b0yle on Twitter and adding the Cosmic Log page to your Google+ presence. To keep up with Cosmic Log as well as NBCNews.com's other stories about science and space, sign up for the Tech & Science newsletter, delivered to your email in-box every weekday. You can also check out "The Case for Pluto," my book about the controversial dwarf planet and the search for new worlds.