When the asteroid known as 2012 DA14 zooms within 17,200 miles of our planet on Friday, it'll mark the closest approach by a massive space rock in more than a century (although the meteor that flared through the skies over Russia's Chelyabinsk region early Friday injured hundreds of people, it is only a fragment of the size of these monsters). Fortunately, the 150-foot-wide object will pose absolutely no risk to Earth — but over the course of millennia, other asteroids have literally rocked our world.

As safe as Friday's encounter will be, it's a reminder that Earth has been vulnerable to cosmic impacts in the past, and will continue to be in the future. That's why NASA and other agencies are spending millions of dollars to detect more of the estimated 1 million near-Earth objects that could be as threatening as 2012 DA14.

"We are looking at all kinds of partnership possibilities, across universities, space institutions and with the Air Force," said Lindley Johnson, program executive for the Near-Earth Object Observations Program at NASA Headquarters. This week, Johnson and other experts are gathering at a U.N.-sponsored conference in Vienna to discuss the creation of an international asteroid warning network.

Vienna is actually one of the places where 2012 DA14 can be seen in the night sky on Friday — not with the naked eye, but with binoculars or a small telescope. The best viewing opportunities will be available in Asia, Australia and Europe. (Follow the instructions at the bottom of this article to find out if it'll be visible from your location.)

The closest approach comes at 2:44 p.m. ET, when the asteroid will be zooming past at a speed of almost 17,500 mph, directly above the eastern Indian Ocean. It'll come 5,000 miles within the ring of communications satellites in geosynchronous Earth orbit, but those satellites are so widely distributed that experts say the chance of a collision is extremely remote.

NASA / JPL-Caltech

A NASA chart shows how asteroid 2012 DA14 will be deflected by Earth's gravitational field. Experts say the space rock will be put into a safer orbit after this week's encounter.

If 2012 DA14 were on a collision course, the shock of its rapid fall through Earth's atmosphere would cause it to explode, unleashing the power of a 2.4-megaton atomic bomb. In the worst-case scenario, that'd be enough energy to destroy an entire city. A similar cosmic blast in 1908 laid waste to 820 square miles of Siberian forest in the Tunguska region.

It's possible that other such blasts have occurred over the course of Earth's history without being recorded. Based on a statistical analysis, NASA estimates that asteroids the size of 2012 DA14 strike Earth every 1,200 years or so. The only reason we know about this encounter is because the capabilities for tracking near-Earth objects have improved so much in recent years.

A Spanish observation team discovered 2012 DA14 just last year during a more distant flyby. "We probably would not have found DA14 10 years ago," said Don Yeomans, the head of NASA's Near-Earth Object Program Office at the Jet Propulsion Laboratory in Pasadena, Calif. Now that they know it's there, astronomers will be monitoring the asteroid with optical and radio telescopes, including the Arecibo Observatory's 1,000-foot-wide dish in Puerto Rico and NASA's Goldstone radio antenna in California.

Radar observations could provide insights into the space rock's shape and spin, while an analysis of the optical data could reveal what 2012 DA14 is made of. Think of Friday's encounter as a practice run for identifying and tracking the unknown asteroids that actually could threaten us in the years to come — and an incentive to figure out ways to deflect them in case we have to.

To get a better sense of how 2012 DA14 rates, here are a dozen more hits and misses involving near-Earth objects:

65 million B.C.: The most infamous asteroid is the 6-mile-wide rock that smashed into Mexico's Yucatan Peninsula 65 million years ago, sparking the global catastrophe that did in the dinosaurs. Experts say the explosion released as much energy as 100 trillion tons of TNT.

35 million B.C.: Geologists say a roughly kilometer-wide (0.6-mile-wide) asteroid or comet struck America's Eastern Seaboard millions of years ago, contributing to the formation of Chesapeake Bay and creating a biological crisis. Studies have shown that microbes deep underground in the blast zone are still adjusting to the ancient shock.

NASA

Barringer Impact Crater in Arizona — also known as Meteor Crater — is captured in this image taken in 1995 by space shuttle astronauts.

50,000 B.C.: A 150-foot-wide iron-nickel meteorite hits Arizona, creating the 0.75-mile-wide Meteor Crater. Asteroid 2012 DA14 is thought to be the same size as this meteorite, but made of less dense stuff that would break up before it hits the ground.

1490: Chinese accounts tell of a meteor shower during which "stones fell like rain" on the Qingyang (Ch'ing-Yang) district of Shaanxi Province (now Gansu Province), killing as many as 10,000 people. Experts are doubtful about the reported death toll, but they don't doubt that a dramatic event occurred, perhaps involving the breakup of an asteroid. 

1908: The Tunguska event in Siberia, which flattened millions of trees, is thought to have been caused by an asteroid similar to 2012 DA14 in size and composition. Tunguska has become a watchword for asteroid activists. "The greatest danger from an asteroid strike is from the ones we haven't yet found," former NASA astronaut Ed Lu, chairman and CEO of the B612 Foundation, told NBC News. "Of the asteroids larger than the one that struck Tunguska in 1908, we know less than 1 percent."

1937: Asteroid Hermes is observed to miss Earth by a distance of just 460,000 miles. Decades later, scientists found out that Hermes occasionally comes even closer to Earth, and in fact consists of two space rocks flying in tandem. Each of the objects is thought to be about 1,300 feet (400 meters) wide.

1972: The Great Daylight Fireball is witnessed blazing over the Rocky Mountains from the U.S. Southwest to Canada. Scientists say it was an Earth-grazing meteoroid that passed within 35 miles (57 kilometers) of Earth’s surface.

1997: Astronomers report that a mile-wide asteroid known as 1997 XF11 had a chance of hitting Earth in 2028. The report touched off a media tempest, but further observations reduced the chance of collision to zero. The news came amid a spate of asteroid disaster flicks, including "Deep Impact" and "Armageddon" as well as the TV miniseries "Asteroid."

2004: An 885-foot-wide asteroid known as 2004 MN4, later named Apophis, is initially given a 1-in-40 chance of hitting Earth in 2029. That collision risk was ruled out relatively quickly, but it took years longer to analyze the risk posed by a later encounter in 2036. Just last month, astronomers announced that Apophis will pose no threat to Earth in the foreseeable future.

2008: Asteroid 2008 TC3 explodes during atmospheric entry above Sudan’s Nubian Desert. The event marked the first time that a near-Earth object’s impact was successfully predicted, several hours in advance. 2008 TC3 was 2 to 5 meters wide, and broke up into fragments that were later recovered from the desert.

2011: Asteroid 2011 CQ1 makes the closest-ever flyby of Earth for a cataloged asteroid, passing within 3,400 miles of Earth’s surface. The asteroid was discovered just 16 hours before its super-close encounter, but because it's only a meter wide, it would have burned up in the atmosphere if it had been on a direct course.

2011: An asteroid the size of an aircraft carrier, 2005 YU55, sails past Earth at a distance of 198,000 miles, which is closer than the orbit of the moon. Earthlings marked the asteroid's passage with a barrage of picture-taking.

More about the asteroid encounter:

• Would-be miners say asteroid is worth $195 billion
• Hooray! Earth will kick asteroid into a safer orbit
• Flash interactive: Close encounters of the asteroid kind

Astronomers say asteroid 2012 DA14 won't be visible to the naked eye, but it is possible to watch it pass by through binoculars or a small telescope — if you know where and when to look. The Heavens-Above website can help you get a fix on the fast-moving rock. First, go to the website's location database and find the nearest city. Click on the link for that city. Then, click on over to the 2012 DA14 sky chart and look for the asteroid's track, with notations that indicate observation times. If you don't see the asteroid's track, you won't be able to see the asteroid. In some cases, the track is shown during daylight hours — which would generally rule out visual observations.

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.