Eight Things You Might Not Know About Killer Asteroids

The idea of a giant asteroid crashing into Earth and destroying life as we know it is more than fodder for Hollywood dramas and science fiction: it's a very real threat that some scientists and officials around the world spend a lot of time thinking about.

After all, there's ample evidence that our planet has been socked by giant space rocks many times in the past -- not only in the Solar System's early days, when

interplanetary debris was still in large supply, but even as recently as 100 years ago.

So what should you know about potential "killer" asteroids, and what experts around the globe are doing to study them? Consider these eight facts:

1. Advances in our understanding of space over recent decades have made it clear just how many asteroids and comets roam our Solar System. That growing awareness prompted the U.S. Congress to tell the National Aeronautic and Space Administration (NASA) to start studying, and counting, the objects that come closest to Earth. Starting in 1990, that task was assigned to a new agency, the Near-Earth Object Program Office at NASA's Jet Propulsion Laboratory.

2. As of this month, NASA's Near-Earth Object Program Office has counted a total of 5,648 Near-Earth Objects, or NEOs. Among those are 755 asteroids or comets that are at least 0.62 miles across, and 978 that are designated as "potentially hazardous asteroids" or PHAs. PHAs earn that status by showing the likelihood of coming within at least 4.65 million miles of Earth, and are at least 500 feet in diameter.

3. The Association of Space Explorers recently finished putting together a report titled, "Asteroid Threats: A Call for Global Response." (PDF). Set to be presented next February to the United Nations' Committee on the Peaceful Uses of Outer Space, the report warns that an asteroid collision with Earth could be avoided "if, and only if, national governments and relevant international institutions understand these inevitable events and act together to prevent their occurrence."

4. Our Earth is pelted with interplanetary debris on a regular basis, though it's not noticeable. While lots of bits of rock and ice enter our atmosphere daily, they burn up or disintegrate well before impact -- as long as they're smaller than about 100 feet or so across, according to an article titled "Planetary Defense: Preventing a World of Trouble."

5. Other, larger objects also zoom by our planet at (astronomically speaking) relatively close distances on a regular basis. The Near-Earth Object Program Office keeps a running tally of these "close approaches" online. Among the NEOs to pass through our neighborhood lately are an object known as 2008 SC, which came within 4.3 million miles of Earth on Sept. 24, and 2008 ST, which approached as closely as 948,000 miles on Sept. 28.

6. A giant asteroid is believed to have hit the Earth some 65 million years ago, possibly contributing to the extinction of the dinosaurs at that time. The remains of that impact is a giant, buried crater on Mexico's Yucatan Peninsula; the crater's name is Chicxulub, which means "tail of the devil" in the Mayan language, notes the University of Arizona's Lunar and Planetary Laboratory. At 90 to 112 miles across, Chicxulub is one of the largest confirmed impact craters on our planet, according to the laboratory.

7. One of the more threatening objects that could cross our path in our lifetime is an asteroid known as 999042 Apophis. This chunk of rock, which is about 1,150 feet long, could approach within 18,300 miles of Earth on -- of all dates -- Friday, April 13, 2029, according to the Near-Earth Program Office at NASA's Jet Propulsion Laboratory. How Apophis interacts with Earth's gravity that time will affect how close it could come on its next pass, expected in 2036.

8. This past summer marked the 100th anniversary of the Tunguska blast, a giant explosion that decimated a large area of forest in a largely unpopulated area of Siberia. Most scientists believe the devastation was caused by an asteroid or comet that burst in mid-air, toppling millions of trees over an area of hundreds of square miles. What's especially disturbing is that recent research at Sandia National Laboratories suggests the object wasn't as big as originally thought.

"That such a small object can do this kind of destruction suggests that smaller asteroids are something we should consider," said Mark Boslough, a principal investigator in the Sandia-Tunguska research. "Their smaller size indicates such collisions are not as improbable as we had previously believed. We should make more efforts at detecting the smaller ones than we have till now.