Coming Soon To A Planet Near You

If you’ve been outside these past few nights watching the Perseid Meteor Shower, you know there’s a lot of space stuff slamming into our atmosphere. During the peak of the Perseids on Tuesday night into Wednesday morning, I counted a good half-dozen fireballs arcing across the night sky over the Black River, along with dozens of short streaks.

Our atmosphere may be thin, but it does a great job keeping small space rocks from making it to terra firma. In fact, tons of meteoroids enter the atmosphere every day. Most burn up and vaporize as they encounter the friction from air molecules. A few reach Earth, though, and even a rock as small as 1 meter in diameter (about 3 feet) can do a lot of damage when it hits the ground going thousands of miles an hour.

Outside of the small rocks that continually bombard our planet, there also are larger asteroids and comets that could pose a real threat to our planet it they hit. Those that are expected to pass close to Earth are called Near Earth Objects. According to NASA’s Near Earth Object Program, 10 identified objects made “close approaches” to Earth between August 2 and 10, 2009.

Of course, space is a big place, and the units used to determine a “close approach” to Earth have been developed with this sense of celestial distance in mind. The two main units of measurement are the AU (Astronomical Unit) and the LD (Lunar Distance).

1 AU is equal to the mean distance between the Earth and the Sun, and is used to measure distances in the Solar System. This is equal to roughly 149,597,870.691 kilometers, which is usually shortened to about 150 million kilometers (about 93.2 million miles).

1 LD is equal to the mean distance between the Earth and the Moon, and is equal to roughly 384,401 kilometers (about 239,000 miles). This number is a lot easier to work with when you’re dealing with objects coming close to Earth.

Of the 10 objects that passed close to Earth between August 2 and 10, none came closer than 8.4 LD, or more than eight times the distance between the Earth and the Moon. In case you’re wondering, the object that came closest was 2007 RQ17, which was discovered in 2007.

This brings up another important point — objects are being discovered all the time that might have orbital tracks that could intersect Earth’s orbit and pose a threat. Many of these objects are small enough or have such wide orbits that they don’t pose much of a threat. Some objects, though, have a much higher probability of posing a threat.

The Torino Impact Hazard Scale has been developed to rank the hazard posed by asteroids and comets. The scale ranges from 0 (no hazard) to 10 (collision with Earth and global climate catastrophe is certain).

An object ranked 0 or 1 is in the White or Green Zone, and considered a “normal” object that is expected to either burn up in the atmosphere or pass close to Earth, but not close enough to pose a threat. Objects ranked 2-4 are in the Yellow Zone, and are calculated to have a 1% or greater chance of a collision with Earth. These are considered “close encounters.”

Objects ranked 5-7 are in the Orange Zone, and considered “Threatening.” These are larger objects that are estimated to have a chance of a close to very close encounter that could lead to a global catastrophe. Finally, objects ranked 8-10 are those expected to collide with Earth and lead to localized destruction or a global climatic catastrophe.

The good news is that of the roughly 250 Near Earth Asteroids being tracked as of August 11, all but one are ranked 0 (no hazard). Object 2007 VK184 is ranked 1 (Normal).

A new NASA Web site called Asteroid Watch (http://www.jpl.nasa.gov/asteroidwatch) makes it easier than ever for you to keep tabs on Near Earth Objects. The site also contains links where you can sign up for NASA’s asteroid widget and Twitter account. If you want more technical information or need scientific data, check out the Jet Propulsion Laboratory’s Near Earth Objects Web site (http://neo.jpl.nasa.gov/).