"To attain any kind of life in this universe of ours appears to be quite an achievement. As humans we are doubly lucky, of course: We enjoy not only the privilege of existence but also the singular ability to appreciate it and even, in a multitude of ways, to make it better. It is a talent we have only barely begun to grasp." -Bill Bryson, A Short History of Nearly Everything

It's easy to forget while going about our daily lives that we're perched on a spinning mass of magma, rock and water that's hurtling through space. Around us for billions of light years in all directions are celestial bodies, including planets, stars, moons, galaxies, comets and asteroids.

Today, one of these galactic objects, a 400-meter wide asteroid named 2005 YU55, will soar within a few hundred thousand kilometers of Earth. The asteroid will be nearest - about 201,000 miles away - at 6:28pm EDT. In a universe with vast amounts of space between its celestial bodies, a comet or asteroid this proximate to Earth is considered a near-Earth object (NEO).

The last time an asteroid of similar size came this close was 35 years ago. Though there's no chance of YU55 plunging it's spherical body into our tender planet's crust, nor the possibility of it entering Earth's atmosphere (at least for the next 100 years), this event can serve as both an opportunity to study NEO's and as reminder of the continued luck necessary to sustain life on on our planet.

"We firmly believe that these sorts of events have been happening for most of the lifetime of the Earth, about 4.5 billion years," wrote Scott Fisher, program director in the Division of Astronomical Sciences at the National Science Foundation, during a live Internet chat. "In fact, early in the life of the Earth we went through a time that astronomers have named the "Era of Heavy Bombardment" where close passes and massive impacts happened much more frequently then now."

Though nothing could really stop an asteroid should it be on a collision course with Earth, scientists are studying the heavens and increasingly able to give advance notice.

"Baseball sized object hit the Earth's atmosphere daily," wrote Donald Yeomans, NASA project scientist for the Japanese mission to land upon Hayabusa, a near-Earth asteroid, during the live chat. "Volkswagen sized objects hit every few weeks but are too small to cause any ground damage. On average, a 30 meter sized object, the smallest that could cause significant ground damage, would be expected to hit every few hundred years, and a larger object of a kilometer in diameter would not be expected to hit but every few hundred thousand years."

Again, none of this is likely with the 2005 YU55 since it's safe orbit has probably been the same since before the dawn of astronomy. What's more likely, is your chance of watching YU55's fly by. All you need is a fairly good telescope and to be located on the East Coast this evening.

Perhaps then this event will inspire you as it has with me to seek out a dimly lit patch of earth from which to gaze outwards and contemplate the complexity our universe and the fragility of our planet.


Who else will be watching:

- NASA Infrared Telescope Facility

- The Gemini Observatory in Hawaii