Toutatis is named after a Celtic-Gallic god who appears in a popular French comic-book series called Aventures d'Asterix. It tours the local planetary neighborhood in a lazy, four-year orbit that will take it out toward the asteroid belt between Mars and Jupiter, then back inside Earth's path around the sun.

During its latest close encounter, Toutatis will not be visible except through a telescope and its influence on the Earth will be so small it will not affect air currents in the atmosphere let alone the tides. It actually is two separate pieces of roundish rock that have been bonded together, one quite a bit bigger than the other, making it look like a malformed baking potato.

The latest Toutatis flyby will be at roughly four times the distance from Earth to the moon -- about 960,000 miles -- which is closer than it has come since at least the 12th century. Though astronomers estimate it will not pass this closely again for more than 550 years, they cannot be sure because Toutatis travels along such an erratic orbit. For that reason, it is classified as a potentially hazardous object.

No other known asteroids of its size or larger are expected to come as close as Toutatis during the 21st century, and there is no chance any known object will collide with Earth for at least five more centuries.

The bad news is the key word is known.

Toutatis, which is about half the size of the asteroid thought to have wiped out the dinosaurs 65 million years ago, possesses enough potential energy to cause havoc on a global scale if it ever did collide with Earth. It is just one of 3,000 known objects currently buzzing nearby. Estimates of the undiscovered objects vary, and the consensus is there probably are about 3,000 more, but some NASA scientists think there could be as many as 1 million of them out there.

Earth's atmosphere burns up most objects smaller than about 30 feet in diameter, which is good because even such a small object striking the surface at a speed of up to 40,000 miles per hour could hit with the energy of a 5 megaton nuclear bomb -- equivalent to the explosion of 5 million tons of TNT. An object up to 0.6 miles in diameter would hit with a force equivalent to several times the world's entire nuclear arsenal.

Something about a mile and a half in size, or half the size of Toutatis, could damage the environment on a global scale. It probably would cause a phenomenon first envisioned by the late astronomer Carl Sagan -- a nuclear winter in which so much dust and debris would be kicked up by the impact that sunlight would be blocked out for years, killing vegetation and leading to mass starvation.

If Toutatis hit Earth, it would drive deeply into the planet's crust, throwing up a huge amount of molten rock and scattering it across a wide area, igniting fires wherever it landed. Tsunamis -- huge tidal waves -- would swamp coastal areas, and tremendous earthquakes would level cities.

Part of what is known about potential asteroid impacts has been gathered by scientists studying previous events. The most recent occurred in 1908, when an object about 100 feet across hit Russia and leveled roughly 700 square miles of Siberian forest.

A less recent object carved the huge, circular crater at Flagstaff, Ariz.

The dinosaur killer, which is thought to have hit the Yucatan region of Mexico, deposited a thin layer of iridium -- a material extremely rare on Earth but plentiful in space -- on practically every square inch of the globe.

Given the huge number of dangerous objects meandering nearby, sooner or later, another is going to threaten Earth. Assuming it was discovered before it hit, the planet's governments could possibly devise an emergency strategy to deflect it.

So far, the European Space Agency has devised the most serious impact-defense plan. Last July, the ESA's Near-Earth Object Mission Advisory Panel recommended developing a mission to move an asteroid. That conclusion was based on the panel's previous consideration of six near-Earth-object mission studies. Of those studies, three involved rendezvous missions, while the other three examined the idea of building space-based observatories for the sole purpose of detecting potentially dangerous objects.

The most popular idea was called the Don Quixote mission. It would consist of two spacecraft, called Sancho and Hidalgo, launched at the same time, perhaps as early as 2010, with Sancho taking a faster route to the object of study. Once it arrived at the target asteroid it would begin a seven-month period of observation, landing penetrators and seismometers on the asteroid's surface to analyze its internal structure.

As Sancho recorded the event, Hidalgo then would smash into the asteroid at high speed. The idea would be to provide information about the asteroid's behavior during the impact, and kick up some of the object's interior for Sancho to observe. After the impact, Sancho and Earth-based telescopes would monitor the asteroid's orbit and rotation to see how they were affected.

Of the two other rendezvous missions, one is called SIMONE, for Small-sat Intercept Missions to Objects Near Earth. It would be a flotilla of tiny, low-cost satellites sent to swarm around a near-Earth asteroid and gather data. The other, ISHTAR, for Internal Structure High-resolution Tomography by Asteroid Rendezvous, would use radar tomography to study an asteroid's internal structure.

These missions would be only preliminaries, however. What remains is something vital but as yet unfulfilled: some sort of space tug, which would latch onto the deadly rock and, using some kind of powerful propulsion, nudge its orbit enough for it to pass by Earth harmlessly.

At present, all aspects of the concept remain open to discussion. The only thing certain is it will be needed someday.

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