Based on observations of portions of the main asteroid belt, between Mars and Jupiter, the new study estimates there are between 1.1 million and 1.9 million of these large space rocks. Many millions or perhaps billions of smaller asteroids travel around the Sun in the same belt.

Asteroids in the main belt are not generally considered to be a threat to Earth anytime in the near future, so the new estimate is not likely to alter calculations of the risk of our planet being hit by one. But it does highlight how little is known about the contents of our solar system.

"I think the ISO results are significant in that they show, once again, just how little we still know about the population of asteroids in our cosmic environment," said Benny Peiser, a researcher at Liverpool John Moores University who tracks asteroid studies.

Here are the firm facts:

As of March 28, exactly 39,462 asteroids had been visually detected and officially catalogued. More have been spotted but not yet put in the books. Of these, about 500 large asteroids have been found nearer to Earth. These so-called Near Earth Asteroids (NEAs) are watched more closely, since over time gravitational tugs from the planets and other factors could change the asteroid's courses enough to send them toward us.

Experts figure there are probably somewhere between 900 and 1,200 total Near Earth Asteroids larger than 1 kilometer (0.6 miles).

"We are still far away from a firm knowledge about the real number of and risks from the asteroids of the solar system," Peiser said.

The new survey, called the ISO Deep Asteroid Search (IDAS), detected heat given off by asteroids, rather than optical light.

Because it is impossible to actually count all asteroids with present technology, the astronomers chose selected regions of the asteroid belt and then used a theoretical model to extrapolate the data to the whole belt. The observations were actually done in 1996 and 1997. Only recently, however, was the data processed.

The infrared survey is better able to spot dark asteroids that might be missed by optical telescopes, said the ISO researchers.

Yet Tedesco said both visible and infrared searches might have their own biases, which is the reason why the new study's estimate was given as a range.

Scientists generally agree that the chance of a large NEA hitting Earth is about 1-in-5,000 over the next century.

The new results do not change these odds, researchers agree.

But Tedesco and Desert said that improved knowledge of the number and size distribution of asteroids in the main belt is essential to understand the population of NEAs, since most are believed to be former main belt asteroids. As understanding improved, odds could change, they indicated.