Devised by a US professor, it assigns a number to the likelihood that an asteroid will collide with the Earth. A zero means you can go back to sleep; a 10 will very definitely ruin your day.

Professor Richard Binzel explains how the Torino scale works

Richard Binzel, a professor of Earth, Atmospheric and Planetary Sciences at the Massachusetts Institute of Technology, has worked on the idea for the last five years. He created the scale to help scientists, the media and the public assess the potential danger of asteroids and comets which scientists sometimes refer to as near-Earth objects (NEOs).

The Torino Impact Hazard Scale, to give it its full title, carries the name of the Italian city in which it was adopted at a workshop of the International Astronomical Union (IAU) in June.

Unnecessary alarm

Binzel hopes the scale will end the sensationalism that currently seems to surround the reporting of NEOs.

"Scientists haven't done a very good job of communicating to the public the relative danger of collision with an asteroid," he said. "Scientist-astronomers who are going to be confronted with this should have some means of clearly communicating about it so as to clearly inform but not confuse or unnecessarily alarm the public."

Many believe an asteroid wiped out the dinosaurs

The IAU announced on Thursday that it had officially endorsed the use of the scale.

"What I find especially important about the Torino impact scale is that it comes in time to meet future needs as the rate of discoveries of near-Earth objects continues to increase," said Hans Rickman, IAU assistant general secretary.

"The Torino scale is a major advance in our ability to explain the hazard posed by a particular NEO," said Carl Pilcher, science director for solar system exploration in the Nasa Office of Space Science in Washington, D.C. "If we ever find an object with a greater value than one, the scale will be an effective way to communicate the resulting risk."

The Torino scale works on different levels of complexity for scientists and the public. It takes into account the speed and size of the NEO and its trajectory compared with that of the Earth. NEOs will be given not only a number but a colour coding as well.

Global catastrophe

A zero, in the white zone, means that the object has virtually no chance of colliding with the Earth or that the object is so small it would disintegrate into harmless bits if it passed through the Earth's atmosphere. A red 10 means that the object will definitely hit the Earth and have the capability to cause a "global climatic catastrophe."

An asteroid bigger than a mile across might hit Earth once every 100,000 to one million years

Close encounters in the green, yellow and orange zones with "scores" from one to seven are categorised as "events meriting careful monitoring" to "threatening events." Certain collisions fall in the red zone, with values of eight, nine or 10, depending on whether the impact energy is large enough to be capable of causing local, regional or global devastation.

No asteroid identified to date has ever made it out of the green zone by having a scale value greater than one.

Space-borne fragments the size of sand constantly bombard the Earth. Objects the size of a small car will hit the planet a few times a year. Scientists calculate that an asteroid bigger than a mile across might hit once every 100,000 to one million years. Such an event would now carry a red 10 on the Torino scale.