1999

Torino Impact Hazard Scale

While evidence for all but a few hundred terrestrial impact craters has been erased by our planet’s dynamic geology and hydrology, we can tell just by looking at the ancient, heavily scarred surface of our planetary neighbor—the Moon—that significant numbers of asteroids and comets have hit Earth in the past. These high-speed impact events released huge amounts of energy, and geologic evidence and the fossil record suggest that they occasionally significantly altered the planet’s climate and biosphere.

The impact rate has decreased exponentially with time over Earth’s history, but even in modern times that rate is not zero—consider, for example, the 1908 explosion of a comet or asteroid in the atmosphere above Siberia (the Tunguska event) and the observation of several large atmospheric fireball explosions every year by military and civilian planetary monitoring satellites.

Fueled by public and political interest in understanding the risks associated with cosmic impacts, the rate of discovery of small asteroids and comets, especially within the population known as near-earth objects (NEOs), has increased over the past few decades. Dedicated telescopic surveys have identified more than half a million main belt asteroids and nearly 1,000 NEOs. The few hundred NEOs that could potentially cause a threat to life on our planet get a special acronym: PHAs, for potentially hazardous asteroids.

As the rate of PHA discovery increased, it became clear that there was no systematic or simple way to understand and communicate the risk of PHA impacts; indeed, there was much potential for confusion or even unfounded panic about this issue. So, in 1999, a group of planetary astronomers developed an index called the Torino Impact Hazard Scale to quantify the risks. Torino values for newly discovered PHAs range from 0 (no chance of impact) to 10 (certain impact with likely catastrophic consequences).

Most PHAs have Torino values of 0. About a dozen have had nonzero values (most were downgraded to 0 with follow-on observations), with the record so far being an original value of 4 (a 1 percent or greater chance of collision) for the asteroid 99942 Apophis, which will pass very close to the Earth on April 13, 2029. The Apophis risk has since been downgraded to a 0, but astronomers still monitor it carefully.

A dark, cratered, near-Earth asteroid—perhaps like 99942 Apophis—approaches Earth in this painting by planetary scientist and artist William K. Hartmann.