Deep impact? New scale will evaluate impact threats.
28 September 1999In recent Hollywood blockbusters, asteroids and comets have threatened to collide with the Earth, only to be destroyed at the last minute by astronaut heroics. But such collisions are more than a topic for fiction - the threat from near-Earth objects (NEOs) is remote, but real.
Thousands of these kilometre-sized objects are known to exist, and many more are being discovered every year. Inevitably, some of these will be found in Earth-grazing orbits. Last year, media hysteria spread around the world when astronomers suggested that asteroid 1997 XF11 might strike the Earth in 2028. Further embarrassment followed when astronomers announced - and then quickly retracted - a prediction that near-Earth asteroid 1999 AN10 was a potential impactor.
In an effort to improve the way in which the odds of such collisions are communicated, Richard P. Binzel, professor of Earth, Atmospheric and Planetary Sciences at Massachusetts Institute of Technology, has produced a scale of colours and numbers. It is named the Torino Impact Hazard Scale after the Italian city in which it was adopted during a June workshop co-sponsored by a number of international organisations, including ESA and NASA. The new scale was officially presented on July 22, at the third United Nations conference on the exploration and peaceful uses of outer space (UNISPACE III) in Vienna, Austria and it was welcomed by IAU and the scientific community.
Binzel's risk-assessment system is similar to the Richter scale used for earthquakes. "If you tell a Californian that an earthquake registering one on the Richter scale was going to hit tomorrow, he would say, 'So what?'" Binzel said. "If you were talking about a six, that would be different."
Binzel hopes that his asteroid hazard prediction scale will be regarded in the same way. While nobody should lose sleep over an asteroid in the zero or one categories, which accounts for the vast majority of them, an asteroid registering six or above should generate considerable concern.
"This is a case of a high-consequence but low-probability event. It's difficult in human nature to figure out what level of anxiety we should assign to an approaching asteroid," said Binzel.
"Scientists haven't done a very good job of communicating to the public the relative danger of collision with an asteroid," he added. "What I hope the scale will accomplish is to put in perspective whether an object merits concern."
The scale has been well received by others in the astronomy community:
"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 step in making the complex problem of the potential hazard posed by a particular NEO accessible to the general public, said Gerhard Schwehm of ESA's Space Science Department. It will hopefully help us to get the message across, that although these are very unlikely events, the search programmes and monitoring programmes are vital and need to be supported".
"Naming the newly proposed hazard scale after Torino is a highly appreciated recognition of the Torino Astronomical Observatory's great deal of work over the past two decades," said Alberto Cellino, an astronomer at the Torino Observatory.
Colours and numbers
The Torino scale is colour-coded. Each colour is given a general description, with white representing the lowest risk and red as the highest risk category. Within these divisions, numbers from 0 to 10 are used to give a clearer indication of the potential risk.
Zero or one means virtually no chance of impact or damage; 10 means that a collision is certain, with global catastrophe as the inevitable result. Zero may also be assigned to an object which is too small to survive passage through the Earth's atmosphere, even if a collision does occur. The full scale is listed as an appendix below.
Although 1997 XF11 and 1999 AN10 would have (briefly) been designated as category 1, no known asteroid is currently assigned to a class higher than 0. This is expected to be the normal state of affairs. However, it is also likely that initial uncertainties in the calculation of an orbit of a newly discovered asteroid may temporarily place it in a higher category. This is not a cause for immediate concern, but merely signals the need for more accurate observations, leading to a better determination of the orbit.
Asteroids are small bodies without atmospheres that orbit the Sun. Since they are too small to be classified as planets, they are often called "minor planets". Tens of thousands of these are known to exist in the main asteroid belt. This vast, doughnut-shaped ring lies between the orbits of Mars and Jupiter, 300 - 500 million kilometres from the Sun.
Asteroids are thought to be made from primordial material left over from the creation of the planets 4.6 billion years ago. Scientists believe that Jupiter's strong gravity prevented them from coming together to form a single body. The estimated total mass of all asteroids would make an object about 1490 km in diameter - less than half the size of the Moon.
Outside the Main Asteroid Belt, it has been estimated that some 2,000 asteroids and 'dead' comets larger than about 1 km in diameter follow orbits which bring them close to Earth. Only 15 to 20 percent of these larger near-Earth objects have been detected to date. Several telescopic discovery programmes are actively searching for more, and the discovery rate is increasing. However, it may be at least 10 years before 90 percent of the total population is revealed.
While more asteroids than ever are being identified in the cosmic shooting gallery inhabited by our planet, Binzel points out that there is no increase in the number of asteroids out there - only in our awareness of them. "This doesn't mean that the Earth is in any greater danger," he said. "Fortunately, the odds favour that newly discovered objects will miss."
On the other hand, space-borne objects do hit the Earth. Tiny fragments the size of sand grains bombard us constantly, and objects the size of a small car reach the surface a few times each year. An asteroid bigger than one and a half kilometres across might hit once every 100,000 to 1 million years. Our planet bears scars from these encounters, such as the famous Meteor Crater in the Arizona desert.
Even larger impacts occur over extremely long time scales. Most scientists believe that the dinosaurs were wiped out by a massive object 65 million years ago. As the recent collision of Comet Shoemaker-Levy 9 with Jupiter demonstrated, impacts still occur in the Solar System today. But there is no need to panic. As Binzel says, "No one has clearly documented deaths from a meteorite impact."
On the other hand, predictions can also be fraught with danger. Once a near-Earth asteroid is detected, scientists try to calculate where it will be in 10, 15 or 100 years, based on the limited sightings available. There is inevitably some uncertainty in such early predictions because the orbit measurements are not perfect and the NEO's path may be altered by gravity if it passes close to the Earth or another planet. As more information is gathered about a particular asteroid, its position on the Torino Scale may need to be adjusted accordingly.