Smaller asteroid impacts, such as the famed Tunguska event that devastated Siberia early last century, are less likely than previously thought, Canadian and American astronomers have found.

Using data collected from military satellites since 1994, Associate Professor Peter Brown of the University of Western Ontario in Canada and colleagues in the United States have calculated that the likelihood of impacts by smaller asteroids that cross the Earth's orbit is about once every 1,000 years - not every 200 years as previously estimated.

The findings have been published in this week's issue of the journal Nature.

Meteors that we see flashing through the night sky are cosmic dust and rock debris hitting the Earth's atmosphere. Those between 50 and 100 metres in diameter don't usually reach the ground in one piece, mostly exploding when they strike the upper atmosphere. It is these objects the group were studying: space rocks (or 'bolides') between one and 100 metres across.

Although too small to be seen by ground-based telescopes, the explosions they cause - giving off light and radiation - can be detected by satellites originally developed to spot nuclear explosions on the ground.

The researchers reviewed satellite records collected by the American departments of Defence and Energy. From the intensity of the air bursts produced by the meteors detected over the past 8 and a half years, the group calculated the size of each of the meteors. Only 300 such airbursts were detected.

"Because there are fewer of these objects than originally believed, we can all worry a little less about the risk of the next hazardous impact," writes Dr Robert Jedicke from the University of Arizona in Tucson in an accompanying commentary.

Although bolides with diameters smaller than 50 to 100 metres mostly break up in the atmosphere rather than hitting the ground as a single body, they can still cause considerable damage.

The bolide that exploded above Tunguska, in the Russian Far East, in June 1908 was no more than 50 metres in diameter - yet was still large enough to flatten 2,000 square kilometres of the Siberian forest. Scientists estimate the Tunguska event had an energy equivalent of 10 megatons (10 million tons of TNT explosive) - greater than the 20-kiloton atomic bomb dropped on Hiroshima in World War II.

The group has calculated that bolides with this energy potential - approximately 10 megatons - are likely to strike the Earth about once every 1,000 years or so. This is five times longer than was feared only 10 to 20 years ago.

The new data fits well with previous studies of asteroids that are much larger - from one kilometre in diameter on up (asteroids of this size have been easier to study because they are visible to ground-based telescopes). There are an estimated 1,000 of these killer asteroids that cross the Earth's orbit; scientists estimate that these strike the Earth an average of once every million years.

Source of the given news and the copyrights belong to a ABC Online News

Back