How often do asteroids strike the Earth_part two?

Trees uprooted by the 1908 asteroid impact at Tunguska. Courtesy NASA and the Leonid Kulik expedition

Trees uprooted by the 1908 asteroid impact at Tunguska. Courtesy NASA and the Leonid Kulik expedition

In part one of this post we looked at a few recent small asteroid impacts on Earth such as one over an Indonesian town in October 2009 by a 5 to 10-metre asteroid and one over the Nubian Desert in Sudan a year earlier by an asteroid with a width of about 4-metre.

The famous Tunguska impact of 1908 was somewhat larger than these. On the morning of 30 June that year an asteroid of about 40-metre width entered the Earth’s atmosphere near the Tunguska River in Siberia. Like in the February 2013 event above the town of Chelyabinsk the rock exploded causing fast shock or pressure waves on the ground. The locals though it was the act of an avenging god and there was no scientific investigation for 19 years. In 1927 Leonid Kulik of the St Petersburg museum reached the area on his second attempt with an expedition. Kulik and his team found a large area with uprooted trees lying radially away from the spot below the explosion. It has been estimated that 80 million trees had been uprooted by the impact.

The last impact that we know of that had serious world-wide consequences was the one over the Yucatán Peninsula of Mexico 65 million years ago. That impact by an asteroid of at least 10-km width led to forest fires all over the globe. This was likely followed by a period of cold temperatures due to dust and smoke blocking sunlight and then high temperatures due to the release of carbon dioxide into the atmosphere. These effects were devastating to the dominant animal species on the planet at the time, the dinosaurs, but they provided an opportunity for other animals such as the mammals to become widespread and to evolve into the variety of species that we know today such as dogs, elephants and humans.

By studying past impacts scientists hope to answer the question of how often they take place. Obviously and fortunately, impacts by smaller asteroids are more common than by larger ones. Hence scientists are trying to establish the rate of impacts or the average time between impacts by asteroids of different sizes. It turns out to be a difficult question to answer with a range of answers coming from different studies.

There are a number of ways of approaching the question. One is to look at the 1400 or so Potentially Hazardous Asteroids that have been found so far. From their brightness, which is observable, their size can be estimated and from the size their mass. Further estimates and calculations then are made to derive the energy with which they hit the atmosphere and what fraction of the energy actually reaches the ground. According to a 2011 paper by John Tonry of the University of Hawaii in the Publications of the Astronomical Society of the Pacific these kind of calculations suggest that the rate of impacts by asteroids 140-metres or larger is one per 20,000 years, for asteroids of Tunguska size one per 1000 years and the rate of 10-metre asteroids is one per decade.

David Asher of Armagh Observatory and colleagues take another approach in a paper published in The Observatory. In their calculations they take into account events like the predicted close pass of the 250-metre wide asteroid Apophis on the morning of 14 April 2029. They also look at the number of meteorites from the Moon found on Earth as these were ejected by impacts on the Moon by Tungaska-sized or larger asteroids. Their results suggest an impact rate of asteroids of the size of Apophis or larger as one every 3000 years while a Tunguska-sized impact could occur every 300 years or less.

These figures, together with recent events, suggest that governments do have to take the possibility of asteroid impacts seriously. In the United States NASA and other organisations do an excellent job in searching for PHAs and most of the larger, and hence more dangerous, ones have been found. There is also a search program at Siding Spring Observatory in Australia, but sadly it only receives small and sporadic funding.

For individuals though, the threat of being hurt in an asteroid impact is very tiny and much less than the threat of car accidents and other more mundane day-to-day occurrences. There maybe plenty of good reasons to look up into the sky, but looking out for an exploding asteroid is not one of them!

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