THE odds of encountering a tsunami kicked up by an asteroid strike have just plummeted. Best to hope, though, that you鈥檙e not underneath the almighty splash such an impact could create.
Small impactors hit us far more frequently than larger ones: a 200-metre asteroid hits Earth about every 10,000 years on average, while 10-kilometre objects like the one that probably killed off the dinosaurs strike every 100 million years. Much of the worry over asteroids has centred on the more likely event of a smaller one splashing down in the ocean and triggering a powerful tsunami.
Now simulations to be presented at an , this month suggest that small asteroids do not after all pose a major tsunami threat.
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of the University of Oslo, Norway, and colleagues used software originally written to simulate the effects of underwater nuclear explosions to hurl a virtual 200-metre asteroid into an ocean 5 kilometres deep. The impact initially sends waves hundreds of metres high spreading from the impact site. However, the very height of the waves makes them prone to collapse even in very deep water: they start breaking immediately, like ordinary waves on a beach.
鈥淭he impact initially sends waves hundreds of metres high radiating from the impact site鈥�
By the time they are 30 kilometres from the impact site, they have shrunk to a height of less than 60 metres. The team did not simulate the waves鈥� propagation much further, but extrapolating the shrinkage suggests heights of less than 10 metres by the time they have travelled 1000 kilometres.
That might not seem very reassuring. Tsunamis with open ocean heights of less than a metre can still be very damaging because they rise up as they come ashore and penetrate far inland, but this is related to their long wavelengths and characteristic periods of 8 minutes or more. Asteroid waves would have shorter wavelengths and periods of less than 2 minutes, says Gisler, and so far less penetrating power.
Steven Ward of the University of California, Santa Cruz, has conducted his own simulations and suspects methods in Gisler鈥檚 calculations meant to smooth away errors are accidentally damping down the waves. Ward鈥檚 own results suggest much slower wave decay. Gisler responds that his simulations are more realistic, pointing out that previous modelling leaves out fine-scale turbulent motion that helps dissipate wave energy.
A strike near a populated coastline would undoubtedly cause major damage, however. 鈥淵ou don鈥檛 want to be close to one of these things,鈥� says Gisler. 鈥淟ocal effects will include hurricane-force winds and enormous amounts of water falling directly from the sky.鈥� His simulations suggest that a 200-metre asteroid would make a splash of billions of tonnes of water, which would descend at up to 300 metres per second within about 20 kilometres of the impact site.
Brian Toon of the Universityof Colorado in Boulder says we should continue surveying for asteroids. 鈥淲e probably have quite a while before we鈥檙e going to get hit by a significantly sized [asteroid],鈥� he says. 鈥淏ut nevertheless one of these is going to come at us.鈥�
