Asteroid Ahoy

Travelling at 38,000 miles an hour, an alien straggler is on 

collision course and quite literally threatens to create waves, reports Biplab Das 

Sixty-five million years ago an asteroid wiped out the mighty dinosaurs and a host of other animals. Now another alien straggler is threatening to do an encore. Scientists at the University of California, Santa Cruz speculate that an asteroid called 1950 DA will slam into earth's atmosphere on 16 March 2880. They say this huge rock, two-thirds of a mile in diameter, will crash into the Atlantic Ocean at 38,000 miles per hour, creating waves as high as 400 feet that would sweep onto the Atlantic coast of the USA. 

Steven Ward, researcher at the Institute of Geophysics and Planetary Physics at UCSC, and Erik Asphaug, an associate professor of Earth Sciences, also at UCSC, paint such an apocalyptic picture in the June issue of Geophysics Journal International. 

In the past, asteroids of this size and larger have periodically hammered the earth with devastating effect. "From a geologic perspective, asteroids the size of 1950 DA have probably struck the earth about 600 times since the age of the dinosaurs," Ward says. He and his colleagues rationally assess the asteroid impact hazards. Asphaug, who organised a Nasa sponsored scientific workshop on asteroids last year, notes that asteroids risks are interesting because the probabilities are so small while the potential consequences are enormous. 

Moreover, the physical laws that govern orbital motions help scientists to detect an asteroid in advance. "It is like knowing the exact time when Mount Shasta will erupt," Ashpaug says. Although the chances of a direct hit from 1950 DA is only about three-tenth of a per cent, scientists cannot neglect the threat factor. Last year, a research team from Nasa's Jet Propulsion Laboratory wrote about the probability of 1950 DA being on a collision course with earth in a paper published in Science. 

It is a low threat, but scientists have generated a computer-aided simulation of the impact that the asteroid will have on earth. This simulation reveals that the asteroid splashes down in the Atlantic Ocean 360 miles off the US coast. With the impact of a 60,000-megaton blast, it creates a cavity measuring 11 miles across. At the point of impact, the cavity is three miles deep. The blast even digs out the sea floor. As a result, water rushes in to fill the cavity, and a ring of waves spreads out in all directions. The impact builds tsunami waves of various frequencies and wavelengths. The longest wavelength equals the diameter of the cavity. The low frequency waves have longer wavelengths. Such waves travel fast. The high frequency waves have smaller wavelengths. Such waves travel slowly. 

The simulation begins with a big wave. "The big one is followed by dozens of waves," Ward says. "The first ones that arrive are pretty small, and they gradually gain in height, arriving at intervals of three or four minutes." As they travel, the waves weaken. The coastal areas receive the heavy pounding from the largest waves. Four hours after the impact, the entire east coast of America will have experienced waves at least 200 feet high, Ward says. After eight hours, 30 to 50 feet-high waves will reach Europe. 

Weathering and other geological activities have buried many craters and other signs of asteroid impact on earth. "Computer simulations not only give scientists a better handle on the potential hazards of asteroid impacts, they can also help researchers interpret the geologic evidence of past events," Ward says. Simulation helped him calculate the speed of the water flows created by tsunami at the bottom of the ocean. He and his colleagues found that water could travel at the speed of three feet per second, reaching as far as several hundred miles from the impact site. 

According to Ward, the waves may destroy undersea slopes, causing massive landslides. Such landslides could trigger secondary tsunamis. "Tsunami travels fast, but the ocean is very big, so even if a small or moderate-sized asteroid comes out of nowhere you could have several hours of advance warning before the tsunamis strike land," he says. "We have a pretty good handle on the size of the waves that would be generated if we can estimate the size of the asteroid." 

Right now, a group of planetary scientists are busy gauging the risks of asteroid impacts. A Nasa-led team keeping an eye on large asteroids in near-earth orbits will detect 90 per cent of the asteroids (larger than one kilometre) by 2008. "Until we detect all the big ones and can predict their orbits, we could be struck without warning." Asphaug says. "With the ongoing research, we will probably be able to sound the all clear by 2030 for 90 per cent of the potential impacts that could trigger a global catastrophe." Despite all these precautions, some asteroids may remain undetected. "Those are risks we may just have to live with," Asphaug concludes.