On Monday, January 7, a 300-metre asteroid came whizzing very close to the Earth. Although asteroid 2001YB5, as astronomers have named it, was a false alarm, staying some 200,000 km further away than predicted, it raises important questions regarding a collision with an NEO (Near Earth Object) and why they are so hard to predict.
Problems with predicting the orbits of NEOs were highlighted in 1998, when astronomers tracked the movements of a close-by asteroid. In this case, the initial data seemed to show a high likelihood of a collision with Earth, but closer analysis thankfully annulled this conclusion. Part of the problem is the lack of communication between scientists, as Dr. Brian Marsden of the International Astronomical Union explained.
University of Western Ontario scientist Peter Brown explains that from the moment an NEO is sighted, there develops a “probability circle” which widens as the comet or asteroid zooms toward Earth. This can be partially due to a phenomenon called outgassing, where interactions with the sun’s gravity can cause ice on an NEO to evaporate quickly, which changes its course.
Brown also explains his notion of a “critical threshold”—a particular distance around the planet, outside of which a collision is unlikely, but inside of which Earth’s gravity would take over and pull an NEO in. It is due to such uncertainties that the 1998 comet, known officially as 1997 XF11, was quoted initially by the Central Bureau for Astronomical Telegrams as passing within 30, 000 miles of the Earth in 2036, with an uncertainty of plus or minus 180, 000 miles—a dangerous “probability circle” that easily encompasses the Earth.
Professor John Percy of the Astronomy Department at the University of Toronto is optimistic that in the near future it will be possible to catalogue all Near Earth Objects bigger than half a kilometre. While this may seem small, the asteroid sighted on January 7, measuring less than that, would have released the energy of over 3000 megatons of TNT on impact, comparable to unleashing a third of the world’s nuclear arsenal at once. This kind of collision, according to the Solar Systems Collisions Calculator, created by astronomer Doug Hamilton at the University of Maryland, predicts such a collision every 51,000 years, creating the probability that man’s early ancestors may have witnessed over 20 such phenomena.
Professor Percy explains that in the early solar system, collisions were much more frequent, planets literally being born from the accumulation of asteroids colliding and sticking together. However, during this process, objects that could not join up were thrust into the Keiper Belt beyond Pluto and some to the even further Oort Cloud. They remain there until the gravitational effects of a passing star send a few at a time on a potentially deadly trip towards Earth. It is even believed by many astronomers that in the early days of the solar system, minerals and complex organic material, many of which are necessary for life, may have been delivered to the Earth from such objects, like “manna from heaven.” Despite the heated debates in this arena, one thing is agreed upon: at this point in the game, scientists must be more concerned about the dangers of NEOs than other academic issues.
The Solar Systems Collision Calculator is available at
http://janus.astro.umd.edu/astro/impact.hml
