13-10-2007 Cosmic wild cards (Article 107)

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Simon Kenny
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13-10-2007 Cosmic wild cards (Article 107)

Post by Simon Kenny » Wed Feb 04, 2009 11:29 am

Cosmic wild cards (Article 107)

Long dismissed by some astronomers as the ‘vermin of the skies’, asteroids and comets have now become a hot topic for two reasons. Firstly, there is growing evidence that asteroid and comet collisions with Earth in the ancient past caused dramatic changes in the evolution of life on the planet. Secondly, the recent discovery of Eris, an ‘asteroid’ beyond Pluto and larger than Pluto itself, has forced astronomers to recently redefine the meaning of ‘planet’, ‘asteroid’ (or ‘planetoid’) and create a new category called ‘dwarf planet’.

The first asteroid was discovered on New Year’s Day, 1801, by Giuseppe Piazzi. Contemporary astronomers had predicted a planet in the large void between Mars and Jupiter. However, Ceres, about 1000 km in diameter, was considered too small to be a planet and within a few years, other smaller bodies were found in the same orbit around the Sun. Discoveries of new asteroids continued, slowly at first due to cumbersome early techniques. The advent of astrophotography and more recently automated search techniques have dramatically increased the number of confirmed discoveries. The total number of objects in the Asteroid Belt stands at 365,000 and increasing at about 5,000 per month. The estimated total is expected to reach well over a million. The first one to be discovered is still the largest, Ceres. By far the largest of its group, Ceres contains one third of the total material in the Asteroid Belt and the only one with enough gravity to be spherical in shape. In the recent reclassification by the International Astronomical Union, Ceres was ‘promoted’ from ‘asteroid’ to ‘dwarf planet’, a classification it now shares with Pluto and Eris.

Eris is the latest significant discovery in a region far beyond Pluto, which is receiving much attention in recent years. In 1950, Dutch astronomer Jan Oort proposed the existence of a vast reservoir of icy rocks of various sizes circling the dark frozen void on the outskirts of the Solar System. The Oort Cloud is now generally accepted to exist, and that it also completely dwarfs the Asteroid Belt in size and numbers of objects. While the asteroids circle the Sun doughnut-like along the ecliptic plane, the icy denizens of the Oort Cloud surround the Solar system like a vast sphere. It has long been suspected that the comets that occasionally glorify our skies, and on very rare occasions collide with the Earth, come from this source. As the tails of comets show, they are composed of volatile icy materials that are solid and stable in the deep freeze of their original home, but rapidly boil off the comet into space as it approaches the Sun’s heat, creating the comets distinctive tail. In time, even the largest comets will be evaporated by successive passes around the Sun. This point originally led to Oort’s hypothesis that a continuous stream of fresh comets is being supplied from the region we now know as the Oort Cloud. Clear skies!

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