Throughout the ages, comets were considered to be bad omens. They always appeared mysteriously out of nowhere and then, within days or weeks, they vanished. Because of this unpredictable behaviour, they were associated with impending disasters.
However, after Newton developed his law of universal gravitation, the mystery was solved. He realized that comets were orbiting the Sun, just like the Earth and other planets, but in orbits that were highly elongated. As a result, comets spent most of the time in the outer Solar System, too far from the Sun to be visible.
Newton's colleague Edmond Halley followed up on this idea. He investigated the historical records of comet sightings and concluded that the comets of 1531, 1607 and 1682 might all be the same one. If his explanation was correct, the comet should reappear in 1758. And that is exactly what it did. Since then, it has been known as Halley's Comet, in honour of the scientist who predicted its return, but who did not live to see it. Halley died in 1742, at the age of 85.
After it was recognized that Halley's Comet appeared at regular intervals of about 76 years, investigations revealed records of its close approaches to the Sun dating back to 240 BC. The most recent appearance of Halley's Comet was in 1986 and the next will be in 2061.
With twentieth century technology, astronomers learned a lot more about comets in general, and about Halley's in particular.
From spectroscopy, a technique for studying chemical composition, we now know that a comet has a nucleus that is made of ice and frozen gases such as carbon dioxide, carbon monoxide, methane and ammonia. As the comet approaches the Sun, it warms up, causing its outer layers to change from a solid to a gas, and stream out into a tail that can be millions of kilometres in length. It is the tail that makes a bright comet such a spectacular sight.
Before the space age, astronomers didn't really know what a comet nucleus looked like. However, they knew that Halley's Comet would return in 1986, so they had time to plan for a spacecraft to observe it at close range: from a distance of 600 km. Many scientists were expectng to find a circular nucleus. It turned out to be peanut shaped, with dimensions of approximately 16 by 8 kilometres. And it was darker than coal, suggesting that it was covered with a thick layer of dust. There were many risks associated with this close approach to the comet, but in the end, the mission was highly successful.
There have been many successful missions to the Moon and planets, as well. Next time we will discuss what we have learned from some of these.
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