Friday, March 21, 2014

Halley's Comet

An image of Halley's Comet
captured during the 1986 sighting.
Since before the beginning of recorded history, humans have been awed by the sight of a bright astronomical object streaking across the sky with a long tail extending from it, Halley’s Comet. These sightings occurred roughly every 75 years, but not until the Enlightenment was it realized that these sightings occurred on such a cycle and that the object seen was the same object each time. Written accounts of sightings of the comet can be traced back to 239 B.C. when Chinese astronomers recorded seeing it pass through the sky. It was noted in other ancient records and throughout the Middle Ages. People often saw the comet’s appearance as an omen of coming disaster or change. For example, Halley’s comet appeared around the time of William the Conquerer’s invasion of England in 1066.

The Bayeux Tapestry depicts William
the Conquerer's invasion of England
in 1066. Halley's Comet can be seen
in the top center of the photo.
During the Renaissance Copernicus published the idea that the solar system revolved around the sun, not the earth. When English Astronomer, Edmond Halley saw the comet himself in 1682, he used the idea that astronomical objects orbit the Sun as well as his friend, Isaac Newton’s newly published laws of gravity to study the comet’s behavior. He used records of comet sightings across several centuries and the fact that the orbits were very similar to conclude that these sightings were all the same “periodic” comet. Up until this point, no one realized that these sightings were linked. 

Halley’s Comet is a short-period comet, meaning it has an orbital period less than 200 years. Halley’s Comet is the only of these short period comets that can be seen easily from Earth with the naked eye, making it the most well known. It is suggested that Halley and other short-period comets were once long-period comets (more common types of comets that have orbital periods of thousands of years) that had their orbits perturbed by the gravity of the giant planets, which sent them flying in toward the inner planets. If this is the case then Halley’s comet probably originated from the Oort Cloud, a field of icy rocks that extends up to a light year from the sun. It is estimated that Halley has been in its current orbit for tens or even hundreds of thousands of years.

When Halley’s Comet came near Earth in 1986, humans finally had the space technology to observe it up close. The European Space Agency and NASA as well as Japan sent spacecraft to capture images of it. These missions allowed scientists the opportunity to determine that Halley contains compounds like carbon monoxide and carbon dioxide which, when the comet approaches the sun, heat to give the comet an atmosphere, or coma. The solar wind blows this coma away from the comet, creating the characteristic tail. The debris left behind in Halley’s path is responsible for the annual Orionid meteor shower in October when the Earth passes through these remnants. Halley’s Comet is predicted to return in 2061. Who knows what new things we will learn about it with the technology that will be available half a century from now?
Clare Isaacson