Comet Swift-Tuttle, also known as Comet 109P, is a pretty old
comet. It is the largest object known to make repeated passes near the
Earth. It is also one of the oldest known "periodic comets" with sightings spanning two
millennia. The best estimate of when it will pass by the sun is July 11, 2126. It was originally forecast that it COULD possibly strike the
Earth on August 14, 2126. Scientists figured it had a 1 in 10,000 chance of hitting the Earth and wiping out all of mankind. It's also responsible for the
Perseid meteor shower.
Discovery
Lewis Swift (Marathon,
New York) discovered this comet in
Camelopardalis (a
constellation) on July 16, 1862 while examining the northern sky with his 11.4-cm Fitz refractor. He described the comet as a somewhat bright telescopic object, but did not report it since he thought he was observing the
comet Schmidt had found two weeks earlier. Without knowledge of Swift's observation, Horace Parnell Tuttle (
Harvard College Observatory,
Cambridge,
Massachusetts) independently discovered this comet on July 19 and noted it was heading northward. Tuttle then made an official announcement. When Swift heard of Tuttle's find, he immediately realized the comet seen on July 16 was not Schmidt's and made his announcement to get credit for his first comet discovery. As a result, they share the name of the comet.
When it was seen in 1862, its parabolic orbit was calculated to have a period of about 120 years. In 1982, astronomers waited for it but couldn't see it anywhere, leading them to conclude that it had disintegrated. As the comet approaches the sun, pieces of it begin to break off, leaving a trail of debris behind it in its trail. As the earth orbits through it, the pieces burn up in the Earth's atmosphere, causing a meteor shower. Since this comet was responsible for one of the biggest showers, it was assumed that it had completely broken up.
Comet Swift-Tuttle was rediscovered in September 1992, almost 10 years away from its expected position. The reason for this large discrepancy wasn't due to inaccuracy or miscalculation, but due to the fact that when a comet passes near to the Sun, some of its matter is evaporated by the Sun's radiation. This evaporation does not necessarily happen symmetrically, but the imbalance can distort its orbit in an unpredictable way. Even taking that into account, it was thought impossible to predict the orbit due to the evaporation.
From revised calculations made using the observations made in 1992 and those of 1862, it has been shown that the comet is identical with Comet Kegler observed in 1737. The comet was last seen on 1995 March 29.48, by observers at Siding Spring Observatory (Australia).
Collision course?
Using the best availible calculations of the comet's
orbit, the best estimate for the time of closest passage to the
Sun during its next approach was 2126 July 11. An error of +15 days would mean that the comet could collide with the Earth on August 14 of that year. Scientists initially feared that it might hit the Earth. Even though the odds were slim, the Earth does pass through the orbit of the comet. If the comet adjusted its speed by about 15 days, according to the calculations of the time, it could collide with the
planet, wrecking
havoc. Relative to the Earth's speed, the comet is traveling at 60 kilometers per second. Going that fast, there is a window of only a few minutes for both paths to cross, and the comet must be at a specific spot in its
trajectory for it to be a possibility.
Later observations have allowed a better orbit for the comet to be determined, and it is now clear that it will not hit the Earth. It will still pass by relatively close, making an impressive sky show similiar to comet Hyakutake or the comet Hale-Bopp. The revised orbit has allowed the identification of comet Swift-Tuttle with observations going back 2,000 years.
The comet has a diameter of 10 kilometers, and if it did hit the Earth going 60km/sec, it would be catastrophic. The collision would be 1 Billion times as powerful as the atomic bomb dropped on Hiroshima. Such an impact is theorized to have caused the dinosaurs to become extinct. An impact would hypothetically cause a cloud of dust to block out the sun, killing all plants and causing an ice age.
What are the chances it will hit? It's difficult to estimate. If we take the known velocity of the comet relative to the Earth, then we can work out that the comet will only collide with the Earth if it is within a three and a half minute time-slot in its orbit. This means, for instance, that an error of only one hour in the timing of the comet will result in it missing the Earth by about 100,000 kilometers. Considering that the last calculations were off by 10 years, you can see how unpredictable it is.
In mid-1993 it was 3 AU away, and in 1998 it was 15 AU away
A list of its sightings is availible at http://www.aerith.net/comet/catalog/0109P/
Picture availible at http://antwrp.gsfc.nasa.gov/apod/ap960219.html and at http://star.pst.qub.ac.uk/~af/swift.html
Sources
http://cometography.com/pcomets/109p.html
http://antwrp.gsfc.nasa.gov/apod/ap960219.html
http://www.oarval.org/section3_16.htm