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Royal Greenwich Observatory
Information Leaflet No. 56(rev): 'Comet Swift-Tuttle'.
Periodic Comet Swift-Tuttle (1992t):
Periodic comet Swift-Tuttle was last seen in 1862. Its orbit was then
calculated to have a period of about 120 years. It was predicted to return
in 1982 but was not observed. Because of this, there was speculation that
the comet had disintegrated. This theory was supported by its association
with the Perseid meteor stream (see the Leaflets
Meteors and Meteorites and
Meteor streams like this are believed to be formed of small parts that have become detached from the main body of the comet during one of its close approaches to the Sun. These small pieces become scattered along the length of the comet's orbit around the Sun. If the Earth passes through the resultant stream of particles then we see them as meteors, or 'shooting stars', as they burn up in the atmosphere.
Comet Swift-Tuttle was rediscovered in September 1992, almost 10 years away
from its expected position. The reason for this large discrepancy is not
due to inaccuracies in the old observations, or in the calculations based
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 can act rather like a variable rocket motor attached to the comet, causing non-symmetrical forces that distort its orbit in an unpredictable way.
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.
Initially it had proved impossible to obtain a definitive orbit that satisfied all the observations, even including terms to allow for the effects of evaporation.
Using the best orbit then available, the comet's position at any time in the future could be predicted. 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 in this date would mean that the comet could collide with the Earth on 2126 August 14.
The chance of a collision would be very small even though the Earth does pass through the orbit of the comet. The comet will be moving at a speed, relative to the Earth, of 60 kilometres per second. This means that there is only a time of a few minutes during which a collision is possible. The comet will have to lie in this very small time interval in its orbit for a collision to occur.
The latest observations have allowed a better orbit for the comet to be determined, and it is now clear that it will not hit the Earth. The revised orbit has allowed the identification of Swift-Tuttle with observations going back 2,000 years.
Although we are confident that the comet will pass by the Earth, it will still be prudent to attempt to follow Swift-Tuttle for as long as possible after the present perihelion passage, so that an accurate orbit determination, uncontaminated by non-gravitational effects, can be made. From mid-1993 (when the distance to the comet would be 3 AU) to, say, 1998 (when its distance will be 15 AU and it will have an apparent magnitude of 26).
The effects of a collision with the Earth of a comet like this, with a diameter of about 10 kilometres, would be devastating. Such a collision is thought by many to have been the cause of the demise of the dinosaurs. The 'explosion' resulting from the collision would be about 1,000 million times greater than the bomb that destroyed Hiroshima. A huge cloud of dust, thrown into the upper atmosphere, would envelop the globe for many years, and this would cause widespread changes in climate, even to producing an ice-age. The effects on civilisation can only be guessed at, but would be very severe.
The chances of such a collision are very 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 thousand kilometres.
If we compare that accuracy with the error of 10 years in this predicted return, we can see how difficult such predictions are!
The comet has now moved into the southern sky and is not visible from the UK.
Produced by the Information Services Department of the Royal Greenwich Observatory.
PJA Thu Nov 25 10:46:17 GMT 1993
Updated: July 27 '97, October 14 '08
Best seen with MS Internet Explorer.
See Perseid Trail (NASA/GSFC Astronomy Picture of the Day, 2008 August 14)
For some illustrative images and excellent texts, link to:
Comets in Calvin J. Hamilton's Views of the Solar System
For some details and images on a comet and its appearance, link to:
Comet 1995 O1 (Hale-Bopp) in ARVAL
Back: ARVAL - RGO Leaflets - The Solar System