The comet's perihelion is just under that of
Earth, while its aphelion is just over that of
Pluto. An unusual aspect of its orbit is that it was recently captured into a 1:11
orbital resonance with
Jupiter; it completes one orbit for every 11 of Jupiter. It was the first comet in a
retrograde orbit to be found in a resonance. In principle this would mean that its
proper long-term average period would be 130.48 years, as it
librates about the resonance. Over the short term, between
epochs 1737 and 2126 the orbital period varies between 128 and 136 years. However, it only entered this resonance about 1000 years ago, and will probably exit the resonance in several thousand years.
Threat to Earth The comet is on an
orbit that makes repeated close approaches to the
Earth–
Moon system, and has an Earth-MOID (
Minimum orbit intersection distance) of . Upon its September 1992 rediscovery, the comet's date of
perihelion passage was off from the 1973 prediction by 17 days. Another close encounter is predicted for the year 4479, around Sept. 15; the close approach is estimated to be less than 0.05
AU, with a probability of impact of 1 in a million. Subsequent to 4479, the orbital evolution of the comet is more difficult to predict; the probability of Earth impact per orbit is estimated as 2 (0.000002%). Comet Swift–Tuttle is by far the largest
near-Earth object (
Apollo or
Aten asteroid or
short-period comet) to cross Earth's orbit and make repeated close approaches to Earth. With a relative velocity of 60 km/s, an Earth impact would have an estimated energy of ~27 times that of the
Cretaceous–Paleogene impactor. The comet has been described as "the single most dangerous object known to humanity". In 1996, the long-term possibility of Comet Swift–Tuttle impacting Earth was compared to
433 Eros and about 3000 other kilometer-sized objects of concern. == See also ==