Comet Swift–Tuttle reaches 51.225 astronomical units from the Sun at aphelion.
Aphelion is the farthest point from the Sun in an object’s orbit. One astronomical unit, or AU, is the average distance between Earth and the Sun, so Swift–Tuttle travels more than fifty Earth–Sun distances from the Sun at its orbital extreme.
The comet’s perihelion distance is much smaller, listed as 0.9595 AU. This highly elongated orbit carries Swift–Tuttle from the outer Solar System into the region near Earth’s orbit and back again. Its eccentricity, listed as 0.9632, reflects that pronounced elongation.
Earth encounters the comet’s dust stream in August even though Swift–Tuttle spends most of its orbital path far away. Dust released during earlier returns remains distributed along the orbit and intersects Earth’s path at the location where the Perseid stream is active.