As you may know, we are right in the heart of the Perseid meteor shower. You can all thank Comet Swift-Tuttle for this beautiful annual spectacle! This comet, that is 26 km in diameter, has been observed for many thousands of years by ancient as well as modern astronomers, and has a fairly stable and well understood orbit. It was rediscovered in 1992, when it last made its closest approach to the Sun, at which point many thought it might be an impact risk to the Earth during its next approach on August 14th 2126. Its orbit has since been recalculated, and chances of an impact occurring are thought to be extremely low.
However, it is the largest Solar System object that makes repeated close passes of Earth. With its size and velocity, a collision with Swift-Tuttle would have approximately 27 times the energy of the K-T event impactor, most commonly known as the cause of the extinction of the dinosaurs. It is for this reason that many astronomers consider Comet Swift-Tuttle to be the single most dangerous object known to humanity. Marvel at the awesome power of the Universe! For more factoids on Swift-Tuttle and the Perseids, visit Space Magazine.