Comet Swift–Tuttle (formally designated 109P/Swift–Tuttle)
It is the parent body of the Perseid meteor shower, perhaps the best known shower and among the most reliable in performance. [5]
An unusual aspect of its orbit is that it is presently captured into a 1:11 orbital resonance with Jupiter; it completes one orbit for every 11 of Jupiter.[6]
The ephemeris is available from NASA's Jet Propulsion Laboratory [1], although it is not listed in NASA Close Approach Tables [2] of Near Earth Objects
Potential threat to Earth:
"The comet is on an orbit which puts it close to the Earth and the Moon.[7] Upon its 1992 rediscovery, the comet's date of perihelion passage was off from the then-current prediction by 17 days. It was then noticed that, if its next perihelion passage (August 14, 2126) was also off by another 15 days, the comet would very likely strike the Earth or Moon. Given the size of the nucleus of Swift–Tuttle, this was of some concern. This prompted amateur astronomer and writer Gary W. Kronk to search for previous apparitions of this comet. He found the comet was most likely observed by the Chinese in 69 BC and AD 188, which was quickly confirmed by Brian G. Marsden.[8] This information and subsequent observations have led to recalculation of its orbit, which indicates the comet's orbit is very stable, and that there is absolutely no threat over the next two thousand years.[9] Astronomers believe that in the 2126 pass it will likely be a great naked-eye comet like Hale–Bopp.[3]
A close encounter with Earth is predicted for the comet's return to the inner solar system in the year 4479, around Sept. 15; the closest approach is estimated to be 0.03–0.05 AU, with a probability of impact of 1×10−6.[6] Subsequent to 4479, the orbital evolution of the comet is more difficult to predict; the probability of Earth impact per orbit is estimated as 2×10−8.[6] As the largest Solar System object that makes repeated close passes of Earth, and which does so at a relative velocity of 60 km/s,[4][10] leading to an estimated impact energy of ≈27 times that of the K–T impactor,[11] Comet Swift–Tuttle has been described as "the single most dangerous object known to humanity".[10]"
Thanks for input. I am a natural skeptic and I haven’t “done the math(s)”, though I’m perfectly capable of doing so. It just seems counter intuitive, especially if its in orbital resonance with Jupiter, it should be in a direct, rather than retrograde, orbit and the vector sum of the velocities should be smaller than if they were in quadrature, i.e., smaller than the sum of the squares.
I think what I am missing is the added energy obtained by falling from “infinity” to earth. There still doesn’t seem to be enough energy in the orbits, though.