Diana, orbital mechanics are well understood and are nothing like weather or climate when it comes to modeling. You can calculate orbits with excellent precision.
Comet 109P/Swift-Tuttle's orbital properties very well known and understood. Every 133 years, it makes a complete orbit around the Sun, coming about 8 million km (5 million miles) interior to Earth's orbit, yet reaching farther away from the Sun than even Pluto does at its most distant. It last entered the inner Solar System in December of 1992, and won't do so again until 2126. We'll get a close pass (within 1,000,000 miles) in 3044, but it ought to miss us. In fact, its next 2,000+ years of orbits are mapped out incredibly well, and the Earth is 100% safe until at least 4479, when it will come quite close to Earth once again. Even then, there's still a 99.9999% chance it will miss us.However, astrophysicists do make some simplifying assumptions leading to model uncertainty:
The way scientists classify the likelihood of a collision between an asteroid-or-comet and a planet is by measuring its MOID, or the Minimum Orbit Intersection Distance. If these were the only two objects orbiting the Sun and there were no mutual gravitational interactions between anything else...I doubt they are ignoring the gravitational effects of the large gas giants, but you never know.
Thanks for putting that in English for me! :)