Comets are classified as either short-period, with orbits that take less than 200 years to complete, or long-period which take longer than 200 years to complete. Long-period comets, like Tsuchinshan-ATLAS, are thought to originate from the Oort Cloud, a region at the farthest edge of our Solar System (more than 40 times further away than Pluto), where the gravitational pull of the Sun is so weak it competes with tidal pull of the galaxy and passing stars. Objects in the Oort cloud are composed of frozen gas, dust, and rock, and occasional gravitational interactions can send these objects toward the Sun, where when they get close enough, we call them comets.
Graphic showing the positions of comet Tsuchinshan-ATLAS on October 12, 19, and 26 based on NASA/ JPL SSD orbital data. Solar system objects not to scale. Image credit: Jason E. Ybarra; CC BY-NC-SA.
The orbital period of Comet Tsuchinshan-ATLAS was calculated to be more than
80,000 years, however, as it travel through the Solar System, gravitational interactions
with other objects can alter the orbit. The most current calculations from NASA's
Solar System Dynamics (SSD) group show that it is on a hyperbolic path, meaning
that it will not return. Either way, it is a once-in-a-lifetime event.