The current estimate is circa 4380 CE. Hale-Bopp's orbital period was altered due to the gravitational effects of approaching Jupiter as closely as 0.77AU (1 AU = average distance between earth and sun) in April 1996. The orbital period before it approached the solar system planets was about 4200 years, which changed to 2380 years following the gravitational interaction with Jupiter.
Like most comets, Hale-Bopp also releases large jets of easily vaporized material when it approaches the sun. While predominantly H2O, many other compounds such as acetaldehyde, prussic acid, and hydrogen sulfide are also present in these jets. Even though such outgassing has been modeled, it introduces additional uncertainties to orbital period estimation.
This page was last updated on July 18, 2015.