If you're waiting for Halley's comet to show up exactly 75 years after its 1986 appearance, you may be disappointed. The ball of ice has an orbit that varies by months or even years.
And new research suggests that Venus is responsible for the comet's variations today, rather than the more massive planet Jupiter.
"Comet Halley has been observed throughout history, all the way back to 240 BC by the Chinese," Tjarda Boekholt, an astrophysicist at Leiden Observatory in the Netherlands, told Discover by email.
With many well-documented appearances, scientists quickly realized that the arrival time of the comet was constantly changing. For instance, although it passed Earth in February in 1986, it won't be back until February 2061. In 45 years, it will instead appear in July.
"It is the variation in the time of sightings that provided the first clue to comet Halley's chaotic orbit," Boekholt said. "The orbit of comet Halley is not static, but it is evolving."
Boekholt led a team that investigated the comet's changing orbit. They found that Venus played an important role in revising the comet's orbit in the past, and will probably continue to do so in the future, despite its small stature. Mighty Jupiter often dominates the influence of gravitational bodies due to its high mass, but Venus currently dominates Halley's movements.
Narrowing down how and why Halley's orbit changes can help improve scientists' understanding of how other bodies move in the solar system. "Performing such analysis to other bodies in the solar system, we can obtain a better overview and understanding of chaotic orbits, stable orbits, and the dynamical evolution of the solar system and other planetary systems," says Boekholt.
This is an extract; the whole story, with video, can be found here.