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. Erratic Schedule 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