Halley's comet is composed mostly of hydrogen, carbon and oxygen. These elements make up the bulk of the sun as well, and scientists believe Halley's and other comets and the sun formed at about the same time in the early solar nebula.
According to NASA, because Halley's comet is made up of hydrogen, carbon and oxygen, which are light elements, it is lighter and less dense than rock found on Earth. Scientists tested the composition of Halley's comet in 1986 when the European Space Agency's Giotto probe passed less than 600 km from the comet's nucleus.
Every year there is the Orionids meteor shower, which occurs when the Earth passes through the debris left behind by Comet Halley. The debris are roughly the size of sand grains, but they hit the Earth's atmosphere at such high speeds, so they appear to be shooting stars. Scientists use the data gathered from the debris to gain insights about the comet.
The orbital period of Halley's comet is approximately 76 to 79 years, varying according to the gravitational pull of other celestial bodies. In 1705 Edmond Halley predicted its reappearance in 1758 using documented appearances throughout history and based on Newton's laws of motion.