What is the X in X-mas?
|Halley's comet |
A short-period comet that orbits the Sun once every 76 years. It is visible to the unaided eye and last appeared in 1986, when close observation by spacecraft showed that its nucleus measures about 16 km (10 mi) by 8 km (5 mi) and is composed of water ice, stony minerals, and organic compounds. Its next appearance will be in the year 2061.