Halley's Comet - http://www.ottisoft.com/Activities/Halleys%20comet.htm
1. Four differences between planets and comets are:
a. Comets are usually made from ______
b. Comets have highly elliptical ______ compared to planets
c. Comets usually travel _____ than ______
d. Comets usually have long ______, made when the heat from the sun melts the vaporises the comet
