Comet Holmes (17P/Holmes), in October 2007 after it brightened many thousands of times and became visible to the naked eye in the night sky. Comets are bodies of ice and dust orbiting the Sun. Those that enter the inner solar system heat up as they approach the Sun. Here, the brightening was due to an explosion that took place as the comet was moving away from the Sun, and the cause is not yet known. Comets form a tail of gas and dust while they are near the Sun. Here, the tail is pointing away from the Earth, so is less visible than with other comets. Comet Holmes is a short-period comet, with an orbit of only 6.9 years. It was discovered in 1892 by the British astronomer Edwin Holmes.