Halley's Comet, in the night sky on 15 April 1986, computer-enhanced image. The galaxy Centaurus A is at bottom left. Halley's Comet orbits the Sun every 76 years, and will return to the inner Solar System in 2061. It was discovered by Edmund Halley (1656-1742). A comet is a lump of ice and rock whose orbit brings it close enough to the Sun for its icy core (red) to melt and boil into space. The Sun's solar wind and radiation blows the dust and ions into a tail (across top) that can be millions of kilometres long. The tail shines by reflected sunlight, and can be visible from Earth. Centaurus A is an active galaxy, and is a powerful source of radio waves, X-rays and gamma rays. It lies around 10 million light years away in the constellation Centaurus.