Comet Hale-Bopp was first observed by Alan Hale in New Mexico and Thomas Bopp in Arizona on July 22, 1995. Their discovery was announced by the International Astronomical Union on July 23, 1995. Hale-Bopp was closest to Earth in March 1997 when it was 122 million miles (196 million kilometers) away. It is a very large comet with a nucleus of approximately 25 miles (40 kilometers) in diameter, making it four times as large as Halley’s comet. Although most comets have two tails, Hale-Bopp exhibited three tails. The two tails typical of most comets are the dust tail and the ion tail. The dust tail, consisting of dust and debris from the nucleus, streams behind the comet in its orbit. The ion tail, consisting of the comet’s material interacting with the solar wind, faces away from the sun. Hale-Bopp’s third tail was composed of neutral sodium atoms. Hale-Bopp was visible with the naked eye for nearly 19 months. It is not expected to return for 4,000 years.