Soho images Comet Hale-Bopp approaching the Sun

A time series of six images of Comet Hale-Bopp taken by the SWAN instrument onboard ESA's Soho satellite in ultraviolet light (110-180 nm), on January 4, February 2 and 18, March 4 and 16, and April 3, 1997, as the comet slowly approaches the Sun, increasing in brightness. Hale-Bopp is clearly visible because of its huge hydrogen cloud produced by photodissociation of water vapour molecules evaporated from the solid nucleus. This nucleus is composed of ice and dust and has an estimated diameter of about 40 km. In contrast, the hydrogen cloud is more than 100 million km across, which makes it the largest object in the Solar System. The analysis of SWAN data of the hydrogen cloud indicates that about 600 tonnes of ice are vaporised and ejected in space each second during closest approach, forming its brilliant tail. [Image Date: 01-04-97] [97.04.011-001]