1951 – USA
‘A typical comet has three parts: a frozen central part called the nucleus, a fuzzy cloud surrounding the nucleus called the coma (or head) and a tail consisting of gas and dust. The nucleus, usually only a few kilometres across is made of grains of frozen water, methane, ethane, carbon dioxide, ammonia and other gases’
Whipple coined the phrase ‘dirty snowball’. In 1986 the European Space Agency‘s craft Giotto proved that Whipple’s theory is fairly accurate when it took close-up photographs (from a distance of 480 kilometres) of the nucleus of HALLEY‘s comet.