Seeing Halley's Comet Up Close

In 1986, Halley's Comet made one of its rare returns to the sun and Earth, opening up an opportunity that had never been present before – namely, the chance to send a spacecraft to examine the comet up close and learn more about its nature and structure. Five craft, manned by the USSR, the European Space Agency, and Japan, went to the comet to study it, snapping pictures and obtaining data that had never been available before. These five craft, known collectively as the Halley Armada, helped formulate much of our modern knowledge of comets in general and Halley's Comet in particular.

One of the craft, the European Giotto, was the first to obtain color pictures of the nucleus of the comet. While many of the observations made were what was expected, others came as a surprise. The theories of Fred Whipple, who had speculated that comets were “dirty snowballs”, were proven mostly right – the cause of the brilliant coma and streaming tail of the comet was indeed the evaporation of ice and the freeing of dust particles. However, observation showed that most of the surface of the nucleus was covered in non-volatile dust, with only small parts of the surface (around 10%) responding in an ‘active' way. This showed that the “dirty snowball” was in fact mostly dirt with only a small amount of snow, surprising scientists.

The center of the comet was in fact only very loosely collected, being mostly a loose collection of rubble held together by gravity. In addition, the core was surprisingly small and jet black, despite the huge and bright appearance of the comet overall – all the extra size and brightness resulted from the emissions of the comet.

The emissions themselves proved to be mostly water, with some carbon monoxide, methane, and ammonia, and slight traces of hydrocarbons. Early speculation held that the composition of the ice was close enough to our ocean's water that comet impacts could have helped build Earth's water supplies, but further study showed that the composition was in fact much different. It is clear Halley's Comet has very different ice than our own.

Halley's Comet has turned out to be one of the most active comets of its type despite its seeming dimness and lack of activity, beating comets much like it (such as Encke and Holmes) by multiple magnitudes of order. This is why it is so vivid and startling in the skies when it arrives.

The Halley Armada is a brilliant example of international cooperation in science, showcasing the ability of humanity to explore the world around itself in new ways through cooperation and technology. The Armada helped us to better understand the universe, confirming old theories and introducing new questions and surprises. Hopefully in 2061 the nations of the world will repeat the effort, allowing us to discover even more new and startling things about Halley's Comet, that spectral visitor who still holds many questions for us to answer.

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