Comets are named for long hair and built like fried ice cream
The Greeks called them long-haired stars, and the word comet still carries that image. Up close, a comet's core is a loose clump of ice, dust and rock only kilometres wide, yet its glowing halo can grow fifteen times broader than Earth and its tail can stretch beyond the Sun-Earth distance.
A comet is a small icy body that heats up as it nears the Sun and starts venting gas. Sunlight and the solar wind shape that escaping material into a hazy coma surrounding the core, often with a streaming tail behind it. Nuclei range from a few hundred metres to tens of kilometres wide. A bright, nearby comet can span as much as 30 degrees of sky, the width of 60 full Moons, and cultures across history have noted them. The symbol for a comet is a small disc trailing three hair-like strands.
Fred Whipple famously called them dirty snowballs, a mix of rock, dust, water ice and frozen gases such as carbon dioxide, methane and ammonia. After NASA's Deep Impact probe smashed into Comet Tempel 1 in July 2005, icy dirtballs seemed a better label. Research in 2014 went further, likening them to deep-fried ice cream: a hard crust of dense crystalline ice laced with organic compounds over a colder, fluffier interior. Surfaces tend to be dry and dusty, with ices hidden metres below. Glycine, an amino acid, turned up in comet dust returned by NASA's Stardust mission, confirmed in 2009.
Most comets follow stretched elliptical paths with periods from a few years to millions. Short-period ones come from the Kuiper belt and scattered disc beyond Neptune; long-period ones are thought to fall inward from the Oort cloud, which reaches halfway to the nearest star and is nudged by passing stars and the galaxy's tidal pull. Some take hyperbolic paths, visiting once before escaping to interstellar space.
Unlike rocky asteroids, which formed inside Jupiter's orbit and grow no tails, comets carry volatile ices. That line has blurred, though, with main-belt comets and so-called Manx comets, 27 of which were found between 2013 and 2017. By November 2021 astronomers had catalogued 4,584 comets, a tiny slice of the roughly one trillion thought to lurk in the Oort cloud. About one a year becomes visible without a telescope, and ESA's Rosetta became the first mission to set a robotic lander on one.
Source: Comet
