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Spectacular comets may be visible only once in a lifetime.
Wandering through the Solar System, comets can be among the most incredible of astronomical sights and, after years of careful observation, astronomers have coaxed out the secrets hidden within their glow.
The heart of a comet is its nucleus, a core of ice laced with rock and dust, a few kilometers wide. Though sometimes called a 'dirty snowball', the ice found on comets is far more exotic than that on Earth.
When the Rosetta spacecraft reached the comet 67P/Churyumov-Gerasimenko in 2014 it performed the first in-situ analysis of the comet's nucleus, finding not only water ice, but also carbon dioxide and monoxide, as well as traces of ammonia, methane and methanol. These highly volatile compounds are usually found as a gas or liquid on Earth, but the frigid depths of space have frozen them to ice as hard as rock.
These snowballs travel in huge elliptical orbits, briefly visiting the inner Solar System at one end before travelling billions of kilometers to the outer regions. 'Long-period comets' travel into deep space, taking thousands of years to complete an orbit, while 'short-period comets' have orbits that take only a few years or decades. Halley's Comet is visible to the naked eye from Earth every 75.3 years.
Comas & Tails
It's thought short-period comets come from the Kuiper Belt, after being knocked out of orbit. Beyond the Kuiper Belt, the Oort Cloud stretches to 3.2 lightyears from the Sun. If a passing star kicks one of its bodies off course, it creates a long-period comet.
For most of these orbits, the nucleus remains an inert lump of ice, but this changes as the comet nears 'perihelion' — its closest approach to the Sun. When close enough, the solar radiation heats the surface, causing the volatile components to boil. As the gas escapes into deep space it lifts off dust, creating a shroud that can stretch out over 50,000km around it — the coma.
As the comet gets closer to the Sun, this envelope begins to feel the solar influence even more acutely, as its wind and magnetic field sweep the dust and gas out into a huge tail, which can extend for millions of kilometers. Some of the tail's debris is left behind in its orbit, forming a meteoroid stream. Several of these cross the Earth's orbit, and when we pass through them every year, we see the debris burning up in the atmosphere as a meteor shower.
Sunlight reflecting off the coma and tail causes these celestial visitors to glow in the night, making them an ever-popular target for astronomers. But, only a handful of comets can be seen every year with the aid of a small telescope. Websites such as www.icq.eps.harvard.edu/cometobs.html or www.ast.cam.ac.uk/~jds will tell you which comets are active and where to find them.
Chasing the Tail
The most alluring part of a comet is surely its huge tail, but it's not always obvious that there are two. The most apparent is the dust tail, swept out in an arc by the solar wind. However, the magnetic field captures the gas, forming a fainter second tail. Sometimes the comet's position relative to Earth means the tails appear to go in two different directions.
Dominating the sky or the landing site for a probe, these are the best-known comets
Closest approach: 136 million km
Orbit: 2,520-2,533 years
Famed for: Visible to the naked eye for a record 18 months in 1996/97, Hale-Bopp will return around the year 4385.
Closest approach: 186 million km
Orbit: 6.4 years
Famed for: Target of the Rosetta mission, which sent the Philae lander to its surface, finding water and organic compounds.
Great Daylight Comet
Closest approach: 19 million km
Orbit: 57,300 years
Famed for: Spotted in January 1910, this comet quickly brightened until it outshone even Venus. Its tail was noticeably curved.
Closest approach: 88 million km
Orbit: 75.3 years
Famed for: The only known short-period comet routinely visible to the naked eye, this regular visitor was observed as early as 240 BC.
Closest approach: 450,000 km
Orbit: 876.7 years
Famed for: Its 1965 close pass of the sun made Ikeya-Seki one of the brightest comets in 1,000 years. It's thought to be a fragment of the Great Comet of 1106.
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