Comets have been spotted throughout antiquity and were often thought to be omens of doom but even in ancient times natural philosophers tried to understand exactly what these hairy stars actually were.
Aristotle thought comets were atmospheric phenomena much like meteors burning up in the Earth's atmosphere.
Over time scientists began to speculate that comets might actually be objects in outer space.
In the 16th century Tycho Brahe demonstrated that the great comet of 1577 had to be at least four times more distant than the moon.
In the 17th century Edmund Halley suspected that the comet sightings of 1456 1531 1607 and 1682 were in fact the same comet following a regular 75 or 76 year pattern.
As a matter of fact this comet showed up in ancient Chinese and Babylonian tablets and was even featured in the Bayeux Tapestry.
Halle used Isaac Newton's laws of gravitation to predict the Comet's return in 1758.
Halle wouldn't live to see his comet come back but sure enough it arrived on schedule just as Halle predicted.
Comets were no longer omens of doom but proper members of our solar system.
Comets are similar to asteroids in that they are both largely primordial bodies that contain rock, ice, water and carbon dioxide but whereas asteroids are mostly rock with a little bit of ice and water comets are mostly ice and water with a little bit of rock.
Comets have highly elliptical orbits that bring them close to the Sun which in turn warms up the comet.
This warming is what causes the comet's ice to sublimate or go directly from a solid into a gas. This is what makes a comet look like a comet.
The dirty snowball at the center of a comet is called the nucleus.
This is the solid part of the comet and when it gets close to the Sun it warms up and forms a giant atmosphere called the coma
The ice in question is water ice but it's also carbon dioxide carbon monoxide ammonia and other ices, as well the rest of the comet is mostly rock.
When the ice sublimates gas and dust are released to form two different tails.
Gas molecules get ionized by the sun's ultraviolet light losing electrons in the process.
The sun's solar wind moves very fast and carries its own magnetic field with it;
This traps the ions and pushes them directly away from the comet forming the ion tail.
The Sun is also very bright and sunlight exerts a tiny but steady pressure on the dust.
Over time this pressure pushes the dust particles away from the comet but because it's a very gentle push the dust falls away from the comet much slower and basically lags behind.
Because they are moving at two different speeds the ion and dust hills point in two different directions, still both tails will point away from the Sun even when the comet itself is moving away from the Sun, in other words, the tails are sometimes in front of the comet.
The tails are the most recognizable feature of a comet and they can grow to tens of millions of kilometers but despite their size tails are about a million billion times less dense than air that's why flying a spacecraft through the comet's tail is really not a big deal.
Comets are generally classified by their orbits.
Comets with orbital periods of less than 200 years are called short-period comets.
These comets tend to orbit the Sun in roughly the same direction as the planets.
Comets with periods greater than 200 years are called long-period comets. These comets tend to have orbits at random inclinations which means that they can come in from any direction.
But every time a comet comes near the Sun it loses mass and that means that over time they should evaporate.
In fact, we got to see this in 2013 when comet Ison pulled an Icarus and fizzled out as it flew around the Sun.
But if comets lose mass every time they approach the Sun how can there still be comets all these billions of years later?
Well, it turns out there's a vast region of the outer solar system. Way beyond the orbit of Neptune where icy bodies have been quietly orbiting the Sun in a cold storage for millions or even billions of years. In fact, there are two such regions the Kuiper belt extends from Neptune's orbit at about 30 astronomical units.
All the way out to about 50 astronomical units from the Sun I see bodies that fall in from the Kuiper belt are the short-period comets.
There's an even larger more distant region called the Oort cloud which is thought to have trillions of comets and may extend out to a couple of light years from the Sun.
The Oort cloud is home to long-period comets.
Objects in both of these regions can remain in stable orbits until something comes along to disturb them then they fall in toward the Sun becoming comets in the process
Comets are beautiful to look at in telescopes and can even be seen by the naked eye if they're close enough to earth but all we can see are the Comet's coma and Tails but that changed in 1986 when the Soviet Union's Vega 1, and the European Space Agency's Giotto spacecraft flew into Halley's Comet and directly imaged a comet's nucleus for the first time.
Even though Halley's coma was about a hundred thousand kilometers across its nucleus is really small at just 15 kilometers long by eight kilometers wide it also has a low mass and an average density of just 0.6 grams per cubic centimeter, in other words, Hollywood float in water if you could find a bathtub big enough.
Giotto also found that despite Halley's bright appearance its nucleus is actually very dark in fact it's even darker than coal with jets of sublimating ice.
These Jets have been spotted on other comets during spacecraft flybys as well; this suggests that comets have surfaces of dark compounds with ice and dust underneath.
But our best view of a comet happened in 2014 when the Rosetta spacecraft entered into orbit around comet 67p Churyumov-Gerasimenko becoming the first spacecraft to orbit a comet rather than just fly past it.
Comet 67p turned out to be a different kind of comet than those studied on previous missions whereas Hallie and others look like giant lumpy snowballs 67p has these two large lobes and could be that 67p lost most of its ice around the neck.
Or perhaps the lobes are two smaller comets that gently collided and stuck together as comet 67p closer to the Sun it began to out gas Jets of water vapor carbon dioxide and carbon monoxide the Rosetta spacecraft carried a small Lander named filet which was supposed to land on what was assumed to be a soft snow like surface instead it encountered a very hard surface and bounced twice.
67p had a much harder surface than expected despite being a low-density rubble pile.
It's not clear why this would be the case, perhaps surface ice melted and then refrozen into a super hard surface, but no one's really sure yet.
When fili finally landed it found 16 types of organic compounds including acid amide acetone methyl isocyanate and propionaldehyde.
I hope I didn't screw up those pronunciations but I probably did anyway.
Rosetta even detected molecular nitrogen and oxygen outgassing from the comet as well.
These gases seem familiar to you, it's because they make up the air that we breathe on Earth but this isn't the first time organic compounds have been detected in comets.
In 2004 the Stardust spacecraft flew into the coma of Comet wild 2 and returned samples to earth but not only to define organics in wild twos atmosphere but it also found amino acids.
These are the same amino acids that are used to build proteins.
Comets have lots of organic ice and even amino acids. They also get very close to Earth.
Billions of years ago comets and asteroids compounds to earth and that means that our oceans the gasses that we breathe and even the building blocks of life may have come courtesy of comets.
This is why we are so interested in comets and asteroids their time capsules revealing the conditions of the early solar system but they also have much to teach us about the origins of life as well
