Comets: Time capsules of the solar system

let us imagine ourselves 5 billion years back in time in the outer part of the Milky way there floated an immense cloud of gases and dust gradually the swirling Cloud began to condense in some places forming several discs one of them was the solar nebula from which our solar system was to [Music] emerge almost all the material in the nebula was pulled into the center where it formed a new star the Sun out of what was left a few smaller bodies took shape far from the Sun these became the planets and their moons by about 4 and A2 billion years ago the solar system had formed that's the theory but we don't know exactly how the Sun and its family came into being we know more more about the evolution of stars than we do about our own Origins why because enormous changes went on in the early history of the solar system chunks of matter left over from the condensing disc bombarded planets and moons some of them melted and later cooled into solid Rocky bodies other planets became huge Globes of gases and liquids these dramatic changes are like a curtain that hides what we want to know how did this solar system really form from our observatories we look for clues in the night sky that will help us draw back that curtain one group of objects in particular may give us some of the answers we're looking for they are the Comets occasionally one of these lonely Wanderers passes near the sun then night after night sometimes for weeks a comet blazes a glowing path across the sky on its Journey around the Sun and back to deep [Music] space the astronomer reaches out to the comet with instruments wanting to understand it better for scientists believe that comets may still contain particles of the original cloud from which the solar system condensed over 4 and A2 billion years ago if that is so then comets may be our best chance to discover how the solar system came into [Music] being comets have long been part of human 's history and legends a medieval artist Associated a comet with a joyful event the birth of Jesus but usually comets were seen as evil Omens Misfortune would surely follow them people feared these mysterious streaks in the night sky believing they were symbols of God's anger but a few saw them as part of the natural world which they wanted to understand through careful observations scientists of the 16th and 17th centuries discovered that comets orbited the sun Edmund Hy an English astronomer was one of these scientists he tracked the great Comet of 1682 and realized that this same Comet might have been seen many times in the past he predicted that it would return about every 76 years some thought the idea was ridiculous but the comet's next return proved that hi was right when this Comet now called hi's Comet returned in 1910 millions of people were fascinated by the Heavenly spectacle scientists studied the comet closely and it was photographed for the first time as it orbited the Sun and disappeared into space for another 76 [Music] years what we know about comets is limited by what we've observed from Earth over 500 comets are well known and with the help of computers we can plot some of their orbits around the Sun [Music] here the computer first draws the orbits of the outer planets then the path of a comet some comets come from enormous distances and may take several million years to circle the [Music] sun they may appear only once in our night sky before escaping from the solar system nearly a 100 comets have short periods meaning that they return in less than 200 years one comes back every 3 years and 4 months here the computer shows the motion of a comet called temple 2 notice that that it speeds up as it approaches the sun then slows down again as it moves away a comet may approach the Sun from any angle Hal's Comet for example orbits the Sun in the opposite direction from the planets some comets never go beyond the orbit of Jupiter others like hi travel nearly to the edge of the solar system before the Sun's gravity pulls them [Music] back the Motions of comets are relatively simple to compute but even the best earth-based instruments can't reveal the exact composition and structure of comets for this reason our present knowledge is mostly Theory the evidence we have suggests that at the heart of a comet is a nucleus of ice and dust the largest probably not more than about 30 km across the nucleus is what scientists would like to study most as the comet moves closer to the Sun gases and dust begin to boil off forming a diffuse Cloud called the [Music] coma the coma may get to be 100,000 kilm across and it practically hides the nucleus the coma Blends into the tail the most spectacular feature of a comet the tail grows longer as the comet moves closer to the Sun and always points more or less away from it that's because the sun's energy acts like a wind that blows gases and dust away from the comet's [Music] nucleus a comet's tail May extend for tens of millions of kilometers farther than light travels in one minute but with each trip around the Sun a comet loses some of its substance eventually a comet dies yet even as old comets are consumed new ones continue to enter the solar system but from where if we could travel away from the Sun leaving all the planets behind until they disappear into the Blackness of space and until the sun itself is just another bright spot in the sky we would find a thin cloud of objects enveloping the solar system these are the original comets billions of them scattered across half a lightyear of space they have been here unchanged since the beginning of the solar system occasionally one of these comets changes Direction perhaps because another star pass nearby and begins its journey toward the sun hundreds perhaps thousands of years later it may become visible from Earth still carrying clues about the early history of the solar system recently we began sending spacecraft to explore other planets and moons hoping to add to our understanding of how they were [Music] formed every such Voyage has added immensely to our knowledge and has tantalized us with new puzzles one day we may be able to study a comet at close range and so get answers we can get nowhere else about the origin of the solar system and perhaps even about the origin of life on Earth [Music] [Applause] [Music]