Category Science

          Comets usually have two tails — one of gas and one of dust. The gas tail is normally blue, and is pushed away from the Sun by particles in the solar wind. The dust tail is yellow or white, and although it too is pushed away from the Sun, the star’s gravity causes the tail to curve.

          The tails appear as the comet approaches the Sun. Sunlight pushes on things, but very gently. Because the comet dust particles are so small, they are pushed away from the Sun into a long tail. Another tail is made of electrically charged molecules of gas (called ions). Very rarely a comet will have a third tail made of sodium, which we usually don’t see with our unaided eyes. In the early time of our solar system, Earth was often hit by comets. Scientists believe comets may have contributed some of the water for our oceans or even some of the molecules from which life eventually evolved. Some believe it may have been a comet hitting Earth that caused the dinosaurs to become extinct. The Stardust mission, as well as other comet missions NASA has planned, will teach us much more about these fascinating solar system objects.

          Comets are believed to be a very old part of our solar system. They are made of the leftover materials that didn’t become part of the Sun, the planets, or the moons. If we knew more about comets, we would know more about how our solar system formed over four billion years ago! The nucleus, or solid part of a comet, is usually less than 10 kilometers (about 6 miles) across. The nucleus is like a dirty snowball. Nobody knows for sure what any comet is like inside. Maybe they are not all similar. Comets seem to contain a lot of ice, some rocks and dust, and some gas. As they get closer to the Sun and start to heat up, some of their materials start to boil off. This material forms a cloud around the nucleus. The cloud is called the coma and may be hundreds of thousands of kilometers in diameter. And trailing out for oftentimes millions of kilometers are the comet’s tails.

          Stardust is the first space mission to capture dust from a comet and return it to Earth. Stardust was launched on February 7, 1999. On January 2, 2004, Stardust met up with Comet Wild 2 (pronounced “Vilt 2”). Before it got there, the spacecraft had to make two trips around the Sun. Then it captured particles of dust using some very weird stuff called aerogel.

          Aerogel looks like frozen smoke It is so light and wispy you can see right through it. It is the lightest known solid material. The comet particles were trapped gently inside the aerogel and stored safely for the trip home.  Stardust made one more trip around the Sun to catch up with Earth again. The samples inside the aerogel, stored in a special reentry capsule, parachuted safely to Earth on January 15, 2006.

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          All comets begin their lives as dirty snowballs. They are relics from the birth of the Solar System around 4.6 billion years ago, and are made up mainly of ice, gas and rock. As the comet approaches the inner Solar System, the Sun’s heat causes the ice to evaporate. It turns into gas and forms a glowing head around the nucleus.

          In order to understand what are comets made of, we need to break down the three main parts of the comet: the nucleus, coma, and tail. Comet nuclei are known to range from about 100 meters to more than 40 kilometers across. They are composed of rock, dust, ice and frozen gases such as carbon monoxide, carbon dioxide, methane, and ammonia. Sometimes called dirty snowballs, recent studies have shown that the ice of a comet is covered by a crust. Comets also contain a variety of organic compounds as well as the gases already mentioned. Some of these are methanol, hydrogen cyanide, formaldehyde, ethanol, and ethane. More complex molecules such as long-chain hydrocarbons and amino acids may also be in comets. Because of their low mass, comets cannot become spherical under their own gravity, and will thus have irregular shapes.

          The coma is the nebulous envelope around the nucleus of a comet. It is formed when the comet passes close to the Sun on a highly elliptical orbit. As the comet warms, parts of it turn from solid to gas (sublimate). Larger charged dust particles are left along the comet’s orbital path while smaller charged particles are pushed away from the Sun into the comet’s tail by solar wind. This helps astronomers distinguish comets from stars because it creates a fuzzy appearance.

          The tail is illuminated by the Sun and may become visible from Earth when a comet passes through the inner solar system, the dust reflecting sunlight directly and the gases glowing from ionization. The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet’s orbit in such a manner that it often forms a curved tail called the antitail. At the same time, the ion tail, made of gases, always points directly away from the Sun, as this gas is more strongly affected by the solar wind than is dust, following magnetic field lines rather than an orbital trajectory. Parallax viewing from the Earth may sometimes mean the tails appear to point in opposite direction.

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          Although all comets seen from Earth have tails, they do not always look like this. When a comet is a long distance from the Sun, it exists purely as a lump of ice, frozen gas and rocky dust. However, as the comet’s orbit takes it closer to the Sun, the temperature rises, and the ice begins to melt. Gas and dust are released, forming a huge cloud around the comet. This cloud is blown by the solar wind to form a tail.

          A comet tail—and coma—are features visible in comets when they are illuminated by the Sun and may become visible from Earth when a comet passes through the inner Solar System. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. Separate tails are formed of dust and gases, becoming visible through different phenomena; the dust reflects sunlight directly and the gases glow from ionization. Most comets are too faint to be visible without the aid of a telescope, but a few each decade become bright enough to be visible to the naked eye.

          In the outer Solar System, comets remain frozen and are extremely difficult or impossible to detect from Earth due to their small size. Statistical detections of inactive comet nuclei in the Kuiper belt have been reported from the Hubble Space Telescope observations, but these detections have been questioned, and have not yet been independently confirmed. As a comet approaches the inner Solar System, solar radiation causes the volatile materials within the comet to vaporize and stream out of the nucleus, carrying dust away with them. The streams of dust and gas thus released form a huge, extremely tenuous atmosphere around the comet called the coma, and the force exerted on the coma by the Sun’s radiation pressure and solar wind cause an enormous tail to form, which points away from the Sun.

          The streams of dust and gas each form their own distinct tail, pointing in slightly different directions. The tail of dust is left behind in the comet’s orbit in such a manner that it often forms a curved tail called the antitail, only when it seems that it is directed towards the Sun. At the same time, the ion tail made of gases, always points along the streamlines of the solar wind as it is strongly affected by the magnetic field of the plasma of the solar wind. The ion tail follows the magnetic field lines rather than an orbital trajectory. Parallax viewing from the Earth may sometimes mean the tails appear to point in opposite directions.

          Some scientists are convinced that the orbits of Uranus and Neptune are being distorted by the gravitational pull of a planet beyond Pluto. The recent discovery of minor members beyond Pluto which could be responsible for this distortion makes the existence of a tenth planet unlikely. New planets are being discovered continually, however, not in our Solar System, but orbiting other stars.

          Pluto has been downgraded from ‘ninth planet’ after we found the rest of the Kuiper belt, including Eris which is more massive, though with a slightly smaller volume. Even before that, the apparent need for an extra planet to explain deviations in the orbit of Neptune no longer applied. Voyager 2 made more accurate measurements and found nothing out of line. (And found that Neptune was more massive than Uranus, despite its smaller radius and volume.)

          Anything large enough to count as a 10th Planet would have been expected to have caused disturbances in the orbits of known planets and in the Voyager and Pioneer probes as they moved outwards.

          There is a small possibility of one or more Earth-sized bodies very far out. Under current rules these would probably not count because they would probably not have cleared their orbits of similar bodies.

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          Beyond the orbit of Neptune, stretching deep into the outer Solar System, lies a belt of celestial bodies – made of rock and ice. The astronomer Gerard Kuiper first suggested the existence of this zone of comet-like objects, and so it was named the Kuiper Belt. There are at least 70,000 minor members in the Kuiper Belt with a diameter of over 100km (62 miles). The largest of these is 1992QBI, otherwise known as Smiley, which is 220km (137 miles) across.

          The Kuiper belt occasionally called the Edge worth–Kuiper belt, is a cicumstellar disc in the outer Solar System, extending from the orbit of Neptune (at 30 AU) to approximately 50 AU from the Sun. It is similar to the asteroid belt, but is far larger—20 times as wide and 20 to 200 times as massive. Like the asteroid belt, it consists mainly of small bodies or remnants from when the Solar System formed. While many asteroids are composed primarily of rock and metal, most Kuiper belt objects are composed largely of frozen volatiles (termed “ices”), such as methane, ammoni and water. The Kuiper belt is home to three officially recognized dwarf planets: Pluto, Haumea and Makemake. Some of the Solar System’s moons, such as Neptune’s Triton and Saturn’s Phoebe, may have originated in the region.

          The Kuiper belt was named after Dutch-American astronomer Gerard Kuiper, though he did not predict its existence. In 1992, Albion was discovered, the first Kuiper belt object (KBO) since Pluto and Charon. Since its discovery, the number of known KBOs has increased to thousands, and more than 100,000 KBOs over 100 km (62 mi) in diameter are thought to exist. The Kuiper belt was initially thought to be the main repository for periodic comets, those with orbits lasting less than 200 years. Studies since the mid-1990s have shown that the belt is dynamically stable and that comets’ true place of origin is the scattered disc, a dynamically active zone created by the outward motion of Neptune 4.5 billion years ago; scattered disc objects such as Eris have extremely eccentric orbits that take them as far as 100 AU from the Sun.

          The Kuiper belt is distinct from the thyeoretical Oort cloud, which is a thousand times more distant and is mostly spherical. The objects within the Kuiper belt, together with the members of the scattered disc and any potential Hills cloud or Oort cloud objects are collectively referred to as trans-Neptunian objects (TNOs). Pluto is the largest and most massive member of the Kuiper belt, and the largest and the second-most-massive known TNO, surpassed only by Eris in the scattered disc. Originally considered a planet, Pluto’s status as part of the Kuiper belt caused it to be reclassified as a dwarf planet in 2006. It is compositionally similar to many other objects of the Kuiper belt and its orbital period is characteristic of a class of KBOs, known as “plutons” that share the same resonance with Neptune.

          The Kuiper Belt and Neptune are noted as one of the ways to define the extent of the Solar System, along with the heliopause and the radius at which the Sun’s gravitational influence is matched by other stars, estimated to be between 50000 AU to about 2 light-years.

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          The solar system does not end at Pluto but stretches outwards in all directions for billions of kilometres. Many scientists believe that the boundary of the Solar System could be an immense cloud of comets, called the Oort cloud, which surrounds the planets like a spherical cage. Scientists believe that there are over ten trillion comets in this spherical halo, stretching nearly 8 million kilometres (5 million miles) from end to end.

          With recent searches of what lies beyond Pluto, astronomers have now detected a small planet some 200 miles in diameter which lies at the incredible distance of some 7.9 billion miles from the Sun. The new object has been named 2015 TG387, a rather bland name for this most distant object. When Pluto was discovered here in Arizona back in 1930, there was much excitement that this object was thought to be at the farthest edge of the solar system. Since that time, many new objects have been discovered at much greater distances than Pluto.

          The names, Eris, Makemake, Sedna, Quaoar, Varuna and Haumea, are not part of most peoples’ vocabulary, as these are a few of the new dwarf type planets that lie within this amazing region in the solar system, known as the Kuiper Belt. Simply, the solar system is packed with so many small planets and the discoveries just keep on coming! When Pluto was first discovered in 1930, many had the belief that a true 10th planet was lurking far beyond Pluto. To this day, there are many in the scientific community who believe that “Planet X” still exists as a large body yet undiscovered.

          Astronomers know of the “Kuiper Cliff,” a region in the Kuiper Belt that seems to end at a distance from the Sun of 48 AU (1 AU is the distance of the Sun from the Earth), around 4.46 billion miles from the Sun! The vast distance of this object at 2.5 times the distance of the dwarf planet Pluto makes this a most interesting object out in a region known as the Kuiper Belt.

          This is very important, as this might signify that there is a much more massive-type object lurking well beyond this distance. Some have labeled this object “Planet 9.” It might be an object as large as the earth or larger in an orbit that takes well over 20,000 years to circle the Sun.

          Beyond the realm of the Kuiper Belt is a large cloud like formation, known as the “Oort Cloud.” This is a region of the distant solar system where comets are thought to be and there is even speculation that a large gas giant planet much like Jupiter may inhabit this region of space.