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National Aeronautics and Space Administration National Aeronautics and Space Administration Supplemental Educational Support Materials for Special Feature: “An Icy Traveler Sweeps Past Mars” Discussion questions Q1: What is a comet, and how does it change as it gets closer to the Sun? Answer: A comet is, at its core, a dusty chunk of ice and rock ranging in size from a few kilometers to tens of kilometers across. The core is called the “nucleus” of the comet. When a comet gets closer to the Sun, the ice near its surface is heated, releasing gas and dust. This expelled gas and dust forms a bright, cloudlike coma around the nucleus that shines brightly in reflected sunlight. The gas and dust in the coma is swept back by the solar wind to form one or more tails. These tails always point away from the Sun. The thicker dust tail, made of the comet’s dust, usually has a slight curve in the direction of the comet’s orbit, but the ion tail, made up of ionized gas, will be thinner and straight. The shape and color of the tails tells astronomers about the composition of the comet. Q2: What is the difference between a short- and a long-period comet? Answer: Short-period comets take less than 200 years to orbit the Sun. These comets originate in the Kuiper Belt — the region beyond Neptune’s orbit. These comets can have their orbits disturbed by the giant planets and end up within the inner solar system. Many short-period comets have orbits in the same region as the asteroid belt, located between the orbits of Mars and Jupiter. There may be as many as 100 million objects in the Kuiper Belt. Long-period comets require more than 200 years to orbit the Sun, generally taking many thousands to a few millions of years. These comets spend most of their time in the vast spherical region in the outer reaches of our solar system known as the Oort Cloud. Unlike the Kuiper Belt, which is a relatively flat belt of material, the Oort Cloud surrounds the solar system. Trillions of long-period Continued … Educational Product Educators & Students Grades 5–8 comets must reside in this region, but we only can observe the ones with very elongated orbits that take them into the inner solar system. They can come from any direction and are usually only observed once. Q3: Choose a short- and/or a long-period comet that interests you, and explain why you chose the one (or ones) you did. Answer: Student answers will vary. Some short-period comets include Halley’s Comet, Comet Holmes, Comet Encke, Comet 9P/Tempel (also known as Temple 1) and Comet 81P/Wild (also known as Wild 2). Famous long-period comets include Hale-Bopp, Comet Hyakutake, Comet West, Comet McNaught, Comet Lovejoy, Comet Shoemaker-Levy 9, and Comet ISON. Students may choose a comet because they have heard of it, as in Comet Halley, Comet Shoemaker-Levy 9, Comet Lovejoy, or Comet ISON. They may have chosen a particular comet because they read about it in the Star Witness News series, which featured Comet ISON (October 2013), Comet Holmes (November 2007), or Comet 9P/Tempel (July 2005) and mentioned Comet Shoemaker-Levy 9 in several stories. Students may have seen, heard, or read about any number of comets, and will have a personal reason for their selection. Vocabulary words Astronomer A scientist who studies the universe and the celestial bodies residing in it, including their location, motion, composition, and history. Many of the scientists at the Space Telescope Science Institute are astronomers. Astronomers from all over the world use the Hubble Space Telescope. Astronomical Unit (AU) The average distance between Earth and the Sun, which is about 150 million kilometers (93 million miles). This unit of length is commonly used for measuring the distances between objects within the solar system. Comet A ball of rock and ice, often referred to as a “dirty snowball.” Typically a few kilometers in diameter, comets orbit the Sun in paths that either allow them to pass by the Sun only once or that repeatedly bring them through the solar system (as in the 76-year orbit of Halley’s Comet). A comet’s “signature” long, glowing tail Continued … is formed when the Sun’s heat warms the coma or nucleus, which releases vapors into space. Hubble Space Telescope An orbiting telescope that collects light from celestial objects in visible, nearultraviolet, and near-infrared wavelengths. The telescope’s primary mirror is 2.4 meters (8 feet) wide. It orbits the Earth about every 96 minutes and is powered by sunlight collected with its two solar arrays. Observatory A structure designed and equipped for making astronomical observations. Observatories are located on Earth and in space. Oort Cloud A vast spherical region in the outer reaches of our solar system where billions of long-period comets (those with orbital periods greater than 200 years) reside. Comets from the Oort Cloud come from all directions, often from as far away as 50,000 astronomical units (4.6 trillion miles). Education Standards Common Core Standards for English Language Arts http://www.corestandards.org/ELA-Literacy/CCRA/R/ College and Career Readiness Anchor Standard for Reading CCSS.ELA-Literacy.CCRA.R.10 Read and comprehend complex literary and informational texts independently and proficiently.