Download Asteroids, Comets, and Meteoroids

Among the planets in the solar system are countless numbers of asteroids, comets, and meteoroids. Let’s take a look at how these solar system objects are different from planets. Asteroids Asteroids are metallic, rocky objects in space. They have no atmospheres and move in independent orbits around the Sun. Tens of thousands of asteroids are found in an area called the asteroid belt – a vast, doughnut-­‐shaped ring located between the orbits of Mars and Jupiter. Some scientists theorize that asteroids may be pieces of a planet that never formed because Jupiter’s great mass exerted too much gravitational force to allow the pieces to combine into one planet. Because asteroids are too small to be classified as planets, they are often called “minor planets”. Some asteroids are the size of a small building. Ceres was the first asteroid scientists observed. Discovered in 1801, it is about 1000 kilometers across and one of the largest known asteroids. Most asteroids are actually less than a kilometer wide. If we could combine all of these asteroids, they would be smaller than half the size of the Moon. Comets A comet is a mass of frozen gas, cosmic dust, and ice crystals. Comets are often described as “dirty icebergs”. They circle the Sun in long, narrow orbits, mainly located in the cold outer reaches of our solar system. They orbit the Sun in the Kuiper Belt, which begins just past Neptune. A trillion more comets may live even farther out in a cold area called the “Oort Cloud.” Some comets leave their orbits in the Kuiper Belt or the Oort Cloud and journey toward the Sun. When a comet flies near the Sun, its ice begins to “boil” away. As it vaporizes, a tail of glowing gases and dust forms behind it, always pointing away from the Sun. If Earth happens to pass through comet dust, burning particles can be seen streaking through the sky in a spectacular display called a “meteor shower”. Meteoroids, Meteors, & Meteorites Meteoroids are pieces of rock and metal dislodged from comets, planets, asteroids, or moons. Most meteoroids are made up of dust-­‐sized particles. When a meteoroid enters a planet’s atmosphere, it burns up due to friction. As it burns, a meteoroid creates a bright streak of light in the sky that we call a “meteor”. Sometimes large meteoroids do not burn up completely – one may make it all the way through a planet, moon, or asteroid’s atmosphere and land on its surface, after which it is called a “meteorite.” A Fiery Necklace It’s not often that you get to see a comet strike a planet. Until July 1994, only one comet strike had ever been observed. That was in 1178, when five English monks reported seeing “a flaming torch” on the Moon “spewing out fire, hot coals, and sparks.” Modern astronomers have confirmed that those monks had seen a comet or a small asteroid hit the Moon, forming the crater that is now named Giordano Bruno. Those 12th century monks weren’t equipped with cameras. But in 1994, astronomers all over the world got a chance to see and photograph a similar event when Comet Shoemaker-­‐Levy 9 collided with Jupiter. In 1993, astronomers Eugene and Carolyn Shoemaker, a husband-­‐and-­‐wife team, and David Levy discovered the comet Shoemaker-­‐Levy 9. The three astronomers were working at the Mr. Palomar Observatory in California. New comets are discovered all the time, but this one made headlines. Shoemaker-­‐Levy 9 was a comet that had been ripped to pieces. Instead of being a single ball, the comet was made up of 21 fragments, each one trailing a large cloud of ice and dust. It looked like a fiery necklace blazing across the night sky. The astronomers calculated that about 9 months before they spotted it, Shoemaker-­‐Levy 9 had passed within about 21,000 kilometers of Jupiter. Jupiter’s gravitational force (2.36 times that of Earth’s) had pulled the comet apart. Next came even bigger news – the pieces of Shoemaker Levy were on a collision course with Jupiter. Astronomers predicted that Jupiter’s gravitational force was about to grab those fragments, once and for all. That set the stage for one of the most photographed events in astronomical history. Between July 16 and July 22, 1994, Shoemaker-­‐Levy 9’s fragments hit Jupiter’s upper atmosphere one by one. Virtually every large telescope on Earth recorded the collisions. The Hubble Space Telescope recorded the event as it orbited Earth, as did the Galileo spacecraft, which was on its way to Jupiter. The 21 fragments hit Jupiter at speeds of more than 60 kilometers per second. The impacts created plumes of hot gas that rose thousands of kilometers high. They left marks on the planet’s surface that lasted for nearly a year. The pieces plowed into the planet’s atmosphere with enormous energy. One astronomer described the impacts as “ringing Jupiter like a bell.”