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---------------------- 2332 - Asteroids, Comets and Russian Roulette
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- The Sun’s gravity pulled the heavier elements toward the center. That is why the four inner planets are terrestrial planets and the four outer planets are the gaseous plants. ( See Review #29, “ Our Gaseous Planets”). Most of the debris in asteroids and comets is unchanged remnants from the formation of our solar system.
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- Asteroids are the heaviest debris and formed between the orbits of Mars and Jupiter. There are over one million asteroids over one kilometer in diameter rotating around the Sun in this region. Most of these asteroids are separated by over one million kilometers, so the asteroid belt is not a dense region as it is often depicted in the movies. All together the mass of all the asteroids would equal 0.1% the mass of the Earth. Asteroids are composed of Carbonaceous, silicate, and metallic material, the heavier elements.
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- On March 18,2004 a 100 foot diameter asteroid passed within 26,500 miles from Earth. That is between Earth and the Moon which is 231,000 miles away. This is the closest recorded near miss by any space rock. The asteroids name is 2004 FH. It made its closest approach at 2:08 pm on March 18th over the southern Atlantic Ocean. If you were in the southern hemisphere at the time you could have seen it with binoculars.
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- Comets were formed much further from the Sun and are composed of the lighter elements. They are frozen liquids of water ice or carbon dioxide ice. The short-period comets are located outside the orbit of Neptune, between 30 and 50 astronomical units (AU = Earth-Sun distance).
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- These comets are distributed in the flat ring of the accretion disk rotating around the Sun with periods of less than 200 years. This region is known as the Kuiper belt of comets. It is estimated to contain over 70,000 comets with diameters greater than 100 kilometers.
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- The long period comets are in a far different region, located 50,000 to 100,000 astronomical units from the Sun. This debris was never part of the accretion disk that formed the solar system and it remains a spherical shape around the Sun. This region is known as the Oort Cloud and it likely contains trillions of icy ball comets. The average period for a long-period comet to orbit the Sun is 11 million years.
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- The nearest star is Proxima Centauri is 4.22 lightyears away. This is equivalent to 267,000 astronomical units. The Oort Cloud extends out to 100,000 AU, nearly half this distance. If a star passes within 3 lightyears, or 190,000 AU, its gravity is strong enough to slow the orbits of a few comets causing them to lose their energy and fall into a long elliptical orbit around the Sun. This event could likely happen to 10 stars every one million years.
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- As a comet approaches the Sun during one of these long elliptical orbits it begins to melt forming a gas tail that stretches out pointing directly away from the Sun. It is the solar wind that stretches the tail out as long as one astronomical unit.
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- The nucleus of the comet is a few kilometers across of frozen ice. The hydrogen gas coma that surrounds the nucleus is over one million kilometers across. And, the tail at 1 AU is 1,500,000,000 kilometers long.
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- If any of this asteroid or comet debris gets into the Earth’s atmosphere it is called a meteor. If it gets through the atmosphere and lands on the Earth’s surface it is called a meteorite. About 100 tons of meteorites accumulate on the Earth’s surface every day. Most of them are as small as a grain of sand.
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- There are three types of meteorites: Iron, Stony, and Stony-Iron. The iron meteorites are easiest to recognize because they are heavy for their size and contain crystal patterns of iron in a slice or etch made in its surface.
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- The stony meteorites are the most common, representing 95% of the total that fall to Earth. However, they are the hardest to recognize. They appear as ordinary rock, but have rounded particles embedded in them called chondrules. These chondrules can contain high levels of carbon and often amino acids, the building blocks for proteins.
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- The stony-iron meteorites have pieces of metal embedded in the ordinary silicate rock. Most meteorites likely come from the asteroid belt because asteroids are large enough to have held their heat for millions of years. Long enough for the materials to differentiate with the iron falling to the center, surrounded by stony-iron, then the stony crust.
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- When these asteroids collide all three types of meteors are formed. Some of these fragments eventually find there way into Earth’s atmosphere to become the three different types of meteorites we see as shooting stars.
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- Over 8,000 of the 1,000,000 asteroids have been named and cataloged with well-determined orbits. Ceres is the largest with a diameter of 900 kilometers. Pallas and Vesta are next biggest asteroids with diameters of 500 kilometers. The Galileo spacecraft took a picture of Gaspra in 1991 on its way to Jupiter. Gaspra is 20 kilometers across. Some asteroids are Earth-crossing and potential impactors.
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- About 500 Earth-crossers are known. Most of these are small, less than 40 kilometers across, likely to strike the Earth over the next 20 to 30 million years. Only 1% or the asteroids over 1 kilometer have been cataloged.
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- A 1000 kilogram rock ( 2,205 pounds ), traveling at 30 kilometers / second ( 2,237 miles per hour ) contains the kinetic energy of a 225 megaton warhead.
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---------------------------------- Kinetic Energy = ½ * mass * velocity^2
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------------ Kinetic Energy = ½ * 1000 * (30,000)^2 kilogram* meter^2/second^2
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--------------------------------- Kinetic Energy = 9 * 10^11 joules
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--------------------------------- One megaton of TNT = 4 * 10^9 joules
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--------------------------------- Kinetic Energy = 225 Megatons of TNT
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- In January 2000 a 3 meter meteorite exploded over Canada’s Yukon Territory with a force of 5,000 tons of TNT. In 1908 a 60 meter rock exploded over the Tunguska region of Siberia with a force of 10 megatons of TNT. The chance of a similar event occurring in the next 100 years is about 10%.
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- Comets constitute only 1% impact threat to Earth. It is the asteroids that are the greater threat and studies are proceeding to use an unmanned space tug to rendezvous with incoming asteroids, attach to their surface, and slowly push them to where they miss Earth.
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- By 2015 NASA plans to test such a spacecraft on a non-threatening asteroid to prove this capability. The Earth moves along its orbit at 12,800 kilometers per second. It takes 215 seconds to move half its Earth diameter. The spacecraft would have to change the asteroid’s orbital period so its rendezvous site is at least 215 seconds earlier or later than Earth’s arrival.
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- Changing the asteroid’s velocity only one centimeter per second 10 years before its expected impact would cause it to arrive 225 seconds before Earth does. If the asteroid is closer, then we would need to give the rock a bigger push with the ion-propulsion thrusters on the spacecraft.
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- How would you like to be the engineer designing this space mission? No pressure!
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- April 13, 2019 30
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-------------------------- Saturday, April 13, 2019 --------------------------
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