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- The oldest astronomical records of comets is dated 300 years before the birth of Christ. The records were unearthed in China. They were Chinese drawings of 29 comets observed in the heavens over a period of 100’s of years.
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- It was not until 1950 that astronomers first proposed that comets were actually “ dirty snowballs”, a chunk of dust and ice. As the gravity of the Sun pulls a comet closer it heats up. Gas and dust evaporate from the surface of the nucleus. A head of escaping gas called the “ Coma” creates the Head of the comet. A trail of diffuse gas and dust gets strung out by the solar wind and creates a Tail for the comet.
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- Since 1995 spacecraft have visited 7 comets. We have learned that the comet nucleus is darker than coal. It has a density of ½ that of water, 500 kilograms per cubic meter. They typically have irregular shapes and are far from perfectly spherical.
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- All types of gas are evaporated off the comet surface: water, methane, ammonia, carbon monoxide, carbon dioxide.
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- Comets contain a substantial amount of water. Therefore, we know that their formation occurred away from the Sun at orbits beyond the “ frost line”. This distance is at least 5 Astronomical Units ( 5 AU = 5 times the Earth-Sun distance.) This is about the distances of Jupiter and Pluto in today’s Solar System.
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- The theories for the formation of the early Solar System have the giant planets, particularly Jupiter, migrating outward in their orbits around the Sun. The gravitational interactions caused smaller comets to get flung into the inner Solar System. This is what caused all the crater impacts found on Mercury, Venus, Earth, Mars and our Moon. These impacts are dated to be 3.8 to 4.1 billion years ago.
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- Other comets were flung outward beyond the orbit of Neptune. Action equals reaction. So, the masses kicked into smaller orbits were matched by masses kicked into longer orbits. These outer comets are orbiting between 30 ad 50 Astronomical Units, known as the Kuiper Belt of Comets.
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- Some of the comets were flung so far as to escape the gravity of the Sun and to drift between the stars as interstellar comets. Trillion of icy comets remain 2,000 to 100,000 Astronomical Units from the Sun barely being held in orbit with the Sun’s gravity. The periods of orbits span 1,000’s to 1,000,000’s of years. These are called the Oort Cloud of Comets.
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- Trillions of these icy bodies populate the Solar System. When collisions, or near flybys , disturb their stable orbits they can get nudged into lower orbits and get pulled towards the Sun. When the Solar Wind and radiation heat up the icy snowball it can become bright with a Coma and a Tail that we can see in the night sky.
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- Asteroids and Comets are much the same. Except Asteroids have lost the water and gaseous composition and remain mostly a clump of rocks. Many asteroids can be viewed as dead or dormant comets.
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- The last good sized comet to strike Earth occurred February 2013 over Chelyabinsk, Russia. This one entered the atmosphere initially 60 feet wide. ( See Footnote 1 ). Compare this with the comet that hit us 65,000,000 years ago that was 6 miles wide. That one is credited for causing the extinction of the dinosaurs.
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- Every time you see a “ shooting star” in the night sky it is either space debris, or a comet, some as small as grains of dust.
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- We tend to view comets as a threat to life on Earth. And, they are. But, they are likely also what allowed the creation of life on Earth. When Earth first formed it was far too hot to retain any water on its surface. It was some time later when it cooled significantly and was bombarded by icy comets that water was brought to the surface of Earth.
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- Comets carried not only water but also many of the other elements needed for life. Comets contain carbon, hydrogen, oxygen and nitrogen, the seeds of life.
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- (1) The comet that flew over Chelyabinsk, Russia, in 2013 was 14.5 miles high traveling 41,600 miles per hour. The main fireball released 90 kilotons of energy. The atomic bomb detonated over Hiroshima released 15 kilotons of energy.
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- The comet, also referred to as an asteroid, slowed during its travel through the atmosphere releasing energy in heat and ionization compressing a giant shockwave. The total energy release over the entire entry was 440 kilotons of energy.
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- A comet that resides inside the orbit of Jupiter is referred to as an asteroid because the Solar winds have long since evaporated its gas and liquids leaving only the rocky core.
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- The Tunguska asteroid in 1908 leveled 770 square miles of forest and released 4,000 kilotons of energy.
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- The small fragments that reached the ground in Chelyabinsk had a density of 3.6 grams/ cm^3. Hard rock. In order to deliver 440 kilotons of energy the asteroid needed to be 60 feet in diameter weighing 11,000 tons.
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- During that very event over Russia an even bigger asteroid passed within 17,200 miles of Earth on its way to the Sun. We have some geocentric communication satellites orbiting that are 23,000 miles in altitude. This asteroid that missed us was 100 feet in diameter and weighed 40,000 tons. The two asteroids appear to not be related.
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- The Chelyabinsk asteroid was composed of Chondrites rich in silicates. 80% of all meteorites have this same composition. The larger Tunguska 1908 asteroid had chondrite composition of carbon, calcium, and aluminum. This evidence says these two asteroids were also not related.
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- The Comet ISON ( See Review #1600) is passing Earth orbit this month, November 2013, rounding the Sun, and again in December passing Earth orbit on its way back into space. It will miss Earth by 40,000,000 miles. We are getting better at spotting these visiting comets. Someday we will spot one that is on target to collide with Earth. Then, what will be do? An announcement will be made shortly , stay tuned.
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RSVP, please reply with a number to rate this review: #1- learned something new. #2 - Didn’t read it. #3- very interesting. #4- Send another review #___ from the index. #5- Keep em coming. #6- I forwarded copies to some friends. #7- Don‘t send me these anymore! #8- I am forwarding you some questions? Index is available with email upon request. Some reviews are at http://jdetrick.blogspot.com Please send feedback, corrections, or recommended improvements to: jamesdetrick@comcast.net. ---- “Jim Detrick” -- www.facebook.com, -- www.twitter.com, -- 707-536-3272 Friday, November 15, 2013
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