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---------------------- 1923 - When will a big asteroid hit the Earth?
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- You may have missed it. About 6:00 PM last night ( January 23, 2017) an asteroid, 39 feet in diameter, flew past the Earth. It was only a miss by 46,603 miles. That is one quarter the distance to the Moon.
- Surprised? Actually so far this year ( 2017) 50 asteroids have passed us at distances closer than the Moon which is 238,855 miles away.
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- If you were to tally up all the “ discovered” asteroids in orbit within 130% the distance between the Earth and the Sun ( 1.3 AU ) then the total NEA’s identified come to 15,197.
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- This last one was 39 feet in diameter and it would likely all burn up as it plunged through the Earth’s atmosphere. If an asteroid made it to the surface it would be called a meteorite. It all depends on the asteroids speed, angle of impact, composition, and of course size if it reaches the surface.
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- The first discovery of a Near Earth Asteroid, NEA, occurred August 13, 1898. It was call 433Eros orbiting within 1.13 AU of the Sun.
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- The most threatening asteroid known was discovered in 1999. Known as “ Bennu” Its projected orbit is calculated to pass between us and the Moon in 2135. ( I will miss it, let’s hope it misses you ). Orbits can get tweaked by other gravity pass bys. A miss is not certain . If Bennu did hit us it would be at least a 3,000 megaton TNT impact.
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- Pieces of asteroids fall to the Earth “all the time”. They are called meteorites. Most that make it to the surface of Earth are composed of rock and metal.
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- Bennu is a special case. It is composed of coal-black mass of delicate organic compounds, carbon-based biochemistry. Most of this chemistry would burn up in our atmosphere. Or, if it languished on the surface for a long time it would be significantly changed by Earth’s environmental conditions.
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- The Japanese “ Hayabusa” mission in 2005 to asteroid Itokawa was designed to capture some pristine samples to study. It did manage to bring back 1,500 microscopic mineral grains collected from the asteroid’s surface. ( much less than expected ).
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- The mission to visit Bennu was launched in September 2016. It will arrive at the asteroid in August 2018, a 2 year journey. Bennu is 1,640 feet in diameter ( 0.5 km).
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- The mission has several objectives. We need to study the instabilities of its orbit to determine the likelihood of a collision with Earth in 2135. Secondly, the mission , OSIRIS-Rex, is designed to collect 60 grams of the surface. These organic molecules could become a story of the formation of our solar system and to life on Earth.
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- The organic molecules and the clay minerals on the surface could serve as feedstock for DNA, RNA proteins. Immense numbers of carbonaceous asteroids must have collided with the pre-biotic Earth.
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- Will Bennu ever collide with Earth? It circles the Sun ever 1.2 years traveling at 62,634 miles per hour. It has a close approach to Earth every 6 years, to within 186,000 miles. Bennu’s orbit will take into account “changes” in the orbit due to the gravity of the Sun, the Moon, the 8 planets, and the dwarf planets, like Ceres, and the Earth. Using these models the closest approach occurs in 2135. I will miss that one, Thank God. One of you will have to write about it.
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- The calculations put the chance of an impact at 1 in 2,700.
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- Of course, the odds of hitting the Sun are 50 - 50. And, then there are the planets Venus, Mercury, Mars, and Jupiter it also must miss.
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- Some science needed to make these collision estimates more accuratly include the “ Yarkovsky Effect”. This is the effect of the Sun’s radiation being absorbed on one side of the rotation and radiating the absorbed energy as heat back into space the other side of the rotation. This thermal radiation acts as a mini-thruster. This effect for retrograde rotations, like Bennu has, causes the asteroid to drift towards us. But, by how much?
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- Current calculations since 1999 tells us the inward drift is more than 100 miles closer. The OSIRIS-Rx mission will measure Bennu’s spin , surface area, and thermal emissions to calculate its acceleration in our direction.
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- Bennu will receive a 5-second touch-and-go maneuver to gather a sample of the asteroid’s surface. This maneuver happens at the end of a 3-year stay in orbit. The communications is 18 light-minutes between satellite and Earth. The touch-down on the surface is designed to have a maximum velocity of only 10 centimeters per second. And, the mission is designed to allow 3 of these sampling attempts.
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- In 2023 the sample of Bennu’s surface collected is to be returned to Earth. The return capsule will hit the upper atmosphere at 28,000 miles per hour. The heat shield will burn off 99 % of the reentry energy. At 2 miles altitude parachutes will be deployed. The soft landing is to occur in Utah’s West Desert. What will we learn?
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- An announcement will be made shortly, well in 8 years from now. Stay tuned. I hope we are all around to get it.
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- See also other Reviews available upon request:
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- #1871 - Asteroids and their Trojan orbits.
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- #1829 - Dwarf planets and asteroids. Also lists 11 other reviews about asteroids. Including:
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- #1825 - Asteroids responsible for evolution on Earth.
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--- Some reviews are at: -------------- http://jdetrick.blogspot.com -----
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- to: ------- jamesdetrick@comcast.net ------ “Jim Detrick” -----------
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----- 707-536-3272 ---------------- Thursday, January 26, 2017 -----
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