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--------------------------- 1871 - Asteroids and their Trojan orbits..
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- 4.6 billion years ago the Solar System was a tumultuous landscape of giant planets mixed with small planets, asteroids , and comet nuclei. The bigger planets consumed millions of these smaller bodies. The Earth consumed such a large body the impact “splashed” the Earth’s surface into orbit that eventually coalesced in to our Moon. Our Moon is made of Earth earth.
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- Some of these smaller bodies were still orbiting in our Solar System. Some have acquired “ stable ‘ orbits and are known as Trojan asteroids. These small bodies occupy gravity balancing points 60 degrees ahead and behind our planet’s orbit. Called the Lagrangian Points because Joseph - Louis Lagrange first discovered them.
(See Footnote 1)
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- In 1765 Leonhard Euler defined these gravity balancing points mathematically. There are 5 balance points. 3 are in “ convex stability”. One point lies between the Sun and the planet (L1), a second just beyond the planet in line with the Sun (L2), a third 180 degrees on the opposite side of the Sun (L3). These 3 balancing points are “ convex” because the slightest shift away and these asteroids will leave their stability regions.
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- The 4th and 5th points are in “ concave stability”. These balance points are preceding and trailing the planet by 60 degrees. These points are “ concave” because if they shift away they will tend to return to the stability area. They can remain in these stable orbits over the age of the Solar System.
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- The first Trojan asteroid was discovered to share Jupiter’s orbit in 1906. Today 6,300 Jupiter Trojans have been identified. Saturn and Uranus have no known Trojans likely because of the massive gravity pulls of Jupiter and Neptune on either side. Earth lacks Trojans because of the pull of Venus, Mars, and Jupiter.
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- Although Earth has captured a small asteroid a few thousand years ago in the L3 balance point. Mars has a few Trojans at L4 and L5 balance points.
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- Studying these balancing orbits astronomers are convinced that the major planets did not always have today’s circular orbits. The early Solar System was a chaotic place and the gas giants were closer together in their orbits. Trojans were captured when the friction of gas drag, or collisions , brought them into a balance region with the right orbit speed.
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- As the gas giant planets navigated out from where they first formed their gravity scattered smaller objects allowing more Trojans to be captured. Planet - planet interactions even suggest there was a massive, eccentric 9th planet a few hundred AU’s from the Sun ( AU = Earth - Sun distance)
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- A pair of asteroid binaries orbiting each other allow the masses to be calculated. Estimating their size gives a density calculations of 0.8 grams per cubic centimeter (water is 1.0 gm/cm^3). This would suggest asteroids are mostly porous ice. This also suggests they are more like comets and less like rocky asteroids. This in turn suggest these Trojans originated in the outer Solar System and migrated inward.
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- Trojans will continue to be studied to help us understand the origins and evolutions of the planets. We live one of eight , or is it nine? We will soon be visiting two asteroids orbiting between Earth and Mars, named Bennu ( ¼ mile across) and Ryugu ( ½ mile across).
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- The NASA mission will visit Bennu in 2019. The Japanese mission, Hayabusa 2, will land 3 times on Ryugu. The first landing in 2018 will come a year before NASA gets to Bennu. Each mission will spend 1.5 years studying the asteroid. Each will attempt landing, collecting samples, and returning them to Earth.
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- The 2 missions hope to learn more about possible asteroid impacts on Earth and secondly how likely life could have evolved due to the asteroid impacts on Earth.
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- Asteroid trajectories are difficult to predict due to the “ Yarkovsky Effect”. The asteroid is rotating. It absorbs sunlight from one direction. The asteroid surface heats up, then radiates infrared energy in a different direction. This release of heat can push the asteroid off its original trajectory. The degree of the effect depends greatly on the composition of the asteroid surface.
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- Each mission has a different method in collecting the composition samples. NASA (OSRIS-Rex) in 2019 lands on Bennu’s surface and extends a mechanical arm that will release a jet of nitrogen gas. Loose rocks and grains will be collected in a chamber on the end of the arm. It plans to collect between 2 ounces and 4 pounds of different sized particles. It is designed to attempt this same landing 3 different times if necessary.
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- The Japan mission to Ryugu , Hayabusa 2, plans to land 3 times on different sites to learn of any variations in surface composition. Japan has done this once before when Hayabusa 1 visited asteroid 25143 Itokawa returning a handful of pebbles in June 2010.
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- Hayabusa 2 will fire a bullet into the asteroid and collect the debris. On the 3rd landing they intend to use a 10 pound explosive to carve a 33 foot crater. The spacecraft plans to be on the opposite side of the asteroid when the explosion occurs, leaving only a camera, then swoop around to collect the freshly exposed rock.
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- In addition to learning enough to better predict the motion of asteroids, 100’s of years into their future to avoid Armageddon, science hopes to learn if life on Earth may have come from space. The theory is that desert Earth gained its oceans from the arrival of ice-laden meteoroids. This water may have contained the first organic molecules.
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- Comets that originate beyond Neptune are unlikely carriers because measurements of water vapor in the tails of these comets are different nuclei, containing ah higher ratio of deuterium ( hydrogen with a neutron).
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- Bennu and Ryugu are carbonaceous chrondites formed in the early Solar System unchanged for 4.5 billion years. We would like to learn if their molecules are the same as found on our planet.
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- Both missions NASA and JAXA have very active websites if you yearn to learn more. The idea of what they bring home could tell us about life on Earth in the past and life on Earth if our future.
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- In the meantime we can learn more from meteorites that visit our planet. Meteorites are meteors that avoid burning up in the atmosphere and manage to reach the surface of Earth. Lawrence Livermore Labs just received two walnut size meteorites the are discovered in the Antarctica.
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- If we are going to be successful in deflecting an incoming asteroid from impacting the Earth, we need to learn more about their composition. These meteorites will be cut and polished down to a thickness of a few hundred microns. Then, they will receive a nanosecond laser pulse that will vaporize the sample. Detailed spectroscopy data will be collected to understand their composition.
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- To deflect an Earth-bound asteroid a nuclear explosion or a hypervelocity projectile will be sued. The goal is not to destruction but a nudge to redirect its trajectory. Knowing the composition will allow scientists to design the best plan to deflect it without shattering it. A near miss rather than hitting the Earth with buckshot.
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- NASA has identified 14,000 of these near Earth objects. ( discovering 1,500 per year). Already 1,600 have been identified as “ potentially hazardous”. Hopefully our planning and research will be completed before we need to use it. Stay tuned, an announcement will be made shortly.
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- Request these Reviews to learn more:
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- #1829 - Dwarf planets and asteroids. Pluto has greatly exceeded expectations with its diversity of land forms and environmental processes. This review lists 11 other reviews about asteroids.
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- Footnote: (1) Joseph Louis Lagrange ( 1736 - 1813) was French but raised in Italian Piedmont. Youngest of 11 children. By age 18 he was teaching geometry in Turin. He became the head f the Berlin Academy at age 40. He published “Analytical Mechanics” in 1788. Newton’s law of gravity works with two bodies. Lagrange worked out the math for 3 bodies. He named them the “ Trojan System”. In 1795 he helped devise the “ metric system of measurements”.
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