--------- #1316 - Comets, The Five We Have Visited?
- Attachment: Comets forming in accretion disk
- If you want to know what a comet is made of you have to go visit one, better yet visit 5. We have sent 4 separate spacecraft to visit comets. (See #1315 about visiting asteroids). These comets have been orbiting the Sun since the beginning of the Solar System and have pretty much remained unchanged. Studying them is like studying the raw material that our early Solar System evolved from. ( See #1303 How did the Earth get its water?)
- The Solar System evolved from a rotating disk of dust and gas. A cloud that gravity gradually pulled together. 99% of it ended up in the Sun But, some pieces broke off to form the planets and even smaller pieces formed the asteroids and the comets. The massive object at the center got so dense it ignited into nuclear fusion. The radiation of the nuclear fusion stopped the further collapse of gravity. The inner circle of the disk contained the heavier material that was most influenced by the pull of the Sun. This material became the 4 rocky planets and the rocky Asteroid Belt. The outer circle contained the more rarified gaseous clouds and formed the 4 gaseous planets and the Kuiper Belt of comets and a few Dwarf Planets including Pluto.
- The edge of the Asteroid Belt beyond Mars is called the “snowline” where liquid water began to freeze due to its distance from the Sun. Hydrogen gas freezes only at extremely low temperatures, so it remains a gas. Oxygen the same. But, when hydrogen and oxygen atoms combine into the molecule of water they form H2O. The snowline outward is where these water molecules began to freeze solid. These icy objects extend all the way out to the Oort Cloud that extends 5,000 to 100,000 AU distances from the Sun. 100,000 AU is 100,000 times farther than the Earth-Sun distance of 93,000,000 miles.
- The spacecraft mission to the Comet 81P/Wild found silicate grains that originally formed close to the Sun. The conclusion from this find was that the comet nuclei are formed from multiple locations in the proto-planetary disk, not just from debris in their current orbit. Some comets had unusual elliptical orbits bringing them closer and farther from the Sun.
- The relative abundance of different elements and molecules changes with the distance from the Sun. This can tell astronomers where the comets first formed. One of the chemical signatures used is the ratio of Carbon-2 and Carbon-3 to the amount of water, H20. The carbon likely came from the molecule acetylene (C2H2) Knowing this ratio tells us the comets first orbit.
- The spacecraft that visited Comet 9P/Tempel sent along a bullet to fire into the Comet surface and measure the debris that flew out of the crater. The crater was 165 to 500 feet in diameter. It was a big bullet. Some 5,000 to 10,000 tons of ice emerged from the impact along with an equal amount of rocks and dust. The debris was found to have an abundance of ethane (C2H6). As the debris fell back to the surface careful measurements allowed the calculation of the force of gravity and therefore the mass of the comet. Once the mass was known the calculation for density discovered the comet to be quite porous. Calculations showed 75% porous material extended 66 feet deep below the surface. Another conclusion made was that the comet was loosing 8 to 20 feet of surface material with each orbit around the Sun.
- Another spacecraft visited the Comet 103PHartley in November, 2010. Strangely it found softball-size chucks of ice surrounding the nucleus. These particles were 19 miles above the surface orbiting at 4 miles per hour. Another surprise was the high ratio of carbon dioxide to water ice. When findings do not match expectations it is the surprises that prove most interesting.
------------------------------------MISSION ----------- COMET ------------ DIAMETER
--------------- 1985 ----------- Giotto -------------- 1P/Halley ------------- 9.3 miles
--------------- 1998 ----------- DeepSpace 1 ------- 19PBorrelly ---------- 5.4 miles
--------------- 1999 ------------ Stardust ------------- 81P/Wild ------------ 3.4 miles
--------------- 2005 ------------- Deep Impact ------- 9PTempel ------------ 4.7 miles
--------------- 2007 ------------- Stardust-NExt------ 9PTempel ----------- 4.7 miles
--------------- 2010 ------------- EPOX1 ------------- 103PHartley --------- ?
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707-536-3272, Tuesday, November 1, 2011
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