Saturday, January 7, 2023

3813 - MERCURY - what spacecraft are learning?

 

------- 3813  -  MERCURY  -  what spacecraft are learning?     Mercury is the closest planet to the Sun and it has suffered the consequences.  It is the densest plane made up of mostly rock and metal.  Mercury has a diameter of 3,032 miles, which is just slightly larger than our Moon.

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-------------------------  3813  -  MERCURY  -  what spacecraft are learning? 

- There are two moons around Jupiter , Ganymede and around Saturn, Titan, that are bigger than Mercury.  It would take 20 Mercury’s to weigh as much as the Earth.  So, being that small Mercury has very little gravity.  A 100 pound girl on Earth would only weigh 38 pounds on Mercury.

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-  With a radius of 1,516 miles Mercury has a  thin crust of only 72 miles (7.6%).  Below that is a rock mantel that is 373 miles thick (40%).  The rest is the core of iron and other metals which is 497 miles radius (53%).  The iron core of Mercury is 43% of its entire volume.  Compare that with Earth’s core which is 17%.  The density of Mercury is 5.3 grams per cubic centimeter.  Earth’s is 4.4 grams per cubic centimeter.

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-  The extreme density of Mercury is caused because it has lost most of its crust.  The extreme heat radiation and solar wind has evaporated than blown away much of the rocky crust .  Or,  a planetesimals has slammed into Mercury knocking most of the crust away.  Our Moon is made of Earth’s crust so it appears the a planetesimals slamming into the Earth 3,400,000,000 years ago  blasted the Moon into its orbit around us.

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-  The temperature of Mercury gets up to 806 F.  But it is not the hottest planet.  Mercury does not have enough atmosphere to retain the heat and hot gases are continuously being blown away by the solar wind.  Venus on the other hand has greenhouse gases in its atmosphere that trap the heat and temperatures there  rise to over 854 F.  Note that Mercury’s total temperature swing is over 1,100 F.

 

-  You would not think that Mercury would get very cold.  But, at its poles it is                 (-292 F.)  This is because Mercury’s rotation on its axis is exactly perpendicular to the plane of its orbit about the Sun so the poles get no direct sunlight at all.  In fact, astronomers believe the craters at Mercury’s poles are filled with water ice.  This water most assuredly was brought by comets later in its evolution because Mercury’s early history was far too hot for it to have retained water in the beginning.

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-  Mercury does have some atmosphere of hydrogen, helium, oxygen, sodium, calcium and potassium, but, it is not a stable atmosphere.  The Sun’s blast of radiation constantly evaporates the solids on its surface but the gases constantly are blown away by the solar wind.  Mercury does not have enough gravity to retain these hot gases.

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-  Mercury’s surface is all cracked from cooling down over the billions of years.  It is also cratered very similar to our Moon.  It too was bombarded by asteroids and comets some 3,800,000,000 years ago.  Like our Moon, and unlike our Earth, there are no atmospheric conditions that create erosion to make the craters disappear.  There is no process that is resurfacing the planet.

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-  In ancient astronomy Mercury was thought to be two planets, one that arose in the low western horizon at twilight and another that arose low on the eastern horizon at dawn.  Astronomer did figure it out to be the same planet that is so close to the Sun it only rises some 28 degrees above the horizon. Your fist at arms length is about 10 degrees, so, have you arm straight out level with the horizon and put 3 fists on top of each other.  That is as high as Mercury ever gets in our western and eastern skies.

-Like the dark side of the Moon, we have never seen the back side of Mercury.  In fact, even our Mariner 10 spacecraft that flew by Mercury in 1974 and 1975 only was able to map, or photograph, 45% of the planet with ½ mile resolution.                                

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-  Messenger spacecraft to arrived in 2008 and it should finally complete the job with much higher resolution.  For many years astronomers thought that Mercury was held in tidal forces with the Sun with 1 to 1 resonance exactly like our Moon.  The same side of the Moon always faces the Earth because one rotation of the Moon on its axis equals one orbital rotation of the Moon around the Earth.

-Finally, radar images discovered Mercury to have a rotation that was not 1 to 1.  Guessipi (Bebi) Columbo figured out that Mercury actually as a 2 to 3 resonance with the tidal forces of the Sun.  It orbits 2 times for every 3 rotations on its axis.  That is one and ½ Mercury days is a Mercury year.   The Mercury day is 58 days long (2/3rds of its year) and the Mercury year is 88 days long.   Mercury also has the most elliptical orbit of all the planets.  Its orbit extends to 43,000,000 miles at its farthest from the Sun and 29,000,000 miles at its closest.

-   Mercury as a small magnetic field which means it has liquid iron at its core, like our Earth. Unlike our Earth it is too small to have a liquid core due to the compression of gravity.  And, even the Sun’s heat is not hot enough for liquid metal to be at its center.  Therefore, it must be the extreme tidal forces of the Sun’s gravity that pull and stretch the planet as it goes through its elliptical orbit that frictionally creates a liquid iron core.

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-  Mercury’s elliptical orbit itself is rotating, or precessing.  If you pick the spot that Mercury is farthest from the Sun.  That spot is moving around the Sun as well. Astronomers have measured it to be moving 574 arc seconds rotation in 100 years. An arc second is about the width of a human hair held at arms length. ( One degree of arc has 60 arc minutes and one arc minute has 60 arc seconds).

-    All the astronomer’s calculations using Newton’s laws of gravity could only account for 531 arc seconds of precession.  These calculations took in the effect of gravity from the Sun, from Jupiter, from Saturn and from all the other planets that pull on Mercury.  Regardless, the math always came out 46 arc seconds off.                                                                                                                                          

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-  It was not until Einstein’s Theory of Relativity that the calculations explained the 46 arc second error.  In the 88 day orbit Mercury has a velocity averaging 107,100 miles per hour. It is traveling faster when it is closer to the Sun and slower when it is farthest away.  Einstein’s equations accounted for the fact that mass increases, lengths decrease and time slows as objects move fast relative to the speed of light, which is constant.  His equations accounted for the 43 arc seconds per century difference  Then, the fact that the Sun is not a perfect sphere, it bulges at its equator, accounted for the remaining 3 arc seconds.

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-  Astronomers had to wait for a Solar Eclipse, where our Moon blocked out the Sun in order to make some of the measurements to prove Einstein was right.  Mercury is very hard for astronomers to study because it is so close to the Sun.  The Hubble Space Telescope can not take pictures of Mercury because the Sun’s radiation would burn up the sensitive instruments.  It is very hard to send a spacecraft to Mercury as well.  To get a spacecraft away from Earth’s gravity it must be traveling 25,000 miles per hour.  Then, to catch up with Mercury that is traveling so fast the spacecraft must reach 84,500 miles per hour, which is 140,870 miles per hour relative to the Sun.

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-When the spacecraft does catch up to Mercury it is going so fast it can not slow down.  It can not brake be skimming the upper atmosphere because Mercury does not have enough atmosphere.  The spacecraft can brake using rocket thrusters, because it can not carry enough fuel to do the job.                                                                                                                            -
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-  So, what Mariner 10 did in 1974 and what Messenger spacecraft was doing is to take a longer trip and use the gravity braking of the other planets to slow it down.  This is the opposite effect of the gravity boost, or gravity slingshot, used by spacecraft going to the outer planets.  When the planet’s orbit is in the same direction as the spacecraft the pull of gravity is added to the speed of orbit to pull the spacecraft in then sling the spacecraft faster as it moves by and leaves the pull of gravity.  The opposite is to do a flyby with the spacecraft going by in the opposite direction of the planet’s orbit.  The effect then is to pull the spacecraft in then to brake the speed of the spacecraft as it leaves the gravity pull of the planet.

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-           For example:  Messenger is carrying 1,323 pounds of fuel.  It is a flying gas tank.  It was launched August 3, 2004, but there are several planet flybys needed to speed it up and slow it down before the rocket thrusters are used to put it into orbit.

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-------------  Launch -------------------------- August 3, 2004

-------------  Earth flyby---------------------- August 2, 2005,  traveling 66,629 mph

-------------  Venus flyby--------------------- October 24, 2006, traveling 78,341 mph

-------------  Venus 2nd flyby ---------------- June 6, 2007

-------------  Mercury flyby ------------------ January 14, 2008, traveling 107,098 mph

-------------  Mercury 2nd flyby -------------- October 6, 2008

-------------  Mercury 3rd flyby-------------- September 30, 2009

-------------  Mercury orbit ------------------ March 18, 2011

-The flybys to Mercury will be 125 miles above the surface.  It will eventually be a 12 hour long elliptical orbit coming within 125 miles of the surface and as far out as 9,440 miles from the surface.  We will be learning starting next year more about Mercury and will be able to photograph the backside of Mercury for the first time.
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-  There is a ring of mountains on this side of Mercury that may suggest that a very large meteorite hit the opposite side of the planet and sent a shockwave through the planet to create the mountains on this side.  This could explain why the planet is so dense and has so little crust.  That big of a hit would have blown most of the crust into space.  We may soon know if this is true.

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-  January 7, 2023   MERCURY  -  what spascecraft are learning?          815    3813                                                                                                                                                                                                                                                               

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--------------------- ---  Saturday, January 7, 2023  ---------------------------

 

 

 

 

           

 

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