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