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Mars Global Surveyor (MGS)
MGS was the first launched spaceecraft of NASA's Mars
Exploration Program.
It was launched on 7 Nov. 1996.
MGS arrived Mars on 11 Sep. 1997, after the Mars
Pathfinder. But its mapping mission is not started yet. At this moment, MGS is
flying around and around Mars to lower its orbit.
The mapping phase of MGS will begin in mid-March 1998.
The journey of MGS towards Mars
JMGS
travelling from Earth to Mars
Launch
The 1,062 kg MGS was lunched on 7 Nov. 1996 aboard a
Delta-7925 rocket.
The Delta rocket is made by McDonnell Douglas
Aerospace. It is a conglomeration of 3 smaller rockets stacked on top of each other and
with the MGS spacecraft at the top.
The Delta weighed 231,325 kg at the time of lanuch and
stood 37 meters tall. An enormous amount of propellant is neeeded to boost the spacecraft
into space.
MGS
Launched by Delta II
This was the first use of the low cost Delta to send a spacecraft to another planet (It will save about $300 million as compare with the Titan launch vehicles used by previous Mars missions).
Cruise Phase
MGS travelled 750 million km over the course of a 300-day
cruise and reached Mars on 12 Sep. 1997.
During this cruise period, MGS coasts around the Sun in an
orbit that will intercept Mars 10 months after launch.
To imagine that if anybody could throw a baseball from San
Francisco to New York and hit the torch on the Statue of Liberty? That is the
accuracy needed for this cruise.
So the navigation team did 4 times Trajectory Correction
Maneuvers to guide the cruise. Each correction involves a short firing of MGS's rocket
engine to gently change the spacecraft's speed and direction of travel.
Mars Orbit Insertion
Upon reaching Mars, MGS fired its main rocket engine for
the 25-minute Mars orbit insertion (MOI) burn. This maneuver slowed down the spacecraft
and allowed Mars' gravity to capture it into orbit.
This operation was successful. MGS is now flying around
the red planet in an elliptical orbit .
Aerobraking Phase
After orbit insertion, MGS will perform a series of orbit
changes to lower the low point of its orbit into the upper fringes of the Martian
atmosphere at an altitude of about 110 km.
During every atmospheric pass, the spacecraft will slow
down by a slight amount because of air resistance. MGS will have to use this trick over 4
months to lower the high point of its orbit from 56,000 km to altitudes near 400 km.
Finally MGS will circle Mars at an average altitude of 378
km.
Mapping Phase
The mapping phase of MGS mission will begin in mid-March
1998 and will last for one Martian year (687 Earth days) until late Jan. 2000.
During mapping operations, the spacecraft will circle Mars
once every 117 minutes and 39 seconds.
It will pass over both the north and south polar regions
of Mars on every revolution around the planet.
This polar orientation will allow the MGS scientific
instrumants to image the entire Martian surface once every 7.2 Earth days as Mars rotates
under the orbit.
Through mapping , MGS will always travel southward over
the day side of Mars and northward over the night side. The orbit is designed so that
every time the spacecraft flies over the equator on the day side, the local Mars time on
the ground will always be 2.00 pm. This orbit characteristic is called "Sun
sunchronous".
The "afternoon orientation" of the orbit will
ensure that the shadows, lighting conditions, and heat properties of the ground and air
will remain as constant as possible throughout the course. Hence it will provide a common
frame of reference for scientists to compare data from different parts of the planet.
After . . .
After the mapping is finished in Jan. 2000, MGS will
function as a communications satellite to relay data back to Earth from surface landers
launched as part of future Mars missions.
Scientific Instruments
The 2 years MGS mapping operations will collect an
enormous amount of data, enough to fill over 130 CD-ROMs.
It will contribute to an extremely comprehensive study of
the martian atmosphere, surface features, mineral distribution, and magnetic properties
etc.
MGS carries 6 main scientific instruments:
Mars Orbiter Camera (MOC)
This is a dual-mode camera.
In narrow-angle mode, its high-resolution telephoto lens
will spot Martian objects as small as 1.4 meters across. these pictures will be sharp
enough to help scientists conduct detailed geological studies without setting foot on the
planet.
Its wide-angle, global monitoring mode will use a
"fish-eye" lens to generate colourful panoramic images spanning from horizon to
horizon. These pictures will resemble weather photos of Earth shown on the night news
broadcasts.
Mars Orbitor Laser Altimeter (MOLA)
This laser altimeter works by measuring the time that a
pulse of light takes to leave the spacecraft, reflect off the ground, and return to
MOLA's collecting mirror.
By multiplying the reflection time by the speed of light,
scientists will be able to calculate MGS's altitude above the local terrain to within 30
metres.
As the spacecraft flies above hills, valleys, craters and
other surface features, its altitude above the ground will constantly change.
A combination of MOLA data with images from the camera
will allow scientists to construct a detailed topographical atlas of mars.
Thermal Emission Spectrometer (TES)
This instrument will conduct infra red scans of the
planet. It will image Mars in regions of the energy spectrum that humans cannot see.
These thermal data from TES will allow scientists to
determine the general mineral composition of patches of ground as small as 9.0 square km
in area.
Eventually to obtain a planet-wide mineral survey of Mars.
TES also will scan the martian atmosphere to provide data
for the study of the clouds and weather.
Hopefully that TES will ultimately yield clues into the
location of clays containing carbonate minerals. And the future missions to Mars might
begin searching for fossil remains of life in areas identified by TES data.
Magnetometer and Electron Reflectometer
This instrument pair will attempt to measure the poorly
understood global magneetic properties of Mars.
Previous data indicates that the Martian magnetic field is
extremely weak and almost non-existent.
Such a sttudy might yield insight into tthe history of the
geophysical forces that shaped Mars and how those forces differed from Earth.
The magnetometer is not attached to the main body of the
spacecraft. Instead, each of the magnetometer's two sensors sit at opposite ends of the
spacecraft's two solar arrays. This placement ensures that the data generated from the
magnetometer sensors will not be "polluted" by the magneetic signal from the
spacecraft.
Mars Relay
This cylindrical-shaped antenna will focus on collecting
data transmitted to MGS from landers on Martian surface, and then transmit the data back
to Earth.
In this case, the future Mars landers will not need to
carry a large antenna anymore.
The relay operates at a UHF frequency of 437.1 MHz
and can listen to stations on the Martian surface up to 5,000 km away from MGS.
In Dec. 1999 NASA will use the relay antenna for the first
time.
Radio Signals
MGS carries a tiny device called Ultra-Stable-Oscillator
(USO). This unit acts as an electronic clock that will allow the spacecraft radio to
broadcast signals at an extremely precise frequency.
When in orbit, tiny variations in the strength of tthe
Martian gravity caused by small "blemishes" in the planet's shape will bump the
spacecraft slightly from its anticipated path. During these bumps, the "tone" of
MGS's radio signal will vary by a slight amount .
By analyzing these tiny changes, scientists will be able
to determine Mars' shape more accurately than ever before.
This data will be combined with the MPLA datta to improve
the accuracy of the topographical maps.
In addition, as MGS passes over the day side of Mars and
approaches the night side, the planet will block radio signals from reaching Earth. An
analysis of the distortion of the signal's sttrength and tones as it fades and reappears
will enable scientists to determine the atmospheric pressure at a specific location on
Mars.
When combined with data from TES, scientists will be
able to gain a greater understanding of the Martian atmosphere than ever before.
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