GRAIL’s
primary objective is to provide a high resolution map of the Moon’s
gravitational field and use it to gain knowledge on the Moon’s interior
structure, to increase understanding in the Moon’s thermal history, it’s
evolution and the evolution of rocky planets in our and other solar systems.
The Moon’s surface geology holds a record of almost the entire history of the solar system. The knowledge about structure of the lunar interior and it’s thermal evolution will enable scientists to reconstruct this history.
To accomplish those objectives, the following engineering objectives have to be met:
The Moon’s surface geology holds a record of almost the entire history of the solar system. The knowledge about structure of the lunar interior and it’s thermal evolution will enable scientists to reconstruct this history.
To accomplish those objectives, the following engineering objectives have to be met:
- Placing two spacecraft into identical, scientific valuable orbits around the moon at a chosen distance to one another
- Carry out delicate measurements and deleting all the noise that contaminate the precise readings of the science equipment
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Orbit
The Orbit of the spacecraft had to be chosen very carefully. It was decided that a circular, polar orbit fits the mission best. During the science orbits, the distances between the vehicles and their orbital altitudes will need to be changed at times. Flying at a close distance and a low altitude, the spacecraft will be able to determine local gravity fields and effects that smaller structures like craters and small mountains cause. Flying at a medium distance to each other and a higher altitude, GRAIL will be sensitive to regions below the Moon’s crust. At their highest distance and altitude, the orbiters will be able to measure global gravitational effects of the Moon and reveal information about the Moon’s core and lower mantle. |
 Source: NASA Numerous Maneuvers have to be performed to achieve the targeted lunar Orbit
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 Photo: Jiyang Chen |
Timing
A mission like GRAIL has to be timed carefully. It’s lunar science mission will take place between two lunar eclipses (December 10, 2011 & June 4, 2012). The spacecraft can’t go without sunlight for the extended time period when the Moon would be flying through Earth’s shadow. GRAIL’s science mission will be 82 days in duration based on a start on March 8, 2012. During those 82 days, the sun will bathe the spacecraft’s solar arrays in direct light so that sufficient power can be generated and the antennas of both orbiters can be pointed to each other. Fixed solar arrays don’t allow the vehicle to adjust to sun position while keeping their antennas aligned. A Mission Extension will allow the mission to continue several months longer. |
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Noise
Reducing Measurements
GRAIL will document the Moon’s gravitational fields by measuring their distance to each other that changes when lunar gravity decreases and increases. Changes in vehicle range rate can also be caused by a number of different factors. Those changes have to be identified and deleted from gathered data sets. Scientists have determined how much data offsets can be expected by pressure from sunlight which is the main factor of all non gravitational forces, heat radiation by the individual vehicles and the moon, gas that is vented from the vehicles in minute quantities and heat expansion and contraction on the orbiters as they pass from sunlight to darkness. The numbers have been determined and will be inserted into actual data calculations. |
 Source NASA Even slight changes in velocity/range that are not caused by gravity will be measured and have to be taken into account
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Science Objectives
1. Map the Structure of the Moon’s Lithosphere
Lithosphere is the crust and upper mantle of a celestial body that shows significant strength over a geological time frame. Lithosphere conditions and its thickness is dependent on the thermal environment and its evolution since the strength of rocks depends on different temperature levels. GRAIL will provide data on how rigid the Moon’s lithosphere was at several locations when characteristic features have formed. This will lead to information on the thermal evolution of the Moon.
 Photo: NASA |
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2. Understand the Moon’s asymmetric thermal evolution
The near and far side of the moon are very different when comparing one to another. The crust on the near side is much thinner than on the far side with only a few exceptions like the South Pole Aitken Basin and various other significant sites. Scientists desire to find out why the Moon has become asymmetric during the course of its thermal evolution.
 Photo: NASA Near Side
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 Photo: NASA Far Side
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3. Determine the subsurface structure of impact basins and the origin of mascons
'Mascons' is short for mass concentrations. Certain impact basins of the Moon show a very high gravitational pull. Scientists suspect that large amounts of mass are located underneath those basins. GRAIL will map those areas and provide more information on their origin, formation and evolution.
 Graphic: Martin Pauer |
To the left is a good example of a mascon.
The top graphic shows the topography of Mare Smythii. Below that, corresponding gravitational data is shown.
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4. Ascertain the temporal evolution
of crustal brecciation and magmatism
Past gravitational analyses of the Moon have shown that newer craters (formed less than 3 Billion years ago) have a smaller gravitational field than surrounding material while older craters present same gravitational forces as surrounding plain. GRAIL will be used to explain this phenomenon and several factors that might contribute to it.
5. Constrain deep interior structure from tides
The Moon is responsible for tides in our oceans by its gravitational force. Earth has the same effect on the Moon. Moon’s rocky surface does not show a deformation like water on Earth does. Lunar Tides are limited to about 9cm on the surface. What’s more important for GARIL: Earth’s gravity will affect the layers beneath the Moon’s surface, all the way down to its core. Different internal composition reacts differently to Earth’s pull. GRAIL will pick up those changes over different areas of the Moon. Analyses of how the Moon’s interior structure deforms at various places in the Moon’s orbit around Earth could reveal more information about the Moon’s internal composition.
Past gravitational analyses of the Moon have shown that newer craters (formed less than 3 Billion years ago) have a smaller gravitational field than surrounding material while older craters present same gravitational forces as surrounding plain. GRAIL will be used to explain this phenomenon and several factors that might contribute to it.
5. Constrain deep interior structure from tides
The Moon is responsible for tides in our oceans by its gravitational force. Earth has the same effect on the Moon. Moon’s rocky surface does not show a deformation like water on Earth does. Lunar Tides are limited to about 9cm on the surface. What’s more important for GARIL: Earth’s gravity will affect the layers beneath the Moon’s surface, all the way down to its core. Different internal composition reacts differently to Earth’s pull. GRAIL will pick up those changes over different areas of the Moon. Analyses of how the Moon’s interior structure deforms at various places in the Moon’s orbit around Earth could reveal more information about the Moon’s internal composition.
 Photo: NASA |
6. Place Limits on the Size of a
possible solid core
The Moon is believed to have a liquid core, however scientists suspect a solid core within this liquid core. GRAIL will find out if there is a solid core and by analyzing data, scientists will be able to place limits on how big it might be. The other core layers are also of particular interest to the team. Summing up all science objectives: GRAIL will map the Moon from crust to core |
Science Techniques
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When a spacecraft is in
orbit around a planet or moon, its velocity is dependent on this body’s
gravity. From one location with a higher gravitational field to another
location with less ‘pull’, the vehicle is sped up or slowed down just a little
bit. These slight changes can be measured from Earth via radio frequency
readings. This phenomenon is known as Doppler Shifting and is a part of many
science missions to other Planets, for example Juno. Why do we have to send two
spacecraft to map the Moon’s gravitational field when one would do the job just
as well? The problem with the Moon is that it only shows one side of it to us
on Earth. To measure changes in radio frequencies, a direct line of sight
between the spacecraft and Ground Stations on Earth has to be established. That
is impossible when the orbiter is in the shadow of the moon. Both GRAIL
Spacecraft track the other’s relative velocity using this Doppler Shift
technique which will show the gravitational field of the far side of the Moon.
Data from hidden passes will be downlinked to Earth when communications are
established. On the near side of the Moon, the Mission will also utilize the
traditional technique of tracking those slight changes in speed directly from Earth. Another
technique called ranging will be used. That measures the exact distance
between a ground station and a spacecraft by using radio transfer intervals.
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 Photo: NASA/JPL  Source NASA |
To increase science
turnout of the mission, GRAIL’s high resolution gravitational map of the Moon
will be compared with other data acquired by other missions. A topographic map
generated by the Lunar Reconnaissance Orbiter will allow scientists to subtract
gravitational effects that are expected from surface features like mountains or
craters from the effects that GRAIL measures to get data from subsurface layers
and interior materials. Other sources will also be incorporated in data
analyses from the GRAIL Mission.
Photo: NASA LRO
The Moon's Topography
GRAIL will provide the most accurate gravitational map of any body that has been studied before. Even Earth's gravitational studies can not be compared with the results GRAIL will provide because Earth's atmosphere prevents spacecraft from flying in low altitudes to get exact gravitational data. The mission will improve our knowledge on gravity on the far side of the Moon by a factor of 1,000. The knowledge on the well studied near side will grow by a factor of 100.
