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GRAIL
Gravity Recovery and
Interior Laboratory
GRAIL
NASA Aims to Look Inside the Moon
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Scientists have long known that the moon's
gravity field is strangely uneven and tugs on
satellites in complex ways. Without course
corrections, orbiters end their missions nose
down in the moon dust! In fact, all five of
NASA's Lunar Orbiters (1966-1972), four
Soviet Luna probes (1959-1965), two Apollo
sub-satellites (1970-1971) and Japan's Hiten
spacecraft (1993) suffered this fate.
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The source of the gravitational quirkiness is a
number of huge mascons (short for "mass
concentrations") buried under the surfaces of
lunar maria or "seas." Formed by colossal
asteroid impacts billions of years ago, mascons
make the moon the most gravitationally lumpy
major body in the solar system. The anomaly is
so great … half a percent … that it actually
would be measurable to astronauts on the lunar
surface.
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A plumb bob held at the edge of a mascon
would hang about a third of a degree off
vertical, pointing toward the central mass.
Moreover, an astronaut in full spacesuit and
life-support gear whose lunar weight was
exactly 50 pounds at the edge of the mascon
would weigh 50 pounds and 4 ounces when
standing in the mascon's center.
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This is a gravity map
of the moon made by
the Lunar Prospector
spacecraft in 1998-99.
Mascons are shown in
orange-red. The five
largest all correspond
to the largest lavafilled craters or lunar
"seas" visible in
binoculars on the near
side of the Moon.
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Created in 1992, NASA's “Discovery
Mission” program sponsors a series of
scientist-led, cost-capped solar system
exploration missions with highly focused
scientific goals. Proposals are evaluated for
scientific merit, science implementation
merit, and technical, management and cost
feasibility
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On December 11, 2007, NASA
announced the selection of GRAIL in
NASA's “Discovery Mission” selection
process. GRAIL beat out 23 elaborate
competitors and will cost at least $375
million for the whole package —
design, launch, and staffing included.
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The study technique GRAIL will use was
pioneered by the joint U.S.-German Earth
observing GRACE (Gravity Recovery and
Climate Experiment) mission launched in 2002.
The GRACE satellites measure gravity changes
related to the movement of mass within the
Earth, such as the melting of ice at the poles and
changes in ocean circulation. As with GRACE,
both GRAIL spacecraft will be launched on a
single launch vehicle.
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GRAIL will fly twin spacecraft, in
tandem orbits, around the moon for
several months to measure its gravity
field in unprecedented detail.
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Mission Objectives:
GRAIL has the following two primary science
objectives:
• Determine the structure of the lunar
interior, from crust to core
• Advance understanding of the thermal
evolution of the Moon
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Additional mission objectives include:
• Extend knowledge gained on the internal
structure and thermal evolution of the
Moon to other terrestrial planets
• Reduce risk to future lunar robotic or
human science and exploration missions
by providing a high resolution, global
gravity field that will eliminate gravity
uncertainties for precision lunar
navigation and landings
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When it comes to understanding what a
moon or a planet is made of remotely —
short of touching it or placing seismometers
on its surface or probes below the surface —
classical physics comes to the rescue. By
measuring the magnetic and gravitational
forces that are generated on the inside and
manifested on the outside of a planet, or
moon, we can learn volumes about the
structure of its interior.
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GRAIL will measure gravity at different
points around the moon, which should reveal
any gravity differences as slight as 1 million
times weaker than Earth's overall gravity.
The data will be about 1,000 times better than
any other measurements of lunar gravity.
Such data can be processed to peer beneath
the moon's surface and locate any significant
structures related to early lunar history.
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A spacecraft in the proximity of the moon can detect
these forces. In the case of gravity, the mass of the
moon will pull on the spacecraft due to gravitational
attraction. If the spacecraft is transmitting a stable
radio signal at the time, its frequency will shift by an
amount exactly proportional to the forces pulling on
the spacecraft.
This is how we weigh the moon and go further by
measuring the detailed distribution of the densities
of mountains and valleys as well as features below
the moon’s surface. This collection of information is
called the gravity field.
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What is GRAIL?
GRAIL stands for Gravity Recovery and
Interior Laboratory. Mission planners will send
twin satellites into orbit around the Moon to
map its quirky gravitational field in
unprecedented detail. By revealing the Moon's
inner structure, GRAIL will shed light on how
the Moon (and, by extension, the Earth and
other rocky planets) formed.
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Who is coordinating
the GRAIL mission?
.
GRAIL is a NASA mission.
The leading scientific
investigator is Maria Zuber,
a geophysics professor at
the Massachusetts Institute
of Technology (MIT).
NASA's Jet Propulsion
Laboratory (JPL) in
Pasadena will manage the
mission and develop the
scientific instruments.
(continued)
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(continued)
Lockheed Martin Space Systems of Denver
will build and operate the spacecraft.
Educational and public outreach for the
mission is led by Dr. Sally Ride, the former
astronaut, through Sally Ride Science, her
science content company.
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What is the timing of the GRAIL mission?
The two satellites, called GRAIL-A and
GRAIL-B, are scheduled to be launched
aboard a robotic Delta II rocket in September
2011. The rocket will follow an indirect route
that will allow it to save fuel by taking
advantage of the Earth's and Moon's gravity.
When the rocket nears the Moon in January
of 2012, the two satellites will be boosted into
orbit.
(continued)
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(continued)
They will send back information during a
three-month "science phase" of the
mission from March through May of 2012.
After their work is done, the satellites
eventually will crash into the Moon's
surface.
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How will the GRAIL satellites map the Moon's
gravity?
The twin satellites will fly in tandem orbit
around the Moon. As they travel, they will
exchange microwave beams to measure the
precise distance between the orbiters. As
one satellite passes through an area of
greater or weaker gravity, . . .
(continued)
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(continued)
. . . the distance between the two will expand
or shrink slightly, and these measurements
will be beamed to Earth. Researchers will use
the fluctuations to map the Moon's
gravitational field with a degree of precision
never before achieved.
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How high and how far apart will the satellites
travel?
The satellites will orbit at an altitude of 50
kilometers, or about 30 miles. The distance
between them will fluctuate from 175 to 225
kilometers, or about 110 to 140 miles.
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Is the GRAIL mission similar to the GRACE
satellite mission?
Yes, GRAIL is a lunar version of GRACE, an
ongoing mission in which twin satellites
orbiting the Earth send back data that allow
researchers to monitor the planet's changing
gravitational field. Some of the scientists
involved in GRACE, which began in 2002,
are taking a role in GRAIL.
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What do we know
about the Moon's
gravitational field?
As noted earlier in this
presentation, scientists
know the Moon has an
extremely uneven field
of gravity. The
mascons tug on
spacecraft, so that
unless course
corrections are made,
orbiters eventually
crash into the Moon.
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What are scientists
hoping to learn from the
GRAIL mission?
By studying GRAIL's
gravitational maps, which
reveal areas of greater or
lesser density, researchers
can decipher the structure
of the Moon's interior.
Insights into the lunar
structure will help
scientists figure out how
the Moon formed and
provide clues about the
beginnings of the Earth and
other rocky planets, which
are thought to have formed
in a similar way.
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How will GRAIL's findings affect future Moon
missions?
GRAIL's mapping, along with high-resolution
pictures of the Moon's surface sent back by the
Lunar Reconnaissance Orbiter (LRO), represents an
important step for future Moon probes. The gravity
maps will help mission planners calculate orbits that
minimize the interference of mascons. Of particular
importance will be GRAIL's discoveries about
mysterious areas such as the Moon's far side and
polar regions.
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A camera will be aboard each spacecraft and
will allow students, and the public, to
interact with observations from the satellites.
Each camera is called a “MoonKAM.”
MoonKAM stands for ‘Moon Knowledge
Acquired by Middle school students. Each
GRAIL spacecraft will carry the cameras to
document their views from lunar orbits.
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Students at middle schools across the country
will be able to get in on the GRAIL mission's
spirit of discovery. The satellites will carry
special cameras, and during the science phase
of the mission, students can send in requests for
the cameras to take photos of specific areas on
the lunar surface. The images will be posted on
the Internet, and students can refer to them as
they study highlands, maria, and other features
of the Moon's topography.
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The MoonKAM images and supporting
educational materials will be available
for public access on the MoonKAM
website,
www.GRAILMoonKAM.com.
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Websites used in this presentation:
http://discovery.nasa.gov/grail.html
https://www.grailmoonkam.com/about/grail_fact_sheet
http://moon.mit.edu/index.html
http://www.jpl.nasa.gov/images/grail/grail.jpg
http://blogs.jpl.nasa.gov/?p=75
http://www.msnbc.msn.com/id/22202797/ns/technology_and_sciencespace/
http://adsabs.harvard.edu/abs/2010AGUFMED31B0619F
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Presented by Tony Owens, DMNS “Space
Odyssey” Galaxy Guide on January 4, 2011.