Gravity Recovery and Interior Laboratory

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G R A I LNASA Aims to Look Inside the

Moon

G R A I LScientists 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.

G R A I LThe 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.

G R A I LA 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.

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 lava-filled craters or lunar "seas" visible in binoculars on the near side of the Moon.

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G R A I LCreated 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

G R A I LOn 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.

G R A I LThe 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.

G R A I LGRAIL 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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G R A I LMission 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

G R A I LAdditional 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

G R A I LWhen 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.

G R A I LGRAIL 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.

G R A I LA 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.

G R A I LWhat 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.

Who is coordinating the GRAIL mission?

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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.

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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.

G R A I LWhat 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.

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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.

G R A I LHow 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, . . .

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. . . 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.

G R A I LHow 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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G R A I LIs 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.

What do we know about the Moon's gravitational field?

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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.

What are scientists hoping to learn from the GRAIL mission?

G R A I LBy 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.

G R A I LHow 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.

G R A I LA 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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G R A I LStudents 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.

G R A I LThe MoonKAM images and supporting educational materials will be available for public access on the MoonKAM website,

www.GRAILMoonKAM.com.

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G R A I LWebsites 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_science-space/

http://adsabs.harvard.edu/abs/2010AGUFMED31B0619F

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