GRAIL twins coax out the Moon's deeper history of the solar system's early, pulverizing bombardment

GRAIL primary mission global gravity map, centered on 0°N, 355°E, from animation released Dec. 5, 2012. Science Visualization Studio (SVS), GSFC [NASA/JPL/MIT].

Jennifer Chu
MIT

Beneath its heavily pockmarked surface, the moon’s interior bears remnants of the very early solar system. Unlike Earth, where plate tectonics has essentially erased any trace of the planet’s earliest composition, the moon’s interior has remained relatively undisturbed over billions of years, preserving a record in its rocks of processes that occurred in the solar system’s earliest days.

Now scientists at MIT, NASA, the Jet Propulsion Laboratory and elsewhere have found evidence that, beneath its surface, the moon’s crust is almost completely pulverized. The finding suggests that, in its first billion years, the moon — and probably other planets like Earth — may have endured much more fracturing from massive impacts than previously thought.

The startling observations come from data collected by NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission. Since March, the mission’s twin spacecraft, named Ebb and Flow, have been orbiting the moon and measuring its gravitational field.

From GRAIL’s measurements, planetary scientists have now stitched together a high-resolution map of the moon’s gravity — a force created by surface structures such as mountains and craters, as well as deeper structures below the surface. The resulting map reveals an interior gravitational field consistent with an incredibly fractured lunar crust.

“It was known that planets were battered by impacts, but nobody had envisioned that the [moon’s] crust was so beaten up,” says MIT’s Maria Zuber, who leads the GRAIL mission and is the E.A. Griswold Professor of Geophysics in the Department of Earth, Atmospheric and Planetary Sciences. “This is a really big surprise, and is going to cause a lot of people to think about what this means for planetary evolution.” 

Zuber and her colleagues detail their findings from GRAIL in three papers published this week in Science.

GRAIL’s lunar gravity map has also revealed numerous structures on the moon’s surface that were unresolved by previous gravity maps of any planet, including volcanic landforms, impact basin rings, and many simple, bowl-shaped craters. From GRAIL’s measurements, scientists have determined that the moon’s crust, ranging in thickness from 34 to 43 kilometers, is much thinner than planetary geologists had previously suspected. The crust beneath some major basins is nearly nonexistent, indicating that early impacts may have excavated the lunar mantle, providing a window into the interior.

Lifting a veil: In addition to mapping the Moon's anisotropic gravity with unprecedented granularity, the GRAIL primary mission uncovered deeper, essentially vertical dyke structures hundreds of kilometers long, the existence of which had been erased at the surface [NASA/JPL/MIT].

To generate the gravity map, GRAIL’s two probes measure the changing distance between themselves as they orbit in tight formation around the moon. As one of the probes flies over a large mass, such as a mountain or dense, underground rock, the stronger local gravity will pull that probe ahead, widening the space between the two spacecraft. Scientists can translate this changing distance into a gravitational map, representing the gravity produced by both the surface structures and the interior.

To find the gravitational field for the moon’s interior alone, Zuber’s team used topographic measurements from another of their instruments, a laser altimeter aboard the Lunar Reconnaissance Orbiter, a separate spacecraft in orbit around the moon. The scientists calculated the gravitational field expected to be produced by the moon’s topography — its surface structures alone — then subtracted that field from the field measured by GRAIL.

“It’s essentially like removing a veil to reveal the gravity due to the inside of the planet,” Zuber says. “And when we saw those maps, we were just speechless.”

The GRAIL primary science mission also succeeded in definitively mapping the thickness of the Moon's outer crust, shown in another global animation centered over the thickest zones on the lunar farside. The South Pole-Aitken basin, oldest and largest identified impact at 4.1 billion years, hosts some of the thinnest outer crust. The mission has increased the likelihood that fragments of the Moon's deeper zones will eventually be found on the surface, excavated by complex craters located on or near the basin's rim [NASA/JPL/MIT].

Compared to the surface, the map of the interior looked extraordinarily smooth. In fact, the team found that most of the moon’s local gravity is due to surface features, such as crater rims and mountains. Except for the large impact basins, the moon’s upper crust, largely lacks dense rock structures, and is instead likely made of porous, pulverized material.

The interior map did reveal long, linear structures of denser material, which Zuber and her team believe to be buried lunar dikes — formed from magma that seeped into large fractures in the crust, and then solidified into dense walls of rock. These dikes represent evidence for expansion of the moon in its earliest history. But overall, 98 percent of the lunar crust is fragmented — a clear remnant of very early, very massive impacts.

“This is interesting for the moon,” Zuber says. “But what it also means is that every other planet was being bombarded like this.” The resulting fractures, she says, affect the way a planetary body loses heat and also provide a pathway for the transport of interior fluids.

David Kring, a senior staff scientist at the Lunar and Planetary Institute in Houston, says knowing the extent of pulverization in the moon’s crust is an essential detail needed to determine the moon’s bulk composition. Such information would go a long way toward identifying the processes the formed the moon and other planets.

“The staggering quality of the data reported by Professor Zuber and her colleagues is amazing,” says Kring, who was not involved in the research. “The data are exciting because they foretell far more insights than are captured in these initial three papers.”

NASA has scheduled a news conference to discuss the planned deorbiting and impact of the GRAIL spacecraft, December 13.

In addition to GRAIL’s discoveries, Zuber says another major accomplishment has been the performance of the spacecraft themselves. To achieve the mission’s science goals, the two probes, which can travel more than 200 kilometers apart, needed to be able to measure changes in the distance between them to within a few tenths of a micron per second. But GRAIL actually outperformed its measurement requirements by about a factor of five, resolving changes in spacecraft distance to several hundredths of a micron per second — one twenty-thousandth the velocity that a snail travels. 

“On this mission, with two spacecraft, everything had to go perfectly twice,” Zuber says, adding proudly: “Imagine you’re a parent raising a twins, and your children sit down at the piano and play a duet perfectly. That’s how it feels.”

Shackleton harbors ice after all

Spoke too soon! When JAXA released this Kaguya Terrain Camera image, showing the deep interior of Shackleton crater for the first time in 2008, scientists claimed it disappointingly showed no indication of ice, though no one yet can say how a slurry of lunar volatiles might appear. Now, however, researchers analyzing laser altimetry returned by the LOLA instrument on-board the Lunar Reconnaissance Orbiter (LRO) cite strong evidence of ice content in the permanently shadowed interior.  The Moon's south pole is serendipitously situated on Shackleton's rim, directly under all of LRO's nearly twenty thousand polar orbits since 2009, affording extraordinary study [JAXA/SELENE]..

Jennifer Chu

MIT News

If humans are ever to inhabit the moon, the lunar poles may well be the location of choice: Because of the small tilt of the lunar spin axis, the poles contain regions of near-permanent sunlight, needed for power, and regions of near-permanent darkness containing ice — both of which would be essential resources for any lunar colony.

The area around the moon’s Shackleton crater could be a prime site. Scientists have long thought that the crater — whose interior is a permanently sunless abyss — may contain reservoirs of frozen water. But inconsistent observations over the decades have cast doubt on whether ice might indeed exist in the shadowy depths of the crater, which sits at the moon’s south pole.

Now scientists from MIT, Brown University, NASA’s Goddard Space Flight Center and other institutions have mapped Shackleton crater with unprecedented detail, finding possible evidence for small amounts of ice on the crater’s floor. Using (the LOLA) laser altimeter on the Lunar Reconnaissance Orbiter (LRO) spacecraft, the team essentially illuminated the crater’s interior with laser light, measuring its albedo, or natural reflectance. The scientists found that the crater’s floor is in fact brighter than that of other nearby craters — an observation consistent with the presence of ice, which the team calculates may make up 22 percent of the material within a micron-thick layer on the crater’s floor.
 

http://youtu.be/j2NAZODSdwM 

The group published its findings today in the journal Nature.

In addition to the possible evidence of ice, the group’s map of Shackleton reveals a “remarkably preserved” crater that has remained relatively unscathed since its formation more than three billion years ago. The crater’s floor is itself pocked with several smaller craters, which may have formed as part of the collision that created Shackleton.

The crater, named after the Antarctic explorer Ernest Shackleton, is more than 12 miles wide and two miles deep — about as deep as Earth’s oceans. Maria Zuber, the team’s lead investigator and the E.A. Griswold Professor of Geophysics in MIT’s Department of Earth, Atmospheric and Planetary Sciences, describes the crater’s interior as “extremely rugged … It would not be easy to crawl around in there.”

Mapping the dark. Slipping past the Moon's south pole on the brightly lit rim of Shackleton crater, the dark of the permanently shadowed interior of the crater quickly overtakes a very steep crater wall, like the terrestrial oceans. LRO has skipped through thousands of polar orbits eventually carrying the vehicle over every area on the Moon's surface and over Shackleton, high at the top of everyone's list of priority targets, during every orbit,   LROC Narrow Angle Camera (NAC) M142464150L, LRO orbit 6128, October 23, 2010, 89.21° angle of incidence, 0.87 meters resolution from 41.91 kilometers [NASA/GSFC/Arizona State University].

The group was able to map the crater’s elevations and brightness in extreme detail, thanks in part to the LRO’s path: The spacecraft orbits the moon from pole to pole as the moon rotates underneath. With each orbit, the LRO’s laser altimeter maps a different slice of the moon, with each slice containing measurements of both poles. The upshot is that any terrain at the poles — Shackleton crater in particular — is densely recorded. Zuber and her colleagues took advantage of the spacecraft’s orbit to obtain more than 5 million measurements of the polar crater from more than 5,000 orbital tracks.

“We decided we would study the living daylights out of this crater,” Zuber says. “From the incredible density of observations we were able to make an extremely detailed topographic map.”

The team used the (LOLA) to map the crater’s elevations based on the time it took for laser light to bounce back from the moon’s surface: The longer it took, the lower the terrain’s elevation. Through these measurements, the group mapped the crater’s floor and the slope of its walls.

A quaking theory.The researchers also used the laser altimeter to measure the crater’s brightness, sending out pulses of infrared light at a specific wavelength. The crater’s surface absorbed some light based on its own natural albedo, reflecting the rest back to the spacecraft. The researchers calculated the difference, and mapped the relative brightness throughout the crater’s floor and walls.

While the crater’s floor was relatively bright, Zuber and her colleagues observed that its walls were even brighter. The finding was at first puzzling: Scientists had thought that if ice were anywhere in a crater, it would be on the floor, where very little sunlight penetrates. The upper walls of Shackleton crater, in comparison, are occasionally illuminated, which could evaporate any ice that accumulates.

How to explain the bright walls? The team studied the measurements, and came up with a theory: Every once in a while, the moon experiences seismic shaking brought on by collisions, or gravitational tides from Earth. Such “moonquakes” may have caused Shackleton’s walls to slough off older, darker soil, revealing newer, brighter soil underneath.

Until very recently luna incongnita, the permanently shadowed 10.3 km-wide interior of Shackleton, shouldering the Moon's south pole (blue arrow), today seems much like hundreds of other lunar craters of similar age and dimension. Its ink-black interior has steadily been brightly unveiled in a steady build-up of laser data points collected over the course of three years in polar orbit by the LOLA instrument on LRO. As it is on Earth, however, in Real Estate, "location is everything" [NASA/GSFC/LOLA].

Zuber says there may be multiple explanations for the observed brightness throughout the crater: For example, newer material may be exposed along its walls, while ice may be mixed in with its floor. Her team’s ultra-high-resolution map, she says, provides strong evidence for both.

Ben Bussey, staff scientist at Johns Hopkins University’s Applied Physics Laboratory, says the group’s evidence for ice in Shackleton crater may help determine the course for future lunar missions.

“Ice in the polar regions has been sort of an enigmatic thing for some time … I think this is another piece of evidence for the possibility of ice,” Bussey says. “To truly answer the question, we’ll have to send a lunar lander, and these results will help us select where to send a lander.”

Zuber adds that the group’s topographic map will help researchers understand crater formation and study other uncharted areas of the moon.

“I will never get over the thrill when I see a new terrain for the first time,” Zuber says. “It’s that sort of motivation that causes people to explore to begin with. Of course, we’re not risking our lives like the early explorers did, but there is a great personal investment in all of this for a lot of people.”

The research was supported by the Lunar Reconnaissance Orbiter Mission under the auspices of NASA’s Exploration Systems Mission Directorate and Science Mission Directorate.

Japan's scientists may have leaped to conclusions when they over-confidently announced there was no ice inside Shackleton (upper left), after releasing the first image of the crater's interior a few years ago, but their iconic high-definition image of an orbital Earthrise from November 2007 still takes the breath away [JAXA/NHK/SELENE].

Second Conference on the Lunar Highlands

First Results: from Figure 2, "Preliminary results on the structure of lunar highland crust from GRAIL and LOLA altimetry," Zuber & Smith, et al, (#9015, Second Conference on the Lunar Highlands Crust, 2012) Preliminary GRAIL gravity field for the 86-km-diameter Tycho crater. Tycho is the prominent structure at upper left. In this GRAIL map reds correspond to mass excesses and blues to mass deficits. GRAIL gravity has a spatial resolution of 18 kilometers [NASA/JPL/MIT].

Clive Neal
Notre Dame

The first conference on the Lunar Highlands Crust was held in 1979. Since that seminal meeting, our knowledge of the lunar highlands has advanced enormously. Unimagined new data have become available, notably in orbital remote sensing of mineralogy, chemistry, topography, and gravity; geochronology; and geochemistry, especially isotopic constraints and the abundances and natures of lunar volatiles. These new data are paralleled by new concepts of solar system science, including the importance and timing of impact events (including the one that formed the Moon) and the nature of the early solar system disk and its dynamical instabilities. 

In light of these advances in the last 34 years, the time seems right for a synoptic reexamination of the lunar highlands crust. The Second Conference on the Lunar Highlands Crust is intended to bring members of the planetary science community together to share their specialized insights into the lunar highlands crust, exchange ideas freely, and perhaps develop new cross-disciplinary ideas and tests of those ideas.

A field trip to the Stillwater Mine will be held on Thursday, July 12, and a field trip to Picket Pin Mountain will be held on Monday, July 16 (departing Bozemanon Sunday, July 15, following the conclusion of the final oral session). More details about the field trips, along with information about registration, accommodations, transportation, and much more, are available in the final announcement.

For more information, visit the conference website, HERE.

"Preliminary results on the structure of lunar highland crust from GRAIL and LOLA altimetry," Zuber & Smith, et al, (#9015, Second Conference on the Lunar Highlands Crust, 2012)

Related: Lunar Picture of the Day (LPOD), "First Results," June 13, 2012, Charles Wood

Students to exhibit GRAIL MoonKAM images

Forward view from a MoonKAM camera on-board GRAIL Ebb looks down into 29.6 kilometer Titov and south-southwestward over surrounding floor of Mare Moscoviense, in the northern lunar farside. MoonKam image 8108, Robert McCall School, May 19, 2012 [NASA/Caltech-JPL/MIT/SRS].

Reporters have been invited to an exhibit of student-directed MoonKam images of the lunar surface captured by NASA's GRAIL twin spacecraft, Ebb and Flow, Friday, June 1, from 10 am until Noon, EDT, at the Ronald Reagan Building and International Trade Center, 1300 Pennsylvania Avenue NW, Washington, DC.

The event will showcase the MoonKAM (Moon Knowledge Acquired by Middle school students) education and public outreach project flying in lunar orbit on the GRAIL spacecraft. 

MoonKAM provides students around the world with an opportunity to identify and choose images of the moon's surface using small cameras aboard NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission. Thousands of images of lunar targets have been selected by fifth to eighth-grade students since.the GRAIL twins arrived in lunar orbit with the New Year.

MoonKAM is operated by Sally Ride Science in collaboration with undergraduate students at the University of California in San Diego. 

Sally Ride, CEO of the science education company (and America's first woman in space) will host the event and provide opening remarks. Event presenters include NASA deputy administrator Lori Garver and GRAIL principal investigator Maria Zuber of the Massachusetts Institute of Technology, Cambridge, Massachusetts.

Meanwhile, it was announced earlier this week that the primary science mission of the GRAIL mission was completed ahead of schedule. An extended mission will continue at least through December.

Ebb & Flow complete primary mission

Ebb and Flow have completed their prime mission earlier than expected. The mission team of NASA's Gravity Recovery and Interior Laboratory (GRAIL) mission, with twin probes named Ebb and Flow, is now preparing for extended science operations starting Aug. 30 and continuing through Dec. 3, 2012.

The GRAIL mission has gathered unprecedented detail about the internal structure and evolution of the moon. This information will increase our knowledge of how Earth and its rocky neighbors in the inner solar system developed into the diverse worlds we see today.

Since March 8, the spacecraft have operated around the clock for 89 days. From an orbit that passes over the lunar poles, they have collected data covering the entire surface three times. An instrument called the Lunar Gravity Ranging System onboard each spacecraft transmits radio signals that allow scientists to translate the data into a high-resolution map of the moon's gravitational field. The spacecraft returned their last data set of the prime mission May 29. The instruments were turned off at 1 p.m. EDT when the spacecraft were 37 miles (60 kilometers) above the Mare Nectaris.

"Many of the measurement objectives were achieved from analysis of only half the primary mission data, which speaks volumes about the skill and dedication of our science and engineering teams," said Maria Zuber, principal investigator of GRAIL at the Massachusetts Institute of Technology in Cambridge. "While there is a great deal of work yet to be done to achieve the mission's science, it's energizing to realize that what we traveled from Earth to the moon for is right here in our hands."

"GRAIL delivered to Earth over 99.99 percent of the data that could have been collected, which underscores the flawless performance of the spacecraft, instrument and the Deep Space Network," said Zuber.

Both spacecraft instruments will be powered off until Aug. 30. The spacecraft will have to endure a lunar eclipse on June 4. The eclipse and the associated sudden changes in temperature and the energy-sapping darkness that accompanies the phenomena were expected and do not concern engineers about the spacecraft's health.

"Before launch, we planned for all of GRAIL's primary mission science to occur between lunar eclipses," said David Lehman, project manager of GRAIL from NASA's Jet Propulsion Laboratory (JPL) in Pasadena, Calif. "But now that we have flown Ebb and Flow for a while, we understand them and are confident they can survive these eclipses in good shape."

The extended mission goal is to take an even closer look at the moon's gravity field. To achieve this, GRAIL mission planners will halve their current operating altitude flying at the lowest altitude that can be safely maintained.

"Orbiting at an average altitude of 14 miles (23 kilometers) during the extended mission, the GRAIL twins will be clearing some of the moon's higher surface features by about 5 miles (8 kilometers)," said Joe Beerer, GRAIL's mission manager. "If Ebb and Flow had feet, I think by reflex they'd want to pull them up every time they fly over a mountain."

Along with mission science, GRAIL's MoonKAM (Moon Knowledge Acquired by Middle school students) education and public outreach program is also extended. To date over 70,000 student images of the moon have been obtained. The MoonKAM program is led by Sally Ride, America's first woman in space, and her team at Sally Ride Science in collaboration with undergraduate students at the University of California in San Diego.

The GRAIL mission is managed by JPL for NASA's Science Mission Directorate in Washington. The mission is part of the Discovery Program managed at NASA's Marshall Space Flight Center in Huntsville, Ala. NASA's Deep Space Network is an international network of antennas that supports interplanetary spacecraft missions and radio and radar astronomy observations for the exploration of the solar system and the universe. The network also supports selected Earth-orbiting missions. Lockheed Martin Space Systems in Denver built the spacecraft.

Zuber shares lunar morphology insights at Harvard

Daniel J. Kramer and Fatima Mirza
The Harvard Crimson

Geophysicist Maria T. Zuber applied her knowledge of Earth science to one of the world’s most universally studied astronomical bodies—the moon—at this year’s annual Neekeyfar Lecture on Math and Science, hosted Tuesday, April 17, 2012 by the (Harvard University) Undergraduate Council’s Student Advisory Board on Science.

“Basically every human who has lived on the rocky earth has observed the moon,” she said. “So it’s not surprising that when humans first stepped on the moon it was viewed as one of the greatest achievements, if not the greatest achievement, ever accomplished by humanity, at least from a technological standpoint.”

Zuber, E.A. Griswold Professor of Geophysics at MIT, focused on her work on planetary phenomena in the moon’s interior and lunar surface modeling at the talk, titled “Journey to the Center of the Moon.”

Zuber, who is also the department head of Earth, Atmospheric, and Planetary Sciences, is the first female department head in the history of MIT and also the first woman to be selected by NASA to head a major planetary spacecraft mission.

Zuber related her experiences with lunar geophysics to the NASA Gravity Recovery And Interior Laboratory (GRAIL) mission, which launched for the moon in September 2011.

In particular, Zuber explained the details of how to go from research projects in the lab to an applied astronomical endeavor.

Zuber explained to the audience, which ranged from curious undergraduates to astrophysics concentrators and professors, that the moon’s surface consists of two types of rock. One type, anorthosite, or white rock, represents the areas of the moon’s surface that were once covered by the lunar magma ocean.

The other type is basalt, or black rock, whose formation is indicative of lava flows from volcanic melting in the moon’s distant past. Zuber mentioned that the discovery of the once-present magma ocean is considered one of the greatest fruits of the Apollo Mission.

Early in the Earth’s history, Zuber said, it was hit by a Mars-sized object that struck at a sharp angle, tearing off a piece of the Earth’s mantle and sending it into the atmosphere.

Over time, the piece of the mantle folded into a disk and then condensed into the modern-day moon.

The moon also has a rich history of “impact events” or being hit by giant rocks, as indicated by the numerous craters scattered across its surface.

The biggest crater, or impact basin, is on the side of the moon not visible from Earth. This impact basin is two-thirds the width of the United States and is 8 km deep.

MIT geophysicist Dr. Maria T. Zuber, principal investigator for the GRAIL mission, shares the stage with Apollo 11 lunar module pilot Buzz Aldrin, Steve Squyres, and James T. Belllingham at the MIT+150 Symposia Earth, Air, Ocean and Space: The Future of Exploration, April 26, 2011 [Flickr].

One still-unanswered question Zuber discussed was why the near side of the moon is topologically different from the far side. One theory is that the differences are caused by internal lunar activity.

“If you’ve sent 109 space crafts that have essentially mapped the surface and brought back samples, and you haven’t found the answer, then you’re probably not looking in the right place,” she said.

This internal activity could include phase changes in matter, causing fluctuations in energy flow and distribution, as well as “moon quakes” that occur more often when the moon is closest to Earth.

In her research, Zuber works to study the moon’s gravitational field by increasing the resolution of imaging on the moon.

“The results are really transformative when you can improve measurements by three orders of magnitude or better,” said Zuber.

However, she feels her most profound contribution to society stems from her ability to engage the minds of the future. As part of her GRAIL mission, she incorporated cameras on her spacecraft specifically devoted to educating students around the country. The students use online software to monitor the location of the spacecraft in orbit and look at images.

“I want to teach kids physics is not only interesting but also fun,” said Zuber.

According to Deana Ste. Marie, Executive Assistant to the Dean of Science and an advisor to SABS, said that this year, students were able to spend time with Zuber in breakout sessions throughout the day, which proved to be “engaging and rewarding.”

Attendee and one of the event’s organizers, Alexander L. Jaffe ’15, who is also a member of SABS, enjoyed hearing about brand new topics from such a renowned expert.

“The subjects covered were some that I had no experience with before,” he said. “To hear about them, especially from a first hand research project, was fascinating.”

Flying Formation - Around the Moon at 5,800 KPH

An artist’s depiction of the twin spacecraft (Ebb and Flow) that comprise NASA’s Gravity Recovery And Interior Laboratory (GRAIL) mission. View full image and caption HERE [NASA/Caltech-JPL/MIT].

D. C. Agle
Jet Propulsion Laboratory

Pasadena - The act of two or more aircraft flying together in a disciplined, synchronized manner is one of the cornerstones of military aviation, as well as just about any organized air show. But as amazing as the U.S. Navy's elite Blue Angels or the U.S. Air Force's Thunderbirds are to behold, they remain essentially landlocked, anchored if you will, to our planet and its tenuous atmosphere. What if you could take the level of precision of these great aviators to, say, the moon?

"Our job is to ensure our two GRAIL spacecraft are flying a very, very accurate trail formation in lunar orbit," said David Lehman, GRAIL project manager at NASA's Jet Propulsion Laboratory in Pasadena, California. "We need to do this so our scientists can get the data they need."

Essentially, trail formation means one aircraft (or spacecraft in this case), follows directly behind the other. Ebb and Flow, the twins of NASA's GRAIL (Gravity Recovery And Interior Laboratory) mission, are by no means the first to synch up altitude and "air" speed  while zipping over the craters, mountains, hills and rills of Earth's natural satellite. That honor goes to the crew of Apollo 10, who in May 1969 performed a dress rehearsal for the first lunar landing. But as accurate as the astronauts aboard lunar module "Snoopy" and command module "Charlie Brown" were in their piloting, it is hard to imagine they could keep as exacting a position as Ebb and Flow.

"It is an apples and oranges comparison," said Lehman. "Lunar formation in Apollo was about getting a crew to the lunar surface, returning to lunar orbit and docking, so they could get back safely to Earth. For GRAIL, the formation flying is about the science, and that is why we have to make our measurements so precisely."

As the GRAIL twins fly over areas of greater and lesser gravity at 5,800 kilometers per hour, surface features such as mountains and craters, and masses hidden beneath the lunar surface, can influence the distance between the two spacecraft ever so slightly.

How slight a distance change can be measured by the science instrument beaming invisible microwaves back and forth between Ebb and Flow?

NASA/Caltech-JPL/MIT
› Full image and caption

How about one-tenth of one micron? Another way to put it is that the GRAIL twins can detect a change in their position down to one half of a human hair (0.00001 centimeters).  For those of you who are hematologists or vampires (we are not judging here), any change in separation between the two twins greater than one half of a red corpuscle will be duly noted aboard the spacecraft's memory chips for later downlinking to Earth. Working together, Ebb and Flow will make these measurements while flying over the entirety of the lunar surface.

This begs the question, why would scientists care about a change of distance between two spacecraft as infinitesimal as half a red corpuscle a quarter million miles from Earth?

"Mighty oaks from little acorns grow - even in lunar orbit," said Maria Zuber, principal investigator of the GRAIL mission from the Massachusetts Institute of Technology, Cambridge. "From the data collected during these minute distance changes between spacecraft, we will be able to generate an incredibly high-resolution map of the moon's gravitational field.  From that, we will be able to understand what goes on below the lunar surface in unprecedented detail, which will in turn increase our knowledge of how Earth and its rocky neighbors in the inner solar system developed into the diverse worlds we see today."

Getting the GRAIL twins into a hyper-accurate formation from a quarter million miles away gave the team quite a challenge. Launched together on Sept. 10, 2011, Ebb and Flow went their separate ways soon after entering space. Three-and-a-half months and 2.5 million miles (4 million kilometers) later, Ebb entered lunar orbit. Flow followed the next day (New Year's Day 2012).

"Being in lunar orbit is one thing, being in the right lunar orbit for science can be something else entirely," said Joe Beerer, GRAIL's mission manager from JPL. "The twins initial orbit carried them as close to the lunar surface as 56 miles (90 kilometers) and as far out as 5,197 miles (8,363 kilometers), and each revolution took approximately 11.5 hours to complete. They had to go from that to a science orbit of 15 by 53 miles (24.5 by 86 kilometers) and took all of 114 minutes to complete."

To reduce and refine Ebb and Flow's orbits efficiently and precisely required the GRAIL team to plan and execute a series of trajectory modification burns for each spacecraft. And each maneuver had to be just right.

More information about the GRAIL mission is online at: http://www.nasa.gov/grail or http://grail.nasa.gov .

NASA's Jet Propulsion Laboratory in Pasadena, California, manages the GRAIL mission for NASA's Science Mission Directorate, Washington. The GRAIL mission is part of the Discovery Program managed at NASA's Marshall Space Flight Center in Huntsville, Alabama. Lockheed Martin Space Systems in Denver built the spacecraft. JPL is a division of the Çalifornia Institute of Technology in Pasadena.

"Because each one of these maneuvers was so important, we did a lot of planning and testing for each," said Beerer. "Over eight weeks, we did nine maneuvers with Ebb and 10 with Flow to establish the science formation. We would literally be watching our screens for a signal telling us about an Ebb rocket burn, then go into a meeting about the next burn for Flow. Our schedule was very full."

Today, the calendar for GRAIL's flight team remains a busy one with the day-to-day operations of keeping NASA's lunar twins in synch. But as busy as the team gets, they still have time to peer skyward.

"Next time you look up and see the moon, you might want to take a second and think about our two little spacecraft flying  formation, zooming from pole to pole at 5,800 kph," said Lehman. "They're up there, working together, flying together, getting the data our scientists need. As far as I'm concerned, they're putting on quite a show."

The first student-requested pictures from GRAIL

Orbiting 50 kilometers over the farside, the MoonKAM camera on-board the GRAIL twin spacecraft "Ebb" captured the subtle colors of the lunar highlands at the behest of Emily Dickinson Middle School, Bozeman, Montana on March 15, 2012. The 31 kilometer-wide crater in the northeast quadrant is Gadomski A (38.3°N, 213.5°E). For comparison, both LROC WAC and NAC images of the same area are presented further below. Ebb MoonKam IMAGE 84, HERE [NASA/JPL/MIT/MoonKAM].

NASA JPL: One of the two NASA GRAIL spacecraft orbiting the Moon has beamed back the first student-requested pictures of the lunar surface from its onboard camera. Fourth grade students from the Emily Dickinson Elementary School in Bozeman, Montana, received the honor of making the first image selections after winning the nationwide competition to rename the two spacecraft, "Ebb" and "Flow."

The images was taken by the MoonKAM, for "Moon Knowledge Acquired by Middle school students."

Previously named "A" and "B," the twin Gravity Recovery And Interior Laboratory (GRAIL) spacecraft are washing-machine-sized each equipped with small MoonKAM cameras. Over 60 student–requested images were taken by the Ebb spacecraft from March 15-17 and downlinked to Earth March 20.

"MoonKAM is based on the premise that if your average picture is worth a thousand words, then a picture from lunar orbit may be worth a classroom full of engineering and science degrees," said Maria Zuber, GRAIL mission principal investigator from the Massachusetts Institute of Technology (MIT) in Cambridge, Massachusetts.

"Through MoonKAM, we have an opportunity to reach out to the next generation of scientists and engineers. It is great to see things off to such a positive start," Zuber said.

Read the full news release HERE.

For comparison with MoonKAM "IMAGE 84," above, here is a segment from a LROC Wide Angle Camera (WAC) monochrome (604nm) mosaic of Gadomski A, stitched from sequential orbital passes, May 7, 2010; incidence angle 53° - 64.4 meters resolution from 46.3 km. The yellow arrow designates the 300 meter crater on the south rim visible in both the MoonKAM image above and in the LROC NAC high-resolution image below [NASA/GSFC/Arizona State University].

A worn 300 meter-wide crater (37.78°N,  213.56°E), high on the south rim of Gadomski A, in a quick comparative study at native resolution with that of MoonKAM IMAGE 84, among the first returned to Earth from the GRAIL mission in low lunar orbit. - LROC NAC frame M136144934L, orbit 5197, August 11, 2011; resolution 0.65 meters per pixel from 63.32 kilometers [NASA/GSFC/Arizona State University].

To view the student-requested images, visit: http://images.moonkam.ucsd.edu

GRAIL mission extended

Stephen Clark
Spaceflight Now

NASA has granted an extension of the GRAIL moon mission until December, allowing scientists to complete a more definitive map of the lunar gravity field from a lower orbit, according to agency officials and researchers. The $496 million mission was due to end in June.

The extension was confirmed by Jim Green, head of NASA's planetary science division, and Maria Zuber, the GRAIL mission's principal investigator from the Massachusetts Institute of Technology.

The twin GRAIL satellites entered orbit around the moon Dec. 31 and Jan. 1, then began taking science observations in early March. GRAIL's baseline mission was supposed to last about three months.

Read the full article HERE.

GRAIL twins begin science mission

Ebb (GRAIL-A) in continuously measuring the distance between itself and Flow (GRAIL-B), when measured against time and place will enable mapping the lunar interior with high precision [NASA/JPL/MIT].

NASA's Gravity Recovery And Interior Laboratory (GRAIL) spacecraft orbiting the moon officially have begun their science collection phase. During the next 84 days, scientists will obtain a high-resolution map of the lunar gravitational field to learn about the moon's internal structure and composition in unprecedented detail. The data also will provide a better understanding of how Earth and other rocky planets in the solar system formed and evolved.

"The initiation of science data collection is a time when the team lets out a collective sigh of relief because we are finally doing what we came to do," said Maria Zuber, principal investigator for the GRAIL mission at the Massachusetts Institute of Technology in Cambridge. "But it is also a time where we have to put the coffee pot on, roll up our sleeves and get to work."

The GRAIL mission's twin, washing-machine-sized spacecraft, named Ebb and Flow, entered lunar orbit on New Year's Eve and New Years Day. GRAIL's science phase began yesterday at 8:15 p.m. EST (5:15 p.m. PST). During this mission phase, the spacecraft will transmit radio signals precisely defining the distance between them. As they fly over areas of greater and lesser gravity caused by visible features such as mountains, craters and masses hidden beneath the lunar surface, the distance between the two spacecraft will change slightly. Science activities are expected to conclude on May 29, after GRAIL maps the gravity field of the moon three times.

"We are in a near-polar, near-circular orbit with an average altitude of about 34 miles (55 kilometers) right now," said David Lehman, GRAIL project manager from NASA's Jet Propulsion Laboratory (JPL) in Pasadena, California. "During the science phase, our spacecraft will orbit the moon as high as 31 miles (51 kilometers) and as low as 10 miles (16 kilometers). They will get as close to each other as 40 miles (65 kilometers) and as far apart as 140 miles (225 kilometers)."

Previously named GRAIL A and B, the names Ebb and Flow were the result of a nation-wide student contest to choose new names for the spacecraft. The winning entry was submitted by fourth graders from the Emily Dickinson Elementary School in Bozeman, Montana. Nearly 900 classrooms with more than 11,000 students from 45 states, Puerto Rico and the District of Columbia, participated in the contest.

JPL manages the GRAIL mission for NASA's Science Mission Directorate in Washington. The GRAIL mission is part of the Discovery Program managed at NASA's Marshall Space Flight Center in Huntsville, Alabama. Lockheed Martin Space Systems in Denver built the spacecraft.

GRAIL returns first video of Farside

Still of far southern farside returned by GRAIL twin spacecraft Ebb.

JPL-Pasadena:  A camera aboard one of NASA's twin Gravity Recovery And Interior Laboratory (GRAIL) spacecraft has returned a first unique view of the far side of the moon. MoonKAM, or Moon Knowledge Acquired by Middle school students, will be used by students nationwide to select lunar images for study.

GRAIL consists of two identical spacecraft, recently named Ebb and Flow, each of which is equipped with a MoonKAM. The images were taken as part of a test of Ebb's MoonKAM, January 19. The GRAIL project plans to test the MoonKAM aboard Flow at a later date. To view the 30-second video clip, visit: http://go.nasa.gov/zZXAPs

In the video, the north pole of the moon is visible at the top of the screen as the spacecraft flies toward the lunar south pole. One of the first prominent geological features seen on the lower third of the moon is the Mare Orientale, a 900 km-wide  impact basin that straddles both the moon's near and far side.

The clip ends with rugged terrain just short of the lunar south pole. To the left of center near the bottom of the screen is the 149 kilometer Drygalski crater with its distinctive star-shaped formation in the middle. The formation is a central peak, created many billions of years ago by a comet or asteroid impact.

"The quality of the video is excellent and should energize our MoonKAM students as they prepare to explore the moon," said Maria Zuber, GRAIL principal investigator from the Massachusetts Institute of Technology.

The twin spacecraft successfully achieved lunar orbit New Year's Eve and New Year's Day. Previously named GRAIL-A and B the washing machine-sized spacecraft received new names from fourth graders at the Emily Dickinson Elementary School in Bozeman, Montana following a nationwide student naming contest.

Thousands of fourth- to eighth-grade students will select target areas on the lunar surface and send requests to the GRAIL MoonKAM Mission Operations Center in San Diego. Photos of the target areas will be sent back for students to study. The MoonKAM program is led by Sally Ride, America's first woman in space. Her team at Sally Ride Science and undergraduate students at the University of California in San Diego will engage middle schools across the country in the GRAIL mission and lunar exploration. GRAIL is NASA's first planetary mission carrying instruments fully dedicated to education and public outreach.

"We have had great response from schools around the country; more than 2,500 signed up to participate so far," Ride said. "In mid-March, the first pictures of the moon will be taken by students using MoonKAM. I expect this will excite many students about possible careers in science and engineering."

Launched in September 2011, Ebb and Flow periodically perform trajectory correction maneuvers that, over time, will lower their orbits to near-circular ones with an altitude of about 55 kilometers. During their science mission, the duo will answer longstanding questions about the moon and give scientists a better understanding of how Earth and other rocky planets in the solar system formed.

The GRAIL mission is part of the Discovery Program managed at NASA's Marshall Space Flight Center in Huntsville, Alabama. Lockheed Martin Space Systems in Denver built the spacecraft.

For more information about GRAIL, visit: http://grail.nasa.gov/ and http://www.nasa.gov/grail.

Information about MoonKAM is available at: https://moonkam.ucsd.edu/ .

GRAIL twins to be officially renamed January 17

Follow GRAIL A and B, along with the full range
of robotic probes in orbit and deep space through
the JPL Eyes on the Solar System web
application [NASA/JPL].

NASA will host a news conference at 1800 UT, Tuesday, January 17, to announce the names selected from a nationwide student contest for twin spacecraft that will study the moon in unprecedented detail. The event will be held at NASA Headquarters in Washington.

Nine hundred classrooms and more than 11,000 students from 45 states, as well as Puerto Rico and the District of Columbia, participated in the contest that began in October 2011.

The agency's twin Gravity Recovery And Interior Laboratory (GRAIL A/B) spacecraft successfully achieved lunar orbit on New Year's Eve and New Year's Day, respectively. The status of the spacecraft and upcoming plans for science operations also will be discussed.

NASA Television and the agency's website will broadcast the live event.The participants will be John Grunsfeld, associate administrator, Science Mission Directorate, NASA HQ; Leland Melvin, associate administrator for Education, NASA HQ; Maria Zuber, GRAIL principal investigator, MIT, Cambridge, MA & Sally Ride, president and CEO, Sally Ride Science, San Diego along with the teacher and students who submitted the selected contest-winning names for the GRAIL spacecraft.

The event will be carried live on Ustream, with a live chat box available, at: http://www.ustream.tv/nasajpl2 .

For more information about GRAIL, visit: http://grail.nasa.gov/ - http://www.nasa.gov/grail/ or the GRAIL science site at MIT http://moon.mit.edu/.

For NASA TV streaming video, downlink and schedule information, visit: http://www.nasa.gov/nasatv .

NASA's Jet Propulsion Laboratory in Pasadena, CA manages the GRAIL mission for NASA's Science Mission Directorate, Washington. The GRAIL mission is part of the Discovery Program managed at NASA's Marshall Space Flight Center in Huntsville, Ala. Lockheed Martin Space Systems in Denver built the spacecraft. JPL is a division of the California Institute of Technology in Pasadena.

GRAIL-B joins New Years dance with GRAIL-A

GRAIL-B was still 6300 kilometers away and closing fast at 2038UT New Years Day and flight directors at JPL prepared for a burn that would begin a tandem flight with GRAIL-A. At the same moment GRAIL-A was reaching the 87.4 km apogee of a highly elliptical third orbit since its own insertion into lunar orbit 30 hours earlier, on New Years Eve. The high precision lunar gravity mapping mission, following five years of preparation, can be followed using NASA's web-based Eyes on the SOLAR SYSTEM GRAIL AT THE MOON module [NASA/JPL].

GRAIL-B performed a 39 minute lunar orbit insertion burn, Sunday afternoon, beginning at 2205 UT. Following the successful insertion of both GRAIL spacecraft mission principal investigator Dr. Maria Zuber of MIT is expected to announce the winners of a contest to formally name GRAIL A and B, having chosen from 20,000 entries submitter by elementary and middle school students.

Follow the story at SpaceflightNOW.

GRAIL twins arrive in lunar vicinity

The long 2.5 million mile tour to the Moon allowed the spin up of mission-sensitive equipment. GRAIL A and B will arrive in lunar orbit 25 hours apart following their more-than three month flight. The science mission will commence as after an 11 hour series of maneuvers bring the vehicles down to their 50 km high circular polar orbit [JPL].

Beginning with New Year celebrations this coming weekend the United States will start 2012 with a total of five unmanned spacecraft in orbit around the Moon simultaneously.

Following a low energy trajectory allowing more than three months flying time the twin lunar Gravity Recovery and Interior Laboratory (GRAIL A and GRAIL B) will be inserted into lunar orbit New Years Eve and New Years Day, respectively. There they will join the ARTEMIS P1 and P2 probes, two salvaged vehicles of the now-completed five vehicle THEMIS fleet subsequently redeployed to the Moon by way of L1 and L2, and the Lunar Reconnaissance Orbiter (LRO) that has been in lunar orbit since June 2009.

Maria Zuber of MIT, principal investigator for the GRAIL mission, joined David Lehman, project manager at NASA's Jet Propulsion Laboratory (JPL) for an audio news conference Thursday afternoon, December 28.

The Gravity Recovery And Interior Laboratory (GRAIL) spacecraft are intended to map the lunar gravitational field with unprecedented accuracy. 

GRAIL-A and GRAIL-B lifted off from Cape Canaveral on September 10. GRAIL-A will arrive in lunar orbit at 2100 UT) New Years Eve and GRAIL-B will arrive New Years Day at 2200 UT.

MoonKAM field of view. Piggy-backing on the small Lockheed Martin-built probes, four cameras in total will be operated by elementary school students signed up through the MoonKAM website [Zuber/MIT].

The science payload on-board each spacecraft is the Lunar Gravity Ranging System, which will measure changes in the distance between the two spacecraft down to a few microns - about the diameter of a red blood cell.

The GRAILs also carry cameras for the MoonKAM project, an educational outreach effort which will allow students and schoolchildren to download lunar imagery directly. The results of a contest to formally name both GRAIL A and B is scheduled to be announced following orbital insertion.

"The Moon is the nearest pristine airless body with a preserved surface intact from the time of the Solar System's formation," Zuber said. "We actually now know more about Mars than our closest neighbor." 

"We don't yet know fully why there is such a difference between the near and far side of the Moon, and the we think the answer lies within the Moon. GRAIL should complete a gravity map of the Moon 100 times as accurate as we have today."

As both spacecraft stay in line of site through June, approximately 50 kilometers above the surface in polar orbit differences in their distance will be measured to within a few microns, allowing the anisotropic nature of the Moon's mass to measured with unprecedented accuracy.

In addition the probes will carry four cameras between them, admittedly with no true scientific value, that will be operated by students signed up through the MoonKAM project under the direction of former astronaut Dr. Sally Ride.

Following the 84 day nominal mission there is a possibility of an extended mission through December 2012.

December 28 NASA/JPL press conference materials available HERE.

Launch of GRAIL twins slips to Saturday

NASA's GRAIL twin spacecraft await launch atop a United Launch Alliance Delta II rocket at Cape Canaveral Air Force Station [NASA].

The launch of a Delta II vehicle carrying NASA’s Gravity Recovery and Interior Laboratory (GRAIL) was scrubbed once again to Saturday morning, to provide "additional time to review propulsion data from Thursday's detanking operation." The Delta II and GRAIL remains safe and secure, and the launch has now been tentatively rescheduled for September 10, from Space Launch Complex-17B at Cape Canaveral Air Force Station. There are two instantaneous launch opportunities at 12:29:45 and 13:08:52 UT.

Emily Lakdawalla of The Planetary Society quotes professional launch photographer Ben Cooper, "This is the final Delta mission from Complex 17 after nearly six decades of launches from this, the oldest active pad still in use at Cape Canaveral. Complex 17 saw its first launch in January 1957, from Pad B, and its first space launch in 1960 when Echo 1, the world's first communications satellite, failed to make orbit on what was the first Delta vehicle by name, having been transitioned from the Thor IRBM."

NASA's Jet Propulsion Laboratory, Pasadena manages the GRAIL mission, and the Massachusetts Institute of Technology, Cambridge, is home to the mission's principal investigator, Maria Zuber. Using a precision formation-flying technique, the twin GRAIL spacecraft will map the moon's uneven mass. The result should be the most accurate gravity map of the moon ever made. The mission also will answer longstanding questions about Earth's moon, including the size and nature of mass concentrations, "voids" of low desity materal and the Moon's possibly liquid inner core.

SpaceflightNow.com is sponsoring live streaming video of the launch. More information about GRAIL is online at: http://www.nasa.gov/grail and http://grail.nasa.gov .

GRAIL set for early September lift-off

Using a precision formation-flying technique, beginning in January 2012, the twin GRAIL spacecraft will map the moon's gravity field.

The twin GRAIL lunar probes are set to launch aboard a United Launch Alliance (ULA) Delta II, from Cape Canaveral Air Force Station (CCAFS), September 8. That date offers two instantaneous launch windows, at 12:37:06 and and 13:16:12 UT, beginning an optimal launch period (last for a Delta II at the Cape) extending through October 19 when launch windows open approximately four minutes earlier each consecutive day.

The twin GRAIL, developed at JPL originally as a "precursor mission" in support of goals for the defunct Constellation program, are designed to more precisely determine the Moon's interior composition, "from crust to core," and to advance understanding of the Moon's thermal evolution.

A prelaunch news conference is scheduled at the Kennedy Space Center's Press Site, Tuesday, September 6, at 1700 UT. Scheduled to participate in the briefing are Ed Weiler, Science Mission Directorate, NASA HQ; Tim Dunn, KSC launch director; Vernon Thorp, NASA Missions program manager, United Launch Alliance; David Lehman, GRAIL project manager, Jet Propulsion Laboratory (JPL) in Pasadena; John Henk, GRAIL program manager for Lockheed Martin and Joel Tumbiolo, launch weather officer, 45th Weather Squadron, CCAFS

A GRAIL mission science briefing at the KSC Press Site is scheduled Wednesday, September 7, at 1400 UT, Participating in that briefing are Robert Fogel, GRAIL program scientist, NASA Washington; Maria Zuber, GRAIL principal investigator, Massachusetts Institute of Technology; Sami Asmar, GRAIL deputy project scientist, JPL and Sally Ride, president and CEO of Sally Ride Science of San Diego

GRAIL mission website

Related Posts:
Testing the GRAIL twins
GRAIL tandem on target for 2011
Lunar GRAIL will study Moon's "Interiors"

GRAIL tandem on target for 2011

The GRAIL (Gravity Recovery and Interior Laboratory) spacecraft concept, designed to further improve maps of the Moon's anisotropic gravity well and the composition of its interior, taking advantage of a stable lunar lithosphere to read the history of the inner Solar System [NASA/JPL/GSFC/MIT].

Engineers at Lockheed-Martin Space Systems in Denver, CO have conducted a fuel tank check of one of NASA's GRAIL mission spacecraft (Gravity Recovery and Interior Laboratory) scheduled for launch in 2011.

"Confirming the size and fit of manufactured components is one of the steps required prior to welding the spacecraft's fuel tanks into the propulsion system's feed lines," according to a news release from the Jet Propulsion Laboratory in Pasadena, CA.

"The GRAIL mission will fly twin spacecraft (spacecraft "A" and "B") in tandem orbits around the moon for several months to measure its gravity field in unprecedented detail. The mission will also answer longstanding questions about Earth's moon, and provide scientists a better understanding of how Earth and other rocky planets in the solar system formed."

The GRAIL spacecraft concept builds on a legacy of similar tandem satellite concepts, including most recently the twin sub-satellites flown with Japan's SELENE-1. Tiny changes in Ka band transmission time between the two spacecraft, which will arrive in lunar orbit 50 hours apart, and also between each of the spacecraft and ground stations on Earth, will allow investigators to detail the uneven composition of the Moon's interior.

The rationale for the mission, and other details, are detailed by the mission's host, the renowned Massachusetts Institute for Technology (MIT).

"The Moon is the most accessible and best studied of rocky, or "terrestrial", bodies beyond Earth. Unlike Earth, however, the Moon's surface geology preserves the record of nearly the entirety of 4.5 billion years of solar system history. Orbital observations combined with samples of surface rocks returned to Earth, show that no other body preserves the record of geological history so clearly as the Moon.

"The structure and composition of the lunar interior (and by inference the nature and timing of internal melting and heat loss) hold the key to reconstructing this history. Longstanding questions such the origin of the maria, the reason for the nearside-farside asymmetry in crustal thickness, and the explanation for the puzzling magnetization of crustal rocks, all require a greatly improved understanding of the Moon’s interior. Deciphering the structure of the interior will bring understanding of the evolution of the Moon itself, and also extend knowledge of the origin and thermal evolution of the Moon to other bodies in the inner solar system. For example, while the Moon was once thought to be unique in developing a "magma ocean" shortly after accretion, and now such a phenomenon has now been credibly proposed for Mars as well.

"This need to understand the internal structure in order to reconstruct planetary evolution motivates the GRAIL primary science objectives, which are to determine the structure of the lunar interior from crust to core and to further the understanding of the thermal evolution of the Moon. The GRAIL mission will accomplish these goals by performing global, regional and local high-resolution (30x30 km), high-accuracy gravity field measurements with twin, low-altitude (50 km) polar-orbiting spacecraft using a Ka-band ranging instrument."

The GRAIL Science Team will conduct six lunar science investigations:

* Map the structure of the crust & lithosphere.
* Understand the Moon’s asymmetric thermal evolution.
* Determine the subsurface structure of impact basins and the origin of mascons.
* Ascertain the temporal evolution of crustal brecciation and magmatism.
* Constrain deep interior structure from tides.
* Place limits on the size of the possible inner core."

GRAIL operations and preliminary data processing will be handled by NASA's Jet Propulsion Laboratory and noted lunar and planetary scientist Maria Zuber is principal investigator for the GRAIL mission.

Former astronaut Sally Ride will also head up a K-12 educational outreach project utilizing live television transmissions from five on-board "Moonkams."

Together with LRO and LADEE, GRAIL was originally funded as a necessary robotic precursor mission ahead of "extended human activity on the Moon," as mapped by the National Academies of Sciences Space Studies Board in 2007.

Engineers conduct checks on one of two NASA GRAIL spacecraft. The image was taken June 29, 2010 during the propulsion subsystem assembly and integration effort in the Space Support Building clean room at Lockheed Martin Space Systems in Denver.