Twin NASA Science Probes Start Lunar Gravity Mapping

by Ken Kremer on March 8, 2012

Twin GRAIL Moon Probes Ebb and Flow Start Lunar Gravity Science Mission
The twin GRAIL probes have just begun their science mission and will use a precision formation-flying technique to map Lunar Gravity, as depicted in this artist's rendering. Radio signals traveling between the two spacecraft provide scientists with exact measurements which will result in the most accurate gravity map of the moon ever made. Credit: NASA/JPL-Caltech

NASA’s twin lunar orbiting GRAIL (Gravity Recovery and Interior Laboratory) spacecraft christened Ebb and Flow have kicked off their science collection phase aimed at precisely mapping our Moon’s gravity field, interior composition and evolution, the science team informed Universe Today.

“GRAIL’s science mapping phase officially began Tuesday (March 6) and we are collecting science data,” said Maria Zuber, GRAIL principal investigator of the Massachusetts Institute of Technology in Cambridge, to Universe Today.

“It is impossible to overstate how thrilled and excited we are !”

“The data appear to be of excellent quality,” Zuber told me.

GRAIL’s goal is to provide researchers with a better understanding of how the Moon, Earth and other rocky planets in the solar system formed and evolved over its 4.5 billion years of history.

NASA’s Dawn spacecraft is currently mapping the gravity field of Asteroid Vesta in high resolution from low orbit.

Despite more than 100 missions to the Moon there is still a lot we don’t know about the Moon says Zuber, like why the near side is flooded with magma and smooth and the back side is rough, not smooth and completely different.

South pole of the far side of the moon as seen as seen in this 1st image from the MoonKAM camera aboard GRAIL mission’s Ebb spacecraft. Credit: NASA/JPL-Caltech

The formation-flying spacecraft will make detailed science measurements from lunar orbit with unparalleled precision to within 1 micron – the width of a human red blood cell – by transmitting Ka-band radio signals between each other and Earth to help unlock the mysteries of the Moon’s deep interior.

“We’ve worked on calibrating the alignment of the Ka-band antennae to establish the optimal alignment. We’ve verified the data pipeline and are spending a lot of time working with the raw data to make sure that we understand its intricacies,” Zuber explained.

The washing-machine sized probes have been flying in tandem around the Moon since entering lunar orbit in back to back maneuvers over the New Year’s weekend. Engineers have spent the past two months navigating the spaceship duo into lower, near-polar and near-circular orbits with an average altitude of 34 miles (55 kilometers), that are optimized for science data collection, and simultaneously checking out the spacecraft systems.

GRAIL A and B gravity mappers rocket to the moon atop a Delta II Heavy booster on Sept. 10 from Cape Canaveral, Florida. View to Space Launch Complex 17 gantry from Press Site 1. Credit: Ken Kremer


Ebb and Flow were launched to the Moon on September 10, 2011 aboard a Delta II rocket from Cape Canaveral, Florida and took a circuitous 3.5 month low energy path to the moon to minimize the overall costs. The Apollo astronauts reached the Moon in just 3 days.

I asked Zuber to describe the team’s activities putting the mirror image probes to work peering to the central core of our nearest neighbor in unprecedented detail.

“Last Wednesday (Feb. 29) we achieved the science orbit and on Thursday (March 1) we turned the spacecraft to ‘orbiter point’ configuration to test the instrument and to monitor temperatures and power.”

“When we turned on the instrument we established the satellite-to-satellite radio link immediately. All vital signs were nominal so we left the spacecraft in orbiter point configuration and have been collecting science data since then. At the same time, we’ve continued performing calibrations and monitoring spacecraft and instrument performance, such as temperatures, power, currents, voltages, etc., and all is well,” said Zuber.

Measurements gathered over the next 84 days will be used to create high-resolution maps of the Moon’s near side and far side gravitational fields that are 100 to 1000 times more precise than ever before and that will enable researchers to deduce the internal structure and composition of our nearest neighbor from the outer surface crust down to the deep hidden core.

As one satellite follows the other, in the same orbit, they will perform high precision range-rate measurements to precisely measure the changing distance between each other. 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.

“GRAIL is great. Everything is in place to get science data now,” said Sami Asmar, a GRAIL co-investigator from NASA’s Jet Propulsion Lab in Pasadena, Calif. “Soon we’ll get a very high resolution and global gravity map of the Moon.”

The data collected will be translated into gravitational field maps of the Moon that will help unravel information about the makeup of the Moon’s core and interior composition.

GRAIL will gather three complete gravity maps over the three month mission which is expected to conclude around May 29. If the probes survive a solar eclipse in June and if NASA funding is available, then they may get a bonus 3 month extended mission.

Ebb and Flow - New Names for the GRAIL Twins in Lunar Orbit
4th Grade Students from Montana (inset) win NASA’s contest to rename the GRAIL A and GRAIL B spacecraft. Artist concept of twin GRAIL spacecraft flying in tandem orbits around the Moon to measure its gravity field Credit: NASA/JPL Montage: Ken Kremer

NASA sponsored a nation-wide student contest for America’s Youth to choose new names for the twin probes originally known as GRAIL A and GRAIL B. 4th graders from the Emily Dickinson Elementary School in Bozeman, Montana submitted the winning entries -Ebb and Flow. The new names won because they astutely describe the probes movements in orbit to collect the science data.

The GRAIL twins are also equipped with a very special camera dubbed MoonKAM (Moon Knowledge Acquired by Middle school students) whose purpose is to inspire kids to study science.

By having their names selected, the 4th graders from Emily Dickinson Elementary have also won the prize to choose the first target on the Moon to photograph with the MoonKAM cameras, which are managed by Dr Sally Ride, America’s first female astronaut.

“MoonKAMs on both Ebb and Flow were turned on Monday, March 5, and all appears well, Zuber said. “The Bozeman 4th graders will have the opportunity to target the first images a week after our science operations begin.”

About 

Dr. Ken Kremer is a speaker, scientist, freelance science journalist (Princeton, NJ) and photographer whose articles, space exploration images and Mars mosaics have appeared in magazines, books, websites and calanders including Astronomy Picture of the Day, NBC, BBC, SPACE.com, Spaceflight Now and the covers of Aviation Week & Space Technology, Spaceflight and the Explorers Club magazines. Ken has presented at numerous educational institutions, civic & religious organizations, museums and astronomy clubs. Ken has reported first hand from the Kennedy Space Center, Cape Canaveral and NASA Wallops on over 40 launches including 8 shuttle launches. He lectures on both Human and Robotic spaceflight - www.kenkremer.com. Follow Ken on Facebook and Twitter

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