NASA Grail: Twin Probes to Crash into Moon Next Week

Artist's concept of NASA's Grail mission. Grail's twin spacecraft are flying in tandem orbits around the moon to measure its gravity field in unprecedented detail. 

CREDIT: NASA/JPL

NASA's twin Grail spacecraft will crash into the lunar surface intentionally next week, bringing their gravity-mapping mission to a spectacular end.

The probes, known as Ebb and Flow, will be commanded to slam into the moon on Dec. 17, NASA officials said.

The agency will host a press conference to discuss the impact and the events leading up to it.

The $496 million Grail mission (Gravity Recovery and Interior Laboratory) launched in September 2011 to map the moon's gravity field in unprecedented detail.

Ebb and Flow arrived in orbit around the moon about one year ago — on New Year's Eve and New Year's Day, respectively.

The washing-machine-size spacecraft were originally tasked with 90-day science missions, which ran from March to May. but NASA extended Grail, allowing the two probes to gather a trove of additional data.

Ebb and Flow have been flying in formation around the moon, detecting the tiny changes in the distance between them caused by lunar mountains, craters and subsurface mass concentrations.

Scientists used these ultra-precise measurements to construct an incredibly accurate map of the lunar gravity field.

This map, unveiled last week at the fall meeting of the American Geophysical Union (AGU) in San Francisco, reveals that the moon's crust is almost completely pulverized.

The surprising find suggests that the moon and other rocky bodies in the inner solar system were pounded by long-ago impacts far more violently than previously believed, researchers said.

The new map was based on data gathered during Grail's original science mission; scientists expect to upgrade it based on measurements the spacecraft made during their extended mission, which brought them even closer to the lunar surface — an average altitude of 14 miles (23 km) as opposed to 34 miles (55 km).

Ebb and Flow are now running low on fuel as expected, NASA officials said, so the end is near for the probes. Mission managers will ensure that they go out in style on Dec. 17.

Video: GRAIL's Gravity Tour of the Moon - NASA JPL

Embedded video from NASA JPL

This movie shows the variations in the lunar gravity field as measured by NASA's Gravity Recovery and Interior Laboratory (GRAIL) during the primary mapping mission from March to May 2012.

Twin NASA probes orbiting Earth's moon have generated the highest resolution gravity field map of any celestial body.

The new map, created by the Gravity Recovery and Interior Laboratory (GRAIL) mission, is allowing scientists to learn about the moon's internal structure and composition in unprecedented detail.

Data from the two washing machine-sized spacecraft also will provide a better understanding of how Earth and other rocky planets in the solar system formed and evolved.

The gravity field map reveals an abundance of features never before seen in detail, such as tectonic structures, volcanic landforms, basin rings, crater central peaks and numerous simple, bowl-shaped craters.

Data also show the moon's gravity field is unlike that of any terrestrial planet in our solar system.

These are the first scientific results from the prime phase of the mission, and they are published in three papers in the journal Science.

"What this map tells us is that more than any other celestial body we know of, the moon wears its gravity field on its sleeve," said GRAIL Principal Investigator Maria Zuber of the Massachusetts Institute of Technology in Cambridge.

"When we see a notable change in the gravity field, we can sync up this change with surface topography features such as craters, rilles or mountains."

According to Zuber, the moon's gravity field preserves the record of impact bombardment that characterized all terrestrial planetary bodies and reveals evidence for fracturing of the interior extending to the deep crust and possibly the mantle. This impact record is preserved, and now precisely measured, on the moon.

The probes revealed the bulk density of the moon's highland crust is substantially lower than generally assumed. This low-bulk crustal density agrees well with data obtained during the final Apollo lunar missions in the early 1970s, indicating that local samples returned by astronauts are indicative of global processes.

"With our new crustal bulk density determination, we find that the average thickness of the moon's crust is between 21 and 27 miles (34 and 43 kilometers), which is about 6 to 12 miles (10 to 20 kilometers) thinner than previously thought," said Mark Wieczorek, GRAIL co-investigator at the Institut de Physique du Globe de Paris.

"With this crustal thickness, the bulk composition of the moon is similar to that of Earth. This supports models where the moon is derived from Earth materials that were ejected during a giant impact event early in solar system history."

The map was created by the spacecraft transmitting radio signals to define precisely the distance between them as they orbit the moon in formation. As they fly over areas of greater and lesser gravity caused by visible features, such as mountains and craters, and masses hidden beneath the lunar surface, the distance between the two spacecraft will change slightly.

Read the full article here: NASA JPL