This artist rendering provided by NASA shows the twin Grail spacecraft mapping the lunar gravity field.
updated 1/1/2012 5:43:16 PM ET 2012-01-01T22:43:16

A pair of NASA spacecraft are ringing in the new year in grand style, with both now successfully circling the moon after journeying through space for more than three months.

The Grail-B probe entered lunar orbit Sunday after firing its main engine at 5:05 p.m. EST (2205 GMT) in a 40-minute-long orbital-insertion burn, NASA officials said. It joins its twin spacecraft, Grail-A, which arrived at the moon Saturday evening after completing a similar maneuver.

The two probes are on a mission to study Earth's nearest neighbor from crust to core. After gradually circling down to super-low orbits, the pair will zip around the moon in tandem, working together to map the lunar gravity field in unprecedented detail, researchers said.

Scientists expect the probes' measurements to help unlock some longstanding mysteries about the moon's composition and evolution — mysteries that have withstood more than 100 moon missions over the years.

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"You might think that, given all of these observations, we would know what there is to know about the moon," Grail principal investigator Maria Zuber, of the Massachusetts Institute of Technology, told reporters Wednesday (Dec. 28). "Of course that's not the case."

The moon's many secrets
Scientists think the moon formed after a Mars-size body struck Earth about 4.5 billion years ago. This titanic impact blasted huge amounts of material into space, and they eventually coalesced.

While this basic outline of the moon's origin is pretty well established, many mysteries remain about how the rocky body has evolved since then.

Chief among these puzzles, perhaps, is why the near side of the moon — the face we always see from Earth — is so different from the far side. For example, plains of volcanic rock (called maria) are much more widespread on the near side, and the far side is higher and more mountainous, with the lunar surface 1.2 miles (1.9 kilometers) higher in elevation, on average, than the near side.

"We don't actually know why the near side and far side of the moon are different," Zuber said. "We think that the answer is locked in the interior."

One possible explanation, proposed by researchers this August, is that the gigantic collision 4.5 billion years ago actually created two moons. The second moon, which was much smaller, later slammed into our moon's far side, the idea goes, spreading itself over the surface rather than creating a crater.

The Grail probes' observations could help determine whether that scenario is accurate, researchers said.

"We believe that we can obtain the information that we need to test this hypothesis," Zuber said.

Artist rendering of lunar probes over the moon's surface
Lockheed Martin
Artist's concept of the two Gravity Recovery and Interior Laboratory (GRAIL) spacecraft orbiting the moon. NASA launched the twin probes in September 2011 to study the moon's gravitational field in unprecedented detail.

Formation flying
The $496 million Grail (short for "Gravity Recovery And Interior Laboratory") mission launched Sept. 10. The two washing-machine-size probes then spent more than three months making their slow, circuitous way to the moon.

By comparison, NASA's manned Apollo 11 mission in 1969 reached the moon in three days. But Apollo 11 prioritized speed, while Grail's path is energy-efficient and has given engineers ample time to assess the probes' health and charge up their scientific gear, researchers said.

Grail-A and Grail-B still have some work to do from their initial lunar orbits to get into position. They'll spend two months circling lower and lower, eventually settling into orbits just 34 miles (55 kilometers) above the lunar surface by March.

Only then will the twin spacecraft begin their science campaign. They'll chase each other around the moon for 82 days, staying 75 to 225 miles (121 to 362 km) apart. [Video: Grail's Mission to Map Moon Gravity]

Regional differences in the lunar gravity field will cause the two spacecraft to speed up or slow down slightly, changing the distance between them as they fly. Bouncing microwave signals back and forth off  each other, Grail-A and Grail-B will gauge these tiny distance variations constantly.

The Grail team will use the twin probes' measurements to construct highly detailed maps of the moon's gravitational field. These maps are expected to reveal a great deal about lunar composition, allowing scientists to draw insights about how the moon formed and how it has changed over time, researchers said.

Public outreach and education are a big part of the Grail mission. Each spacecraft is carrying four cameras and will snap photos of the lunar surface at the direction of middle-school students across the country. Students will get to pore over these pictures after they're beamed back down to Earth.

The photo project — which is called MoonKam and is run by former NASA astronaut Sally Ride — has already generated lots of enthusiasm. More than 2,100 schools have signed up to participate, according to Zuber.

"We've had a great response to the MoonKam project," Zuber said. "We're still accepting applications. We can accommodate another several hundred schools into the program if there's interest in doing that."

You can follow SPACE.com senior writer Mike Wall on Twitter: @michaeldwall. Follow SPACE.com for the latest in space science and exploration news on Twitter @Spacedotcom and on Facebook.

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Video: NASA hopes to answer questions about the Moon

  1. Closed captioning of: NASA hopes to answer questions about the Moon

    >>> finally tonight here on earth this weekend is all about new year's eve. but for a team of nasa scientists, the focus will be a quarter of a million miles away . they will be monitoring a pair of spacecraft orbiting the moon, hoping to answer some age-old questions in what's being described as the biggest moon mission since man first landed there more than 40 years ago. here's nbc's george lewis .

    >> reporter: millions of years ago, earth may have had not one, but two moons. then, according to theory they collided -- an event known as the big splat. that would explain the irregularities on the present day moon. all the valleys on the side we see. all the mountains on the other side.

    >> scientists have not had a good way to track that down yet. this is the going of solving that mystery.

    >> zero, and liftoff of the delta ii , journey to the center of the moon.

    >> reporter: in september, nasa launched two lunar probes aboard the same rocket. they'll chase each other in orbit around the moon to study it from surface to center. the mission will be run from this control room in paspasadena, california. a critical point comes this weekend when the two craft fire to get into orbit around the moon.

    >> this is the big crunch time. this weekend is going to be the time when it comes together or falls apart .

    >> that's one small step for man --

    >> reporter: for nasa it will be the most intense focus on the moon since the days of the apollo program .

    >> it's very challenging but we have done a lot of practicing and we're ready to do that.

    >> reporter: former astronaut sally ride will organize thousands of middle school students to tell nasa what pictures the spacecraft should take with their on-board cameras.

    >> hopefully it will encourage kids to get more involved in science and engineering.

    >> reporter: so this weekend while millions are watching the new year's ball drop in times square , nasa scientists will be watching the other illuminated ball that holds so many mysteries. george lewis , nbc news, pasadena, california.

Photos: 50 years of views from the moon

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  1. Up-close exploration of the moon, Earth's only natural satellite, began in 1959 when the Soviet Union launched its Luna 1 spacecraft on a flyby mission. NASA quickly followed up with missions of its own. Since then, the Europeans, Japan, China and India have launched their own lunar exploration programs. This view shows the moon as seen from the international space station. Click the "Next" arrow above to check out 11 images from the moon made over the last 50 years. (NASA) Back to slideshow navigation
  2. 1959: Far side in full view

    In October 1959, the Soviet Union's Luna 3 spacecraft - the third successfully launched to the moon - made history as the first probe to image the far side of the moon. The photos were fixed and dried on the spacecraft and beamed back to Earth. Though fuzzy by today's standards, the images showed stark differences from the near side, including relatively few dark areas, called lunar maria. (RSA via NASA) Back to slideshow navigation
  3. 1966: A restored ‘Earthrise'

    In 1966 and 1967, NASA sent a series of Lunar Orbiter spacecraft to collect detailed images of moon's surface in preparation for the Apollo program. The tapes were then put in storage. Decades later, researchers with the Lunar Orbiter Image Recovery Project collected the vintage hardware required to play back the imagery. That imagery was digitized , reproducing the images at a much higher resolution than previously possible. On Nov. 11, 2008, the project researchers released this enhanced photograph of Earth rising above the lunar surface, originally made by Lunar Orbiter 1 in 1966. (LOIRP / NASA) Back to slideshow navigation
  4. 1968: The most famous 'Earthrise'

    On Christmas Eve 1968, Apollo 8 astronauts Frank Borman, Jim Lovell and William Anders made history as the first humans to orbit the moon. They were scouting its surface for a suitable landing spot for future missions. But the sight of Earth rising above the moon's horizon caught their - and the world's - attention. The photograph, called "Earthrise," is among the most famous ever made from the moon. (NASA) Back to slideshow navigation
  5. 1969: One small step

    On July 20, 1969, an estimated 1 billion people around the world were glued to television screens to watch astronaut Neil Armstrong, commander of Apollo 11, climb down from the lunar module spacecraft for a stroll on the moon. As his foot touched the lunar surface, he famously said "That's one small step for (a) man, one giant leap for mankind." This image is a black-and-white reproduction from the telecast, showing Armstrong stepping down from the lunar module's ladder. (NASA Johnson Space Center) Back to slideshow navigation
  6. 1969: Man on the moon

    Apollo 11 astronaut Buzz Aldrin, shown here, accompanied Armstrong for the famous walk on the moon. This iconic image is one of the few that shows Armstrong on the lunar surface - as seen in the reflection on the spacesuit's visor. The astronauts walked around on the lunar surface for about two and a half hours. (NASA) Back to slideshow navigation
  7. 1994: Looking for ice

    This mosaic image of the moon's southern polar region, made by the Clementine spacecraft in 1994, suggested that the region could harbor water ice within regions of its craters that are never lit by the sun. The water ice would be left over from impacting comets. Scientists have debated the evidence for and against water ice at the poles ever since the Clementine discovery. The current era of lunar exploration could resolve the debate. If water ice exists, it could help quench the thirst of future human colonists and be used to make fuel for rockets. (NASA / JPL / USGS) Back to slideshow navigation
  8. 2006: European moon probe crashes

    On Sept. 3, 2006, the European Space Agency's SMART-1 spacecraft went out with a bang - a planned crash landing into a volcanic plain called the Lake of Excellence. The impact, shown here, was captured by the Canada-France-Hawaii Telescope. (The black lines are a processing error due to the brightness of the event.) The spacecraft was launched in 2003 primarily to test an ion propulsion system, which uses energy captured by the sun to produce a stream of charged particles. The slow-and-steady propulsion system may be used on future interplanetary missions. (Christian Veillet / CFHT via AP) Back to slideshow navigation
  9. 2007: China targets the moon

    China made its first major strides in the lunar exploration game with the launch of the Chang'e 1 spacecraft in October 2007. The orbiter was sent to make a detailed, 3-D map of the moon's surface. Premier Wen Jiabao unveiled the first image at a ceremony in Beijing, shown here. Chang'e 1's 16-month mission ended with a controlled crash. The country reportedly plans to launch lunar rovers in 2010 and 2017, and a manned mission to the moon by 2020. (Huang Jingwen / XINHUA NEWS AGENCY) Back to slideshow navigation
  10. 2008: India joins the lunar club

    The Indian Space Research Organization successfully launched its Chandrayaan 1 spacecraft on Oct. 22, 2008, for a mapping mission to the moon. A probe released from the mothership took this picture of the lunar surface during its descent to a planned crash landing at the south pole. The Indian space agency plans to use this and other data for a lunar rover mission in 2011 and, eventually, a manned mission. (ISRO via EPA) Back to slideshow navigation
  11. 2009: Japan orbiter watches eclipse

    Some eclipse enthusiasts travel the globe to glimpse alignments of the sun, Earth and moon. Japan's Kaguya probe did them one better: It shot this sequence of a Feb. 10, 2009, eclipse from its lunar orbit. The image shows the view of the sun from the moon mostly covered by Earth. The "ring" appears dark at the bottom because it is obscured by the night-darkened limb of the moon. The Kaguya orbiter was launched in September 2007 to study the moon's origin and evolution. It made a controlled crash landing on the moon in June 2009. (JAXA / NHK) Back to slideshow navigation
  12. 2009: NASA goes back to the moon

    On June 18, 2009, NASA launched two spacecraft to the moon to map its surface in unprecedented detail, scout for future landing sites, and smash probes into a permanently shaded crater in hopes of resolving a longstanding debate over whether such regions contain water ice. NASA's Lunar Reconnaissance Orbiter will orbit both poles for a year, and its mission could be extended to serve as a communications relay for future lunar missions. This is one of the first pictures sent back by the orbiter. LRO's sibling, the crater-smacking LCROSS probe, is due to impact the moon's south pole in October. (NASA / GSFC / ASU) Back to slideshow navigation
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