NASA Launches Grail Moon Probes After Delays

Two identical space probes are on their way to the moon, after launching Saturday on NASA's newest lunar science mission aimed at unlocking mysteries of the moon that are hidden beneath its surface.

The two Grail spacecraft blasted off this morning on an unmanned Delta 2 rocket at 9:08 a.m. EDT (1308 GMT) from the Cape Canaveral Air Force Station in Florida, after windy weather and a technical glitch delayed the mission's launch by two days and threatened to again. A first try earlier today at 8:29 a.m. EDT (1229 GMT) was called off due to high-altitude winds, but weather conditions improved, and the agency was able to take advantage of the day's second launch opportunity.

The $496 million Grail mission (short for Gravity Recovery And Interior Laboratory) will closely study the interior of the moon, from crust to core, and will map the moon's gravitational field in unprecedented detail. The three-month mission is expected to help scientists better understand the composition of Earth's natural satellite and its evolutionary history since it was formed 4.6 billion years ago.

"We've used gravity science before, however these have been very primitive attempts compared to what Grail will be able to accomplish," Robert Fogel, Grail program scientist at NASA Headquarters in Washington, D.C., said in a news briefing on Wednesday (Sept. 7).

Understanding how the origin of the moon and how it evolved will shed light on how other rocky planets in the inner solar system formed, said Maria Zuber, Grail principal investigator at MIT.

"We have orbital reconnaissance of the surface, we have lunar samples which we can analyze in Earth labs," Zuber said. "The piece of the puzzle that has been missing in trying to reconstruct lunar evolution is understanding of the lunar interior."

The two spacecraft, called Grail-A and Grail-B, are flying on an energy-efficient path to the moon, and are expected to arrive at their lunar target around New Year's Day (Jan. 1).

The twin probes will then enter into tandem orbits around the moon, separated from each other by a distance of about 75 to 225 miles (121 to 362 kilometers). The spacecraft will circle the moon about 34 miles (55 km) above the surface.

As the Grail spacecraft chase each other around the moon, regional differences in the lunar gravitational field will cause the probes to speed up or slow down, changing the distance between them, explained Sami Asmar, Grail deputy project scientist at NASA's Jet Propulsion Laboratory in Pasadena, Calif.

Microwave signals bounced back and forth between Grail-A and Grail-B will measure this distance, which will help researchers construct accurate maps of the moon's gravity. The instruments onboard the Grail probes are so precise they will be able to calculate the distance between them to within less than the width of a human red blood cell, Asmar said. [Video: Grail's Mission to Map Moon Gravity]

Understanding how the moon formed and evolved will help scientists piece together clues of how other large objects in the inner solar system came to be.

"The moon is a fantastic body … in terms of learning about early planets," Zuber said. "It's nearby, it's accessible, and it preserves the record of what early planets are like. Other planets in the inner part of the solar system have gone through the same processes that the moon has gone through."

The Grail mission is also expected to raise public awareness about the moon, and special cameras aboard the probes will be used to encourage middle school students to participate in lunar science and follow along with the Grail expedition.

The so-called MoonKam project, which will capture pictures of the lunar surface for students on Earth, is being led by former NASA astronaut Sally Ride and her educational company Sally Ride Science.

As part of their public outreach efforts, NASA also invited 150 Twitter fans to attend the launch and share their experiences with the public through social media.

*   20 Most Marvelous Moon Missions
  *   How NASA's GRAIL Probes Will Map the Moon's Gravity (Infographic)
  *   Lunar Timeline: Humanity's Exploration of the Moon 

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