A Step Closer to the Lunar Holy GRAIL

An artist's concept of the twin GRAIL spacecraft, Ebb and Flow, in action around the Moon. Image Credit: NASA/JPL-Caltech/MIT

We’re a little closer to unraveling the mysteries of the Moon. Last week, NASA’s Gravity Recovery and Interior Laboratory (GRAIL) mission finished the first phase of its mission a week early, enabling the space agency to extend its science operations through December 3, 2012.

GRAIL’s twin probes were launched on a United Launch Alliance Delta II rocket at 9:08 AM EDT on September 10, 2011 during the second window of the day. GRAIL-A, since named Ebb, reached the Moon on New Year’s Eve 2011 while GRAIL-B, Flow, arrived New Year’s Day 2012. The twin washing machine sized spacecraft entered into a near polar, nearly circular orbit with an average distance from the surface of 34 miles (about 55 kilometers). Then on March 8, they sprung into action and began collecting data.

NASA's GRAIL twin spacecraft await launch atop a United Launch Alliance Delta II rocket at Cape Canaveral Air Force Station, Florida. Image Credit: NASA/JPL-Caltech/United Launch Alliance, Thom Baur

Ebb and Flow were designed to answer longstanding questions about the moon and give scientists a better understanding of how the Earth and other rocky planets in the solar system formed. They do this by studying the Moon from crust to core. They collect information – mission scientists boast the spacecraft’s ability to gather unprecedented levels of detail –  about the Moon’s internal structure and evolution.

For the last 89 days the probes have operated around the clock, covering the entire lunar surface three times. The Lunar Gravity Ranging System onboard each spacecraft sent radio signals to Earth that scientists can translate into a high-resolution map of the moon’s gravitational field. The spacecraft returned their last data set of the prime mission at 1 PM EDT on May 29. At the time, the spacecraft were 37 miles (about 60 kilometers) above the Sea of Nectar.

“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 Maria Zuber, principal investigator of GRAIL at the Massachusetts Institute of Technology.

NASA has shut down both spacecraft’s instruments until August 30. During this resting stage, Ebb and Flow will have to endure the June 4 lunar eclipse, which will bring sudden changes in temperature and energy-sapping darkness. But GRAIL engineers have planned for the hazards of an eclipse, and they don’t expect any damage to the spacecraft’s health.

NASA's twin GRAIL spacecraft are lowered onto the second stage of their Delta II launch vehicle at Space Launch Complex 17B at Cape Canaveral Air Force Station in Florida on August 18, 2011. Photo Credit: NASA/KSC

“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. “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.”

GRAIL’s upcoming extended science mission goal is to take an even closer look at the moon’s gravity field. To do this, mission planners will halve their current altitude and put the spacecraft in the lowest orbit they can safely maintain. It will be an ongoing close shave. “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 of JPL, 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.”

We can expect great return from GRAIL’s extended science mission after this very promising start. “It’s energizing to realize that what we traveled from Earth to the moon for is right here in our hands,” said Zuber. By the end of the year, and GRAIL’s second operational period, we should have a much better understanding of the permanent fixture in our sky.

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