Navigating a three-and-a-half month journey from the earth to the moon, NASA’s Gravity Recovery And Interior Laboratory (GRAIL) twin spacecraft successfully entered lunar orbit on New Year’s Eve and on New Year’s Day. Their four-million-kilometer “low energy” trajectory was designed to minimize the propellant required to decelerate the tandem spacecraft by only 190 meters per second in order to slip into polar orbits around the moon. The twin GRAIL spacecraft were launched from Cape Canaveral, Fla., on Sept. 10.
GRAIL’s principal investigator is Maria Zuber, the E.A. Griswold Professor of Geophysics and Planetary Science at MIT. Zuber’s proposal for the $496 million mission was selected by NASA in 2007.
“Although I fully expected that this day would come, it hardly seems real,” said Zuber. “All of our preparation dealt with how to respond when things went wrong and we never considered a scenario where everything proceeded perfectly. It's probably why we are where we are right now.”
GRAIL’s 82-day scientific mission to map the moon’s gravitational field will begin on March 8 after each spacecraft’s orbit has been circularized at an altitude of 55 kilometers above the lunar surface. A total of seven main engine firings on each spacecraft will be required to circularize their lunar orbits.
By precisely measuring changes in distance between the twin orbiting spacecraft, scientists will construct a detailed gravitational model of the moon that will be used to answer fundamental questions about the moon’s evolution and its internal composition.
"As the first spacecraft passes over a mass anomaly beneath the surface of the moon, it will accelerate or slow down and its distance will change with respect to the second spacecraft. By measuring tiny distance changes, we will be able to determine the moon’s internal structure," Zuber added. "We can measure the distance of these two spacecraft to less than the size of a red blood cell — a few tenths of a micron per second in the relative velocity of these two spacecraft.”
Zuber is currently writing a proposal to NASA for an extended mapping mission to take place between September and December. The GRAIL spacecraft orbits would be lowered from 55 kilometers to only 25 kilometers above the lunar surface, improving their spatial resolution of the moon’s surface and subsurface geological features. “We believe we can get that low … the spacecraft would actually be very close to the highest peaks … so it could be rather exciting,” commented Zuber.
The GRAIL spacecraft would first have to survive a lengthy solar eclipse in early June. Based on better-than-expected performance of the spacecraft solar cells and lithium ion batteries, it now looks very probable that they will survive the eclipse.
One of GRAIL’s secondary goals is to encourage middle school students to participate in the mission. The GRAIL spacecraft carry MoonKAM imaging systems that will allow students from around the world to obtain images of lunar surface features they select. More than 2,100 schools have signed up to participate in MoonKAM. Later this month, the twin GRAIL spacecraft will receive their new names based on a student competition held last fall.
"My resolution for the new year is to unlock lunar mysteries and understand how the moon, earth and other rocky planets evolved," said Zuber. "Now, with GRAIL successfully placed in orbit around the moon, we are one step closer to achieving that goal."
"NASA greets the new year with a new mission of exploration," said NASA Administrator Charles Bolden. "The twin GRAIL spacecraft will vastly expand our knowledge of our moon and the evolution of our own planet."