Satellite Twins Aim to Map Moon's Gravity Field
LAUREL, Md. Two microsatellites developed for an Air Force lab have taken on a NASA mission to map the moon's gravity by using precise formation flying.
The Gravity Recovery and Interior Laboratory (GRAIL) mission can gauge the lunar gravity field by seeing how gravity changes the distance between its two spacecraft. That requires special moon tech in the form of a radio signal-based system that detects distance changes as small as a few millionths of a meter per second.
"It's the first planetary formation-flying mission where we line these [spacecraft] up for precise coordination between the two instruments," said Maria Zuber, MIT geophysicist and principal investigator of the GRAIL mission.
The spacecraft heritage comes from XSS-11 satellites each about the size of a washing machine built by Lockheed Martin for the Air Force Research Laboratory. The Air Force lab wanted the satellites for experimental space missions involving coordinated spaceflight operations, such as what GRAIL requires.
Keeping in touch
Both GRAIL spacecraft are slated to enter a polar orbit about 34 miles (55 km) above the lunar surface, but could range in separation distance from 40 miles (60 km) to almost 140 miles (225 km). They use an S-band antenna to send a time-synchronization code back and forth between the spacecraft, as well as a Ka-band antenna to measure the precise distance between them.
That system works not unlike how GPS can estimate locations based on the timing of received satellite signals . But GRAIL's moon mission cannot take advantage of the GPS satellite constellation that hovers around Earth.
"I was trying to have as few technological miracles as possible to pull it off," Zuber told the attendees at the International Academy of Astronautics' ninth Low-Cost Planetary Missions Conference here yesterday. "I'd love to use GPS around other planets, but if we could figure that out we'd have done something else."
Fly me to the moon
If launch goes as planned in September 2011, GRAIL would arrive at the moon after two to four months of flight time. The twin spacecraft would then enter orbit under the lunar South Pole about 25 hours apart, so that mission planners don't have to deal with two orbital insertions in one day.
By contrast, the Apollo missions that took astronauts to the moon in three or four days. But the slow pace allows the twin spacecraft to use up almost all their minimal fuel supply of just 200 grams, as well as vent any gas that might interfere with the gravity-measuring mission.
"We actually want to start mapping the moon with a practically empty fuel tank, because fuel sloshing around in the fuel tank is a nonconservative force and it perturbs gravity modeling," Zuber explained.
GRAIL has the ability to map the moon's interior from crust to core within a month of arrival. The trickier task of measuring the gravity field is expected to take the entire 90-day mission at the moon .
Solid mission planning has enabled GRAIL to stay on schedule and under budget so far.
"The launch window opens on Sept. 8, and we're going to the moon," Zuber said.