Lunar duo to tackle lingering questions about our Moon
BY JUSTIN RAY
Posted: September 5, 2011
Men have walked on it and satellites have surveyed it from every angle, but another mission to the Moon launching this week seeks profound new insights about our nearest neighbor in the night sky.
"There have been many missions that have gone to the Moon, orbited the Moon, landed on the Moon, brought back samples of the Moon. But the missing piece of the puzzle in trying to understand the Moon is what the deep interior is like," said Maria Zuber, principal investigator of the upcoming GRAIL mission.
"What was going on in the interior? How did it melt? Is there a core? How did the core form? How did the interior convect? Why are the impact basins on the near side flooded with magma and give us this man-in-the-moon shape whereas the back side of the Moon doesn't have any of this? These are all mysteries that despite the fact we've studied the Moon before we don't understand how that has happened. GRAIL is a mission that is going to tell us that."
The GRAIL duo will create an unprecedented lunar gravity map that scientists can follow in their quest to determine the Moon's interior structure from crust-to-core.
"To really understand the Moon and understand what makes it special, we need to study what's inside," said Zuber of the Massachusetts Institute of Technology.
Two precise moments in time are available daily for liftoff to occur during the mission's launch window. The dual opportunities each day are driven by which azimuth the rocket is sent on -- either 93 or 99 degrees. The window closes in October to ensure GRAIL's mapping is completed before next June's lunar eclipse that is expected to be lethal to the satellites.
For Thursday's launch, liftoff is planned for 8:37:06 a.m. EDT (1237:06 GMT). If weather or a technical problem prevents the rocket from getting off the ground, the team can load the different flight azimuth program aboard the Delta and retarget for liftoff at 9:16:12 a.m. EDT (1316:12 GMT).
A complete listing of the launch times can be found in our window chart.
The two-stage launcher sports nine strap-on solid rocket boosters slightly larger in diameter than normally used, which gives the vehicle its "Heavy" designation. This is the last scheduled launch of the venerable Delta 2 from the Cape after 22 years of service.
The public can view the launch from the Jetty Park pier located just three miles south of pad 17B. It's the closest you can get to witness a rocket blasting off from the Cape.
Directions to the viewing spot can be found here.
About 80 minutes into flight, the rocket's second stage will deliver the final boost necessary to escape Earth orbit and then deploy the two satellites eight minutes apart to begin their independent journeys to the Moon.
But the craft won't head straight to their destination, instead taking a circuitous route lasting three-and-a-half months across 2.6 million miles of space before braking into lunar orbit.
"If you leave Earth and go barreling to the Moon, you need a lot of fuel to slow down (and enter lunar orbit). We want to use small spacecraft with small fuel tanks. So as a consequence of that, we use something called a low-energy trajectory where we go out to a point called the Earth-Sun Lagrange Point, which is like a void in the ocean," said Zuber.
The lengthy trip also allows the satellites to "out-gas" on the way, ensuring this phenomena isn't mistaken for gravity measurements during the science-gathering portion of the mission later.
"We need this time to do out-gassing. There's little tiny particles on the spacecraft and those need to out-gas because that's inducing a force on the spacecraft, like internal gravity," said David Lehman, GRAIL project manager from the Jet Propulsion Laboratory.
"The other reason is we have two small spacecraft on one medium-size rocket. We did that for cost savings. If we had two bigger spacecraft, we would need two rockets," Lehman added.
"In addition to saving fuel, what this trajectory allows to happen is these two spacecraft will arrive at the Moon on the same day no matter when we launch," said Zuber.
Approaching from below the lunar south pole, the spacecraft will fire the main thrusters for 38 minutes to slow their speed by 427 miles per hour, just enough to slip into preliminary elliptical orbits stretching 11.5 hours.
A subsequent month of maneuvers will circularize the orbit, followed by another month's worth of work to get the two birds synchronized in precise polar orbits chasing each other around the Moon.
"The big challenge of GRAIL is actually getting into the science orbit. We call that the transfer to science formation phase. We have to get the spacecraft very tightly coordinated one around the other. Our navigators are a quarter-million miles away from the Moon trying to orchestrate these two spacecraft into very, very precise orbits. I'm a retired Navy Reserve officer and to me it's like the Blue Angels at the Moon," said Lehman.
The 82-day science phase begins on March 8 with GRAIL-B leading the GRAIL-A spacecraft while flying at an altitude of 34 miles above the Moon. The separation distance between the two will change from 62 to 140 miles over the course of the mapping to dig deeper into the Moon.
"The separation distance is what we call tuning for what wavelength gravity field you're after. So if you want to see -- gravitationally -- small features, craters, mountains, you want to be close to each other and low. But if you want to see global features such as the core of the Moon that we're after, then you want to be high and separate from each other," said Sami Asmar, GRAIL deputy project scientist from JPL.
The mirror-image craft will bounce radio signals off each other to measure subtle changes in the distance between the two formation-flying satellites caused by the lunar gravity field, enabling scientists to deduce the Moon's origins and the composition of its interior structure.
"It's a little bit like doing a CAT scan of the Moon, but we use gravity instead of acoustic waves. It provides us essentially with a picture of the lunar interior just like you'd make a picture of the inside of your body if you were trying to understand what your state of health is," said Zuber.
"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 recover what the interior structure of the Moon looks like," said Zuber.
"Measuring the distance between two points is not very hard...but we can measure the distance of these two spacecraft two less than the size of a red blood cell -- a few tenths of a micron per second in the velocity of these two spacecraft. It pays to learn how to measure the distance between two points and learn it well!"
Built by Lockheed Martin, the GRAIL spacecraft are based upon the U.S. military's Experimental Satellite System-11 (XSS-11) platform launched in 2005.
About the size of a washer and dryer, the GRAIL satellites weigh 677 pounds at launch. Each stand 3.58 feet high, 3.12 feet wide and 2.49 feet deep. They are equipped with two solar arrays that unfold after deployment from the Delta rocket and they have a 22-Newton hydrazine-fueled main engine and eight 0.9-Newton warm gas attitude control system thrusters.
Asmar says the satellites keep things simplistic. "Single payload, single instrument. It only does one thing and it does it very well."
Although they are twins, they're not exactly alike.
"They are mirror images of each other, so they cannot be interchanged. They are designed such that this one points this way and that one points that way. If you reverse them, it's not going to work," said Asmar.
"GRAIL is the Moon-version of the extremely successful Earth science GRACE mission that is mapping the Earth right now. We were able to take the GRACE instrumentation and adapt it for orbit around the Moon," said Zuber.
Other ways of measuring the gravity field are not possible at the Moon since the far side is never visible from Earth. With two spacecraft linked together like GRAIL, they can track the gravity changes continuously around the entire lunar globe and relay all the data back to scientists.
"We have a decent gravity map for half the Moon, with practically zero information on the far side. We need to cover the Moon globally," said Asmar.
GRAIL promises to improve knowledge of the near side by 100 times and the far side by 1,000 times.
"This will be the highest resolution gravity field for any planet, including Earth. On Earth, you can't get down low enough to make the kind of measurements we're making because of the atmospheric drag," said Zuber.
GRAIL is the first formation-flying mission to another world. The team hopes one of the legacies of the project will enable similar satellite tandems to explore other questions in the solar system.
"Imagine mapping currents in the ocean beneath Europa using a concept like this," said Zuber.
"Technologically, it's blazing the trail for other missions that will want go to other planets and use coordinated observations to make measurements."
GRAIL is facing a finite lifespan, its mapping mission sandwiched between lunar eclipses in December and June. The science phase will be finish by May 29, followed by a final calibration and brief decommissioning before the June 4 eclipse that will starve the spacecraft of solar power.
"Right now, we're thinking we won't survive that. It wasn't designed to do that. Not big enough batteries. We'd just get too cold and lose power and that's it," said Lehman.
Within days, the satellites' altitude will naturally decay lower and lower until crashing into the Moon.
From launch until the mission's ending, the GRAIL-A spacecraft is expected to travel 13.2 million miles and GRAIL-B some 12.8 million miles.
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