After leaving Earth six months ago and entering lunar orbit on New Year's, NASA's tandem GRAIL spacecraft commenced their Moon-mapping mission this week.

"The initiation of science data collection is a time when the team lets out a collective sigh of relief because we are finally doing what we came to do," said Maria Zuber, the GRAIL principal investigator. "But it is also a time where we have to put the coffee pot on, roll up our sleeves and get to work."

The mirror-image craft are bouncing 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.

GRAIL's science instrument is called the Lunar Gravity Ranging System, which uses the Ka-band frequency to detect the distance changes between the two spacecraft as they pass above terrain of different densities.

"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!"

The duo launched atop a ULA Delta 2 rocket from Cape Canaveral on Sept. 10, taking a low-energy, circuitous route to the Moon that required less fuel to brake into lunar orbits on Dec. 31 and Jan. 1.

Over the past two months, flight controllers have systematically maneuvered the washing-machine-sized spacecraft into their science orbits to prepare for the 84-day mapping mission.

"We are in a near-polar, near-circular orbit with an average altitude of about 34 miles right now," said David Lehman, GRAIL project manager from NASA's Jet Propulsion Laboratory. "During the science phase, our spacecraft will orbit the Moon as high as 31 miles and as low as 10 miles. They will get as close to each other as 40 miles and as far apart as 140 miles."

Officially known as the Gravity Recovery And Interior Laboratory (GRAIL) spacecraft, a contest with U.S. schoolchildren renamed the two satellites Ebb and Flow.

Mapping began Tuesday at 8:15 p.m. EST, two days earlier than previously scheduled. NASA said science activities are expected to conclude in late May, after GRAIL maps the gravity field three times.

The twin NASA spacecraft currently looping in orbits around the Moon to reveal profound new insights about our nearest neighbor in the night sky have been given symbolic names through a contest with U.S. schoolchildren.

More than 11,000 students and 900 schools from 45 states, as well as Puerto Rico and the District of Columbia, participated in the contest that began in October to submit essays.

The winning name duo: Ebb and Flow.

"We chose Ebb and Flow because it's the daily example of how the Moon's gravity is working on the Earth," said mission principal investigator Maria Zuber.

A 4th grade class at Emily Dickinson Elementary School in Bozeman, Montana submitted the winning names.

Originally dubbed GRAIL-A and GRAIL-B, the twins were launched atop a single Delta 2 rocket from Cape Canaveral on Sept. 10 and successfully braked into lunar orbits on New Year's Eve and New Year's Day, respectively.

Now, they are methodically working their way into a highly precise tandem orbit just 34 miles above the lunar surface to begin bouncing signals of each other to measure the changing gravity field as they fly. The science mapping is scheduled to start March 8.

Ringing in the new year with back-to-back successes, a second NASA science satellite slipped into orbit around the Moon Sunday, one day after an identical spacecraft braked into the same polar orbit for a $496 million mission to map the Moon's interior by precisely measuring how its gravity affects the trajectories of the twin spacecraft.

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When you gaze up at the Moon tonight, know there's now two new spacecraft orbiting to unveil the hidden lunar interior. The GRAIL twins have reached lunar orbit to join forces in a gravity-mapping tandem.

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.

GRAIL's science instrument is called the Lunar Gravity Ranging System, which will use the Ka-band frequency to detect the distance changes between the two spacecraft as they pass above terrain of different densities.

"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 Maria Zuber, the GRAIL principal investigator.

"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!"

Flight controllers have confirmed a nominal orbit for GRAIL-B, a double success for getting the mission's two spacecraft maneuvered into lunar orbit the past two days.

Now the work begins to position the probes into 34-mile-high science mapping orbits to start surveying the Moon's gravity field for 82 days starting on March 8.

Passing an altitude of 500 miles.

The burn has been completed as scheduled. However, it will take project officials a little bit to confirm a "successful" lunar orbit insertion, So we will stand by for that announcement to come.

Some 38 minutes into the burn, altitude 439 miles. Standing by for cutoff.

No reports of any problems in this critical lunar orbit insertion burn for GRAIL-B. Altitude over 380 miles now as the maneuver enters its final minutes.

Six minutes of propulsion left, altitude 300 miles.

The technique of using two satellites for gravity mapping that GRAIL will perform was borrowed from the Earth-orbiting mission, called GRACE, launched in 2002.

"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 Maria Zuber, the GRAIL principal investigator.

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 Sami Asmar, GRAIL deputy project scientist.

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."

Now 198 miles above the Moon, heading into a highly elliptical orbit with a period of 11.5 hours.

Less than 12 minutes remaining in the burn by the MR-106L hydrazine engine on the bottom of the spacecraft, which is designed to produce 22 Newtons of thrust during large, critical maneuvers during the mission. The craft also has warm gas thrusters for small-scale attitude changes.

Today's burn caps a 113-day trek from the Earth to the Moon for GRAIL-B, a circuitous route 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," explained Maria Zuber, GRAIL's principal investigator.

The lengthy trip also allowed 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.

"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.

The closure rate bottomed and now the spacecraft is climbing away from the lunar surface for the rest of this planned 39-minute-long burn.

Half-way through the burn!

Still closing down on the Moon, albeit at a much slower rate now, range currently 84 miles.

Fifteen minutes into the burn, now 106 miles above the Moon and everything is looking good.

Twelves minutes elapsed. Distance between the spacecraft and Moon now 151 miles.

A quarter-way through the burn. GRAIL-B is just 190 miles in distance from the Moon.

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 weighed 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 unfolded 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.

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 Sami Asmar, GRAIL deputy project scientist from JPL.

Five minutes have elapsed in this operation for the Moon to capture GRAIL-B. Altitude 327 miles.

Two minutes into the burn. The spacecraft continues to close on the Moon, altitude now just 426 miles.

Ready to join its twin, the lunar orbit insertion by GRAIL-B is underway! Mission Control now confirms that the main hydrazine-fueled thruster on the spacecraft has ignited for this crucial 39-minute-long firing that will slow the satellite by about 430 mph to slip into orbit around the Moon.

The big moment has arrived for GRAIL-B. Standing by for official confirmation of ignition for the lunar orbit insertion maneuver.

One minute. The spacecraft is in the proper orientation for the burn. Range 535 miles.

T-minus 2 minutes. GRAIL-B is 560 miles above the Moon.

Five minutes and counting. The Moon is just 675 miles away, and the good luck peanuts are being eaten in JPL's mission control. That's a tradition dating back to Ranger 7 in 1964.

The spacecraft is turning to its proper orientation for the burn.

Ten minutes to go, range down to 894 miles. Ignition occurs at a range of 497 miles.

Just 15 minutes left to wait until the GRAIL-B spacecraft executes pre-stored commands to ignite its braking engine. Now only 1,115 miles from the Moon.

Now 20 minutes from the burn, distance to the Moon just 1,310 miles.

All reported nominal aboard the spacecraft.

Now 30 minutes from the burn, distance to the Moon just 1,750 miles.

GRAIL-A is passing the 2,000-mile marker from the Moon.

Just 45 minutes from ignition of the GRAIL-B's lunar orbit insertion maneuver, the spacecraft is 2,330 miles from the Moon.

Now 60 minutes from the burn and GRAIL-B has closed to 2,900 miles from the Moon. The spacecraft will be less than 500 miles in altitude when ignition occurs at 5:05 p.m. EST.

Now inside 3,700 miles from the Moon for GRAIL-B and 80 minutes from the burn.

The mirror-twin, GRAIL-A, is flying in its egg-shaped lunar orbit and currently making a close approach less than 100 mile above the Moon.

Approaching from below the lunar south pole at 4,400 mph, the GRAIL-B spacecraft will ignite the main thruster today at 5:05 p.m. EST (2205 GMT) for 39 minutes, slowing the craft by about 430 mph to slip into a preliminary elliptical orbit stretching 11.5 hours.

Confirmation of successful lunar orbit insertion will be announced sometime after the burn is finished, once the navigation team finishes its computations. We'll provide updates on this page as information becomes available.

Three hours and counting, range to the Moon just under 7,000 miles.

With less than five hours from the burn, GRAIL-B is 9,500 miles away from the Moon and closing in.

For this New Year's Day, it's a case of rinse and repeat as the GRAIL team gets ready to do it all again with the GRAIL-B spacecraft performing a carbon-copy of yesterday's GRAIL-A lunar orbit insertion maneuvers.

"With GRAIL-A in lunar orbit we are halfway home," said David Lehman, GRAIL project manager at NASA's Jet Propulsion Laboratory in Pasadena. "(Today) may be New Year's everywhere else, but it's another work day around the moon and here at JPL for the GRAIL team."

Yesterday's successful 40-minute burn allowed GRAIL-A to swoop into orbit around the Moon, entering an elliptical one stretching from 56 miles at its closest point to 5,197 miles at the farthest. It takes approximately 11.5 hours to complete an orbit.

"My resolution for the new year is to unlock lunar mysteries and understand how the Moon, Earth and other rocky planets evolved," said Maria Zuber, GRAIL principal investigator at the Massachusetts Institute of Technology in Cambridge. "Now, with GRAIL-A successfully placed in orbit around the Moon, we are one step closer to achieving that goal."

GRAIL-B begins its 39-minute firing at 5:05 p.m. EST (2205 GMT) today.

After a round-about 2.6-million-mile voyage, the first of two small NASA spacecraft successfully braked into orbit around the Moon Saturday, a welcome New Year's Eve milestone for an ambitious $496 million mission to map out the hidden lunar interior.

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Welcome to lunar orbit, GRAIL-A! The first-half of the gravity-mapping tandem has successfully braked into orbit about the Moon, setting the stage for GRAIL-B to follow suit tomorrow starting at 5:05 p.m. EST (2205 GMT).

There it is! Already, flight controllers have confirmed a nominal orbit for GRAIL-A.

The spacecraft has begun re-orienting itself from the burn attitude to a better position for Sun pointing and solar power generation.

And now confirmation that the burn did conclude as scheduled.

The burn should be complete by this time. However, NASA project officials advised reporters earlier this week that confirmation of a "successful" lunar orbit insertion wouldn't been announced for approximately an hour after the maneuver's conclusion. So we will stand by for that announcement.

Some 39 minutes into the burn, altitude 430 miles.

Passing an altitude of 410 miles.

Now into the final five minutes of the burn. GRAIL-A is 311 miles above the Moon.

All still looking good. Altitude 269 miles.

A contest for schoolchildren to name the GRAIL-A and GRAIL-B spacecraft received about 1,000 entries in the form of short essays. Officials will unveil the winning names after the twins successfully get into orbit.

Ten minutes of propulsion left, altitude 187 miles.

Now 130 miles above the Moon, heading into a highly elliptical orbit with a period of 11.5 hours.

About 15 minutes remaining in the burn by the MR-106L hydrazine engine on the bottom of the spacecraft, which is designed to produce 22 Newtons of thrust during large, critical maneuvers during the mission. The craft also has warm gas thrusters for small-scale attitude changes.

No reports of any problems in this critical lunar orbit insertion burn for GRAIL-A. Project officials had said if the spacecraft failed to perform the maneuver there was a backup opportunity in April.

The closure rate bottomed out at 68.6 miles above the Moon and now the spacecraft is climbing away from the lunar surface for the rest of this planned 40-minute-long burn.

Half-way through the burn! Altitude 68.6 miles.

This burn caps a 112-day trek from the Earth to the Moon, a circuitous route 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," explained Maria Zuber, GRAIL's principal investigator.

The lengthy trip also allowed 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.

"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.

Everything continuing to go well. Altitude 88 miles.

Now 101 miles above the Moon.

Twelves minutes elapsed. Distance between the spacecraft and Moon now 149 miles.

A quarter-way through the burn. GRAIL-A is just 190 miles in distance from the Moon.

Parameters aboard the spacecraft looking good, according to folks at the Jet Propulsion Laboratory. Range now 209 miles.

Still closing down on the Moon, range now 238 miles.

Six minutes have elapsed. Altitude 303 miles.

Two minutes into the burn. The spacecraft continues to close on the Moon, altitude now just 438 miles.

The lunar rendezvous by GRAIL-A is underway! Mission Control now confirms that the main hydrazine-fueled thruster on the spacecraft has ignited for this crucial 40-minute-long firing that will slow the satellite by about 427 mph to slip into orbit around the Moon.

The moment of truth has arrived for GRAIL-A. Standing by for official confirmation of ignition for the lunar orbit insertion maneuver.

One minute. Range 550 miles.

The spacecraft is turning to its proper orientation for the burn.

T-minus 3 minutes. GRAIL-A is 625 miles above the Moon.

Five minutes and counting. The traditional good luck peanuts have been eaten in mission control. The Moon is just 705 miles away.

Ten minutes to go, range down to 910 miles.

Just 15 minutes left to wait until the GRAIL-A spacecraft executes pre-stored commands to ignite its braking engine. Now only 1,115 miles from the Moon.

Now 20 minutes from the burn, distance to the Moon just 1,330 miles.

Now 30 minutes from the burn, distance to the Moon just 1,750 miles.

GRAIL-A is passing the 2,000-mile marker from the Moon.

With only an hour from ignition of the GRAIL-A's lunar orbit insertion maneuver, the spacecraft is 2,950 miles from the Moon.

About 90 minutes from the burn and GRAIL-A has closed to 4,000 miles from the Moon. The spacecraft will be less than 400 miles in altitude when ignition occurs at 4:21 p.m. EST.

Now inside 5,000 miles from the Moon for GRAIL-A and two hours from the burn.

The mirror-twin, GRAIL-B, is lagging some 29,250 miles behind GRAIL-A on a separate trajectory and 33,100 miles from the Moon. Its lunar orbit insertion burn occurs tomorrow starting at 5:05 p.m. EST.

Approaching from below the lunar south pole at 4,400 mph, the GRAIL-A spacecraft will ignite the main thruster today at 4:21 p.m. EST (2121 GMT) for 40 minutes, slowing the craft by about 427 mph to slip into a preliminary elliptical orbit stretching 11.5 hours.

Confirmation of successful lunar orbit insertion is expected approximately an hour after the burn is finished, NASA officials advise. We'll provide updates on this page as information becomes available.

With about three hours away from ignition of the GRAIL-A's lunar orbit insertion maneuver, the spacecraft is 6,900 miles from the Moon and closing in.

Just 24 hours remain until GRAIL-A reaches its destination. The spacecraft crossed is 30,758 miles from the Moon, speeding toward a 4:21 p.m. EST ignition time tomorrow.

A spacecraft tandem that will deduce the Moon's interior from crust to core is lined up to enter lunar orbit this weekend after a 2.6-million-mile, 3.5-month trek from Earth.

Flying independently since their joint launch atop a Delta 2 rocket from Cape Canaveral on Sept. 10, the GRAIL-A and GRAIL-B spacecraft will brake into orbits about the Moon in critical maneuvers Saturday and Sunday, respectively.

The $496 million Gravity Recovery and Interior Laboratory project will eventually settle into 34-mile-high orbits by early March and begin creating an unprecedented lunar gravity map that scientists can follow in their quest to determine the Moon's interior structure.

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