An unmanned Delta 2 rocket blasted off from Cape Canaveral Air Force on Saturday to send a pair of NASA science probes on their way to the moon.
After being delayed two days by poor weather and to review technical data after Thursday’s launch scrub, the 124-tall rocket bolted off its seaside pad at 9:08 a.m. EDT on Saturday, darting through partly cloudy skies as it headed into orbit.
The twin satellites, each weighing 677 pounds, are headed to a point in space about 1.5 million kilometers (932,0570 miles) away where gravitational pull from the sun and Earth balances out.
From there, the Gravity Recovery and Interior Laboratory, or GRAIL, satellites will make a long, slow approach to the moon, arriving on Dec. 31 and Jan 1.
More than 100 spacecraft already have been to the moon, including six with U.S. astronauts, but one key piece of information about Earth’s natural satellite is still missing — what’s inside. That’s the focus of the GRAIL mission.
After a few months to maneuver into the proper orbit, the pair will spend 82 days flying over the lunar poles, linked by radio waves.
When one spacecraft flies over a region of higher gravity, it will speed up, momentarily changing the distance between itself and its sibling probe. Less dense regions likewise will impact the satellites’ positions. Using the radio waves as a ruler, changes as tiny as a micron — the width of a red blood cell — can be detected.
The gravity maps will be compared with topographical features and other data to piece together the moon’s history.
Overall, the moon has about one-sixth the gravity of Earth, but it is not evenly distributed. On the moon, a mountain actually might be a molehill, gravitationally speaking.
“Sometimes you’ll see a big mountain and you’d expect a high gravity signal, but in reality you get no (extra) gravity signal,” said Sami Asmar, GRAIL deputy project scientist at NASA’s Jet Propulsion Laboratory in Pasadena, Calif.
“That’s where it gets interesting,” he added. “The planet (moon) has compensated for the weight of this load for a net zero effect.”
Likewise, gravity maps of lunar flatlands show unexplained pockets of extra heft, an indication of subterranean deposits or structures. Learning the interior structure of the moon is considered critical to piecing together the story about what happened to the moon since its formation some 4.5 billion years ago.
Scientists believe the moon’s building blocks were large chunks of debris jettisoned from Earth after a collision with an object as big as Mars. In addition to unraveling the moon’s history, GRAIL scientists expect to extrapolate their findings to other rocky bodies, both in our solar system and eventually to those beyond.
Image (top): From Earth to the moon — Delta 2 rocket carrying NASA GRAIL satellites lifts off Saturday morning (Spaceflightnow.com for Discovery News). Image (bottom): A view from the launch pad (United Launch Alliance).