At the Jet Propulsion Laboratory in La Cañada Flintridge, the New Year’s celebration will have to wait — until 2:05 p.m. Sunday, to be exact.
That’s when the second of two NASA spacecraft is expected to enter the moon’s orbit on a $496-million mission scientists hope will provide unprecedented insight into the interior composition of Earth’s closest neighbor.
On Saturday afternoon, the first satellite constituting the Gravity Recovery and Interior Laboratory successfully entered the moon’s orbit after a 3 1/2-month trip from Earth.
The solar-powered GRAIL twins, each about the size of a washing machine, are expected to map the moon’s gravitational field. The data will allow scientists to deduce the moon’s structure down to its core and shed light on how Earth and the solar system’s other rocky planets developed.
“Five billion years ago, the Earth was formed, and all the other terrestrial planets were formed, and the moon was formed,” said David Lehman, GRAIL project manager at JPL. “But since that time, the Earth evolved — there’s earthquakes, there’s erosion, there’s lots of rain. And so when we study the Earth now, we really don’t have a good feeling for what happened because it changed so much.”
The GRAIL satellites will log some 12 million miles orbiting the moon over the three-month data collection phase, moving in tandem and separated by an average of 124 miles.
As they pass 34 miles above, mountains, craters and below-ground masses will cause changes in gravity that will slightly alter the velocity of the satellites and the distance between them.
Those measurements will be beamed back to Earth, where they’ll be combined with existing data to produce a precise gravitational map of the moon — information that will vastly increase our knowledge of a neighbor that’s often taken for granted.
More than 100 scientific missions have targeted the moon since 1959. The Apollo program landed 12 people on its surface.
“You might think given all these observations that we know all there is to know about the moon. But that is not the case,” said geophysicist Maria Zuber, the mission’s principal science investigator. “We actually know more about Mars 100 million miles away than we do our own moon.... GRAIL is a journey to the inside of the moon.”
Both GRAIL spacecraft launched aboard a rocket from Cape Canaveral, Fla., on Sept. 10. After separating, the solar-powered twins took the long route to the moon: 2.5 million miles, about 30 times the distance traveled by Apollo astronauts.
The extra time allowed the mission’s single measurement tool — an ultra-stable oscillator — to be powered continuously and reach the proper temperature before reaching the moon’s orbit.
On Saturday the GRAIL-A satellite neared the moon at about 750 mph. At 1:21 p.m. its rockets fired, and over the next 40 minutes the craft’s velocity decreased by more than half, placing it into a stable orbit around the moon.
Its twin is expected to arrive later Sunday.
“Like all good twins, GRAIL-B will do exactly the same thing,” Lehman said. “They arrive about 25 hours apart, which we believe is just enough time to get some breathing room between these important milestones.”