It has been described as a cosmic ballet — two spaceships in a delicate, silent dance 230,000 miles from Earth, correcting their course in tandem with air thrusts softer than a human breath, their instruments so fine-tuned they can detect a shift in gravity that pulls them no farther than the width of a strand of hair.
As soon as Thursday, NASA expects to launch its Gravity Recovery and Interior Laboratory, or GRAIL, from Cape Canaveral, Fla. Shortly after launch, two spacecraft will peel away from NASA’s rocket for a meandering journey to the moon. GRAIL-A and GRAIL-B are scheduled to arrive on New Year’s Eve and New Year’s Day, respectively, then spend three months making 12 polar orbits of the moon each day.
The result will be a comprehensive map of the moon’s gravitational field, data that will help scientists calculate the composition of its crust, mantle and core — adding to their understanding of the evolution of the rocky planets, including our own.
It’s the method used to collect that data, however, that could mark the onset of a revolution in space, even in the search for life across the galaxy. GRAIL will mark the first time that scientists use a technique known as “precision formation flying” — studying the same object using multiple, coordinated spacecraft that can speak directly to one another, at times bypassing scientists altogether — beyond Earth’s orbit.
The technology was long viewed as science fiction, even among those who spend their days dreaming of theoretical advances in space exploration.
But standing on the shoulders of GRAIL, scientists now envision a day when they are freed of the practical constraints of having to stuff every bit of machinery aboard a single rocket. Multiple spaceships could take off separately, then join forces to create unified technology “platforms” that enable scientists to study space in once-unthinkable depth and detail.
“People think of it as the Blue Angels. This is more like blue whales — very big spacecraft moving very slowly and deterministically and settling into a precision formation that they will maintain for years,” said Daniel P. Scharf, a senior engineer at the Jet Propulsion Laboratory in La Cañada Flintridge, which is managing the $496-million GRAIL mission. “It’s not zooming around — we’re crystallizing the formation.”
For instance, 30 synchronized spacecraft equipped with telescopes could fly together in a formation as wide as the distance from Honolulu to Houston — creating, effectively, a single, massive telescope that could peer into unexplored pockets of space.
In recent years there has been an explosion of planet discoveries in the Milky Way, including dozens that are considered Earth-like “candidates” — close enough to their star, as we are to the sun, to harbor life, at least in theory.
“Now you can take pictures of Earth-like planets to the point where you can see continents, weather systems, maybe even a forest,” Scharf said. “We can look for bioindicators: methane, ozone, water in the atmosphere of these planets.”
The same technology, give or take, could be used in missions that would represent significant leaps — to map the currents in the underground oceans of one of Jupiter’s moons, for example, or to chart the changes in snowfall on Mars with the turn of its seasons.
One recent academic paper said the technology could yield so much progress that it would be akin to recording a football game with video cameras rather than still photographs.
After they learn more about teaching spacecraft to work together autonomously, scientists even hope to send up “swarms” of iPod-size spaceships, flying in formation. That technology is expected to yield contributions in communication — they could one day replace satellites — and to measure complex radar and radio frequencies.
And if a meteor or a piece of space debris were to crash through the swarm? No problem — the spacecraft would be so small and so cheap that the swarm would be flanked by idle replacements, which would then be told by the other spacecraft where to go to fill in the gaps.
The technology is critical to the GRAIL mission because of an unusual quirk in our corner of the solar system: The moon spins at the same rate it orbits Earth.
The moon is “synchronously locked,” which means in lay terms “that we only see one side of the moon,” said JPL’s Sami Asmar, GRAIL’s deputy project scientist. That’s why there are significant unanswered questions about the moon — half of it, anyway — even though 12 humans have walked on it and many spacecraft have visited, including three that are in orbit today.
A single spacecraft studying the moon can relay data to scientists on the ground only when it has a direct line of communication. That’s possible only when the craft is over the “near side” of the moon — the one facing Earth — as opposed to the “far side.” (A note to Pink Floyd fans: the “far side” is not the same thing as the “dark side,” which refers to the hemisphere of the moon facing away from the sun.)
In many respects, scientists actually know more about Mars than the moon because Mars spins in a fashion that is more conducive to study. Scientists do not know, for example, why the near side of the moon has huge basins that appear to have been flooded with lava at one point, while the far side does not appear to have the same features.
“So what happened to the inside of the moon?” asked Maria Zuber, professor of geophysics at the Massachusetts Institute of Technology and the principal investigator for the GRAIL mission.
But two spacecraft, when they are on the far side of the moon, can speak to each other through a high-frequency signal rather than to scientists on the ground.
“Formation flying is the key to the success of the mission,” Asmar said. “We are no longer dependent on a line of sight to Earth.”
The two GRAIL spacecraft will fly in formations separated by distances ranging from 37 miles to 140 miles. All along, each will know precisely how far away the other spacecraft is flying.
As they pass together over a large mass, such as a mountain on the moon, the subtle shift in gravity will push or pull one of the craft away from the other — data that will be recorded by the spacecraft. The formation will then be locked again using air thrusters so highly calibrated that their force is measured in millinewtons — equivalent, roughly, to one-thousandth of the force of Earth’s gravity on a stack of 20 nickels.
What’s more, on the far side of the moon, the craft will have their own conversation — storing information about how they’ve been affected by gravity until they are back on the near side of the moon and can download that information to scientists back on Earth.
“The two spacecraft are always tracking each other,” said Tom Hoffman, a JPL engineer and the deputy project manager of GRAIL. “With two vehicles, you can set them off on their science-gathering mode and then communicate with them when it’s convenient.”
The result is expected to yield exponential leaps in the understanding of the moon. Scientists expect to develop a timeline of its thermal evolution, for instance, by determining whether the moon has a solid core and whether there is a layer of liquid inside. They also expect to resolve a dispute about whether Earth once had two moons, which collided and formed a single moon.
“People are worried that we are not doing technologically challenging things in space anymore,” Zuber said. “This is exciting.”