The pull of gravity feels rock steady, yet it wobbles from place to place. Even a single mountain or a shrinking aquifer can tip the scales ever so slightly.
Now, a new satellite instrument from NASA’s Jet Propulsion Laboratory is designed to catch those minute shifts. Coined the Quantum Gravity Gradiometer Pathfinder (QGGPf), it packs a physics lab into a box no bigger than a carry-on suitcase and is slated to fly later this decade.
Inside the device, two clouds of rubidium atoms are chilled to just a fraction above absolute zero. At that temperature, the atoms act more like overlapping waves than tiny balls. As the satellite loops Earth, each atom cloud “falls” within the chamber. If one cloud accelerates just a fraction faster than the other, the instrument records a stronger local pull.
“With atoms, I can guarantee that every measurement will be the same,” said Sheng Wey Chiow, an experimental physicist at JPL. “We are less sensitive to environmental effects.”
Current gravity measuring missions such as GRACE-FO rely on paired satellites that measure tiny changes in their separation to chart gravity. QGGPf aims to gather comparable details from a single craft approximately 0.3 cubic yards (0.25 cubic meters) in volume and weighing only about 275 pounds (125 kilograms). If the pathfinder works, a full-scale mission could detect features 10 times smaller than today’s systems can resolve.
Why the data matters
Gravity maps reveal what lies beneath the surface. Hydrologists watch them to gauge groundwater loss during drought. Geophysicists follow the numbers to see where ice sheets are thinning. Emergency managers study the data for clues that magma is on the move.
“We could determine the mass of the Himalayas using atoms,” said Jason Hyon, chief technologist for Earth Science at JPL and director of JPL’s Quantum Space Innovation Center. Hyon and colleagues laid out the concepts behind their Quantum Gravity Gradiometer Pathfinder (QGGPf) instrument in a recent paper in EPJ Quantum Technology.
Being able to track any changes with a solo, compact satellite would not only trim costs but also let NASA revisit the same spots more often.
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The test-run launch will be a shakedown cruise for several untried technologies, including lasers that steer and measure the atom clouds. JPL is working with AOSense, Infleqtion, Vector Atomic and NASA’s Goddard Space Flight Center on the components.
If all goes well, the approach could be adapted for missions to the Moon or Mars, where mapping underground ice and rock layers is high on science wish lists.
“No one has tried to fly one of these instruments yet,” said Ben Stray, a postdoctoral researcher at JPL. “We need to fly it so that we can figure out how well it will operate, and that will allow us to not only advance the quantum gravity gradiometer, but also quantum technology in general.”
