When NASA's Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission launches on May 19, it will be able to map the Earth's gravitational field with unprecedented accuracy. Thanks to a new technique called laser ranging interferometry, the two satellites that make up the five-year mission will be able to chart gravitational anomalies with a ten-fold greater precision than the previous GRACE mission.
The original GRACE mission came to a close in October 2017, when the two orbiters shut down operations before making a controlled reentry into the Earth's atmosphere. Launched in 2002, the German/American project consisted of the spacecraft GRACE-1 and GRACE-2, which flew in close formation in low-Earth orbit.
Using a microwave ranging system, the pair could measure the distance between them to a few microns, or a fraction of the width of a human hair. This was combined with GPS tracking data, attitude data from star trackers, and an accelerometer to eliminate such external factors as atmospheric drag, orbital decay, and solar-wind pressure, to provide a precise record of the pair's velocity and separation distance.
By doing this, scientists could measure how much each sped up and slowed down in response to changes in the Earth's mass below them. Such anomalies acted like an X-ray into the planet's interior as the spacecraft mapped out the structure of mountains, the shift of magma beneath the crust, the effects of earthquakes, and the global flow of water and ice sheets.
However, where GRACE relied on microwaves to measure the gap between them, GRACE-FO is upping the game by bringing lasers into play. Just as the GRACE spacecraft used the way in which microwaves interfered with one another to make measurements, GRACE-FO will use the interference of lasers for even more precise results.
Developed by NASA's Jet Propulsion Laboratory (JPL) in California and the Max Planck Institute for Gravitational Physics (Albert-Einstein Institut) in Germany, the new Laser Ranging Interferometer (LRI) uses wavelengths ten times shorter than microwaves to deliver an increase in precision that, until now, has been confined to the laboratory.
"With GRACE-FO, we're taking something cutting-edge from the lab and making it ready for space flight," says Kirk McKenzie, the LRI instrument manager at JPL. "The reason we spend decades working in the lab is to see our technology enable a new type of measurement and result in scientific discoveries."
NASA says the tricky bit will be for the GRACE-FO satellites to acquire one another while traveling 137 mi (220 km) apart. It's a bit like trying to lock onto a star using a high-powered telescope – not easy unless it has a lower powered sighting scope. In this instance, the lasers will flash signals at one another when first activated and then analyze any received through all the possible configurations of the twin craft, which will take about nine hours. However, it will take only a millisecond to set up the optical link once its acquired and locked.
This new technology will also help future missions with the potential to achieve resolutions of better than 200 mi (300 km) in diameter.
"The laser ranging interferometer on GRACE-FO is potentially an enabling technology for future missions around Earth or even to look at the universe," says Frank Webb, GRACE-FO's project scientist at JPL. "This new, higher precision measurement should enable more efficient missions in the future with lower mass, power and cost. We're eager to see how it performs and what new signals we might be able to tease out of the data."
The video below explains the principle behind GRACE-FO.
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