In May 2018, NASA and the German Research Centre for Geosciences (GFZ) launched the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO) mission, the next generation of satellites in one of the more innovative Earth-observing programs ever launched. GRACE-FO engineers and operators have been testing, calibrating, and checking out the spacecraft in order to begin full science operations in January 2019. This is a sample of their test data.
The original GRACE mission (which ended in 2017) and the new one monitor miniscule, month-to-month changes in Earth’s gravity field. More than 99 percent of the planet’s gravitational pull remains unchanged from one month to the next because it represents the mass of the solid Earth itself. But a tiny fraction is constantly on the move, and it is mostly water: Rain is falling, dew is evaporating, ocean currents are flowing, ice is melting.
GRACE-FO maps variations in gravity that show scientists where that small fraction of planetary mass is moving. Such measurements provide insights into changes in ice sheets and glaciers, the effects of droughts and earthquakes, and the impacts of human activities such as groundwater pumping.
There are two GRACE-FO satellites (exact twins) that orbit in formation approximately 220 kilometers (140 miles) apart. Each satellite has an accelerometer, which measures forces on the satellites other than gravity—such as atmospheric drag or solar radiation pressure. And each satellite has a microwave ranging instrument that precisely measures the distance between the two satellites as they orbit.
Variations in Earth’s gravity field, caused by changes in the distribution of mass on and beneath the surface, cause the distance between the two satellites to vary ever so slightly. The GRACE-FO ranging instruments measure this change in distance with a precision better than one micron—less than the diameter of a blood cell, or one-tenth the width of a human hair.
On the map above, the yellow line marks the path of the GRACE-FO satellites on June 14, 2018, with numbers denoting their positions at various points in the orbit. The middle plot shows the change in distance between the spacecraft—just a few tens of microns—which is affected by changes in the gravity field below. The final plot shows the elevation of the Earth beneath the satellite. Notice how the relatively smooth rise over Antarctica disturbs the satellite orbits much less than the abrupt changes over the Himalaya mountain range. Note also the changes as GRACE-FO passes over higher elevations of Central America versus the lower elevations of the Caribbean Sea and Pacific Ocean.
In addition to the accelerometer and microwave ranging instrument, GRACE-FO has an experimental laser ranging interferometer. “The LRI is a breakthrough for precision distance measurements in space,” said instrument manager Kirk McKenzie of NASA’s Jet Propulsion Laboratory. “It is the first inter-spacecraft laser interferometer and the culmination of about a decade of NASA- and German-funded research and development.”
Gerhard Heinzel, leader of the space interferometry research group at the Albert Einstein Institute, explained the challenge: “There are coin-sized holes on each satellite through which the laser has to be precisely pointed towards the holes in the other satellite over a distance of more than 200 kilometers, while both spacecraft race around Earth at 27,000 kilometers an hour. It is truly mind-boggling.”
NASA Earth Observatory image by Joshua Stevens, using data courtesy of Felix Landerer/NASA JPL and the GRACE Science Team. Story by Michael Carlowicz, based on reporting by Alan Buis and Esprit Smith, NASA JPL.
