When a Falcon 9 rocket took off from Vandenberg Space Force Base in California in May 2018, a spellbound Benedikt Soja was there on the ground to see it. Back then, the researcher was employed by NASA, which had joined forces with the German Research Centre for Geosciences (GFZ) to launch two satellites into space. Today, Soja is Assistant Professor of Space Geodesy in the Department of Civil, Environmental and Geomatic Engineering at ETH Zurich, where he and his team analyse data from this satellite pair. “The GRACE Follow-On mission, or GRACE-FO, is designed to map Earth’s gravity field with unprecedented accuracy,” he says. “With its help, we can track the changing pull of gravity at every point on Earth.”
It is gravity that makes objects fall to the ground and holds the Earth at a distance from the Sun that is conducive to life. Yet the pull of Earth’s gravity actually varies from place to place: our planet is not a perfect sphere, and areas of greater mass exert a stronger gravitational pull. By measuring gravity, GRACE-FO can therefore determine how mass is distributed around the planet.
Keeping an eye on climate change
“We’re very interested in tracking changes in Earth’s mass distribution,” says Soja. “And that’s especially true in regard to water, now that climate change is such a major issue.” Using satellite measurements, researchers can monitor melting ice sheets in Greenland or Antarctica and dwindling levels of groundwater in parts of California or India. “Even heavy rainfall is enough to cause a perceptible change in the local gravitational field, because you suddenly have all this water accumulating in one place,” Soja explains. “We’ve seen some significant changes in mass redistribution over the recent years due to climate change.”
Earth’s gravitational field can also be measured from the ground, but only in certain places. “Trying to use ground-based methods to cover the entire planet, including its oceans, would be impossible,” explains Soja. That’s why measurements taken from space are so important. A satellite’s orbit is partly determined by gravity, so Earth’s gravitational pull can be calculated by simply determining the precise position of each satellite along its orbit. “This method works, but the results aren’t detailed enough for scientific purposes,” says Soja. Fortunately, more accurate information is now available from the twin satellites of the GRACE-FO mission, a follow-up to the GRACE pair of satellites that launched in 2002 and have since burned up in Earth’s atmosphere.
GRACE-FO’s twin satellites follow each other in orbit around the Earth, separated by about 220 kilometres. Measuring devices on board each spacecraft constantly monitor the distance between them. As they pass over areas of greater mass concentration, the gravity anomaly causes this distance to change. On the previous GRACE mission, a microwave ranging system calculated changes in intersatellite distance with a micrometre-per-second precision. But the follow-on mission has taken this tracking performance to a whole new level: by using a laser ranging interferometer, which uses superimposed light waves of a shorter wavelength, it delivers measurements in the nanometres-per-second range.
However, not all the variations in a satellite’s orbit are caused by Earth’s gravity field. At an altitude of about 500 kilometres, space is not a perfect vacuum, and satellites are constantly being slowed down by atmospheric particles. Variations in solar wind can also cause changes in orbit. The decision was therefore made to equip the GRACE-FO satellites with accelerometers. “These high-precision measuring devices enable us to determine all the non-gravitational effects, so we can be confident that we’re only measuring accelerations caused by gravity,” says Soja.