The grace to tackle climate change
Climate change is the largest existential threat facing humanity. Melting ice in polar regions and in mountains contributes to rising ocean levels worldwide; warming air disrupts jet streams and precipitation patterns, making severe storms more likely. Tracking these disruptions is essential for understanding how rapidly climate change is happening. However, Earth is big and many of the important fluctuations can be hard to measure without intensive local observations year-round.
But Earth-observation satellites present another extremely effective way to track climate change. Since 2018, a pair of spacecraft has been recording data that allow scientists to measure the melting of polar ice and the depletion of water tables during droughts. The satellites, together known as the Gravity Recovery and Climate Experiment Follow-On (GRACE-FO), track small fluctuations in Earth’s gravity as water moves from place to place. As its name suggests, the joint project between the US and Germany succeeds the original 2002-2017 GRACE mission. Both have proven so successful that researchers are now planning a third mission.
GRACE, GRACE-FO, and the Gravity Recovery and Interior Laboratory (GRAIL) mission that circled the Moon in 2012 each consist of two spacecraft orbiting in tandem. Fluctuations in gravity tug on the spacecraft differently, changing the separation distance between them. Precision microwave and laser ranging instruments measure this separation, which lets scientists reconstruct the gravitational variations that caused them. Monthly flyovers of the same regions show how local gravity changes over time, a branch of Earth science known as geodesy.
“If the Earth were a billiard ball that is uniform throughout, and you had two spacecraft following each other, the separation would never change,” says William Klipstein of NASA’s Jet Propulsion Laboratory, who led the development of the laser instruments for GRACE-FO. “[But] the lumpiness of the mass distribution of the Earth causes the lead spacecraft to speed up a little bit, then slow down, and the rear spacecraft to speed up a little bit, then slow down.”
This map of fluctuations in gravity is known as the geoid, and the power of the two GRACE missions lies in their ability to measure changes in that map. Ours is a dynamic planet, and most of the measurable changes happening on short time scales are movements of water: specifically, patterns of snow- and rainfall deposit water on land, while ice melt and drought deplete those water supplies. As Earth warms, more ice and snow melts than is replenished, shifting concentrations of water from land into the oceans. By measuring these shifts, GRACE and GRACE-FO together offer 20 years of data on climate change.
Changes in total water storage on Earth in 2007, as measured by GRACE. Credit: NASA/JPL-Caltech/University of Texas-CSR
Gravity is the weakest of the fundamental forces of nature, but paradoxically that’s what makes GRACE-FO effective. The tug from matter deep beneath Earth’s surface passes through all the dense rock above without being affected by it the way a stronger force would, providing a look into the planet’s interior. Most importantly, GRACE-FO can detect groundwater levels and changes in ice density through their gravitational influence, phenomena that can be hard to measure otherwise.
The twin spacecraft chase each other in a polar orbit, which means they pass from North to South Pole and back as the planet rotates beneath them. They orbit Earth at an altitude of roughly 500 km, with an average separation of about 220 km between the probes. Each orbit takes 94.5 minutes, which means they can map the entire planet about once per month. The instruments are sensitive enough to measure changes in 1-cm-thick ice.
“We’re interested in trying to detect if there’s an acceleration in how fast ice sheets are changing,” says Michalea King, who studies the effect of climate change on Greenland at the Polar Ice Center at the University of Washington. “With GRACE and GRACE-FO, we’re getting mass change estimates [with] about monthly resolution, which, for resolving seasonal changes and mass changes, is really important.”
The basic GRACE-FO design, like the original GRACE, consist of two identically sized spacecraft about 3-m-long and 1.9-m-wide, with a trapezoidal cross-section 0.72 m tall. Solar panels on the angled surfaces facing away from Earth provide the primary electrical power. The mission launched on a SpaceX Falcon 9 rocket on 22 May 2018 and began scientific operations on 28 January 2019.
The spacecraft rely on a suite of instruments to measure the distance between them as precisely as possible. The probes carry GPS receivers to determine where they are over Earth, and accelerometers to measure nongravitational influences. The primary range-finding apparatus consists of two-way microwave horns on each spacecraft, operating at 670 and 500 KHz. These are driven by an ultrastable oscillator; the shift between emitted and received frequencies are due to spacecraft separation. The microwave ranging system is accurate to about 10 µm.
New to the GRACE-FO mission is the Laser Ranging Interferometer (LRI) that provides both a major advance in sensitivity for range measurements and a first demonstration of the technology in space. LRI involves the same basic technology used in the gravitational wave observatories LIGO (Laser Interferometer Gravitational Observatory) in the US, Virgo in Europe, and KAGRA (KAmioka GRAvitational wave observatory) in Japan.
Originally developed by physicist Albert Michelson to perform experiments on the fundamental nature of light in the late 19th century, Michelson interferometers use phase shifts between light traveling along two paths to compare differences. One famous example is the Michelson-Morley experiment of 1887 with chemist Edward Morley, which demonstrated that the speed of light is the same in every direction, the first experimental hint of the need for a theory of relativity later developed by Albert Einstein. Gravitational wave interferometers detect passing disturbances in spacetime from colliding black holes or neutron stars by how they perturb mirrors. LIGO is sensitive enough to pick up disturbances significantly smaller than the width of a proton.
The LRI is a slight modification of the basic Michelson design. The primary spacecraft sends a 1,064 nm (infrared) laser beam across open space to the secondary spacecraft, which reads the phase information and locks its own laser to that phase. The secondary then sends that laser signal back to the primary, where a photodetector checks the relative power, determining the degree of interference between the two lasers. Thanks to the smaller wavelength of infrared light compared with microwaves, LRI has improved measurements of the separation distance fluctuations by more than a factor of 10.
The LRI is “not measuring the absolute range very precisely, it’s measuring fluctuations very precisely,” Klipstein says. The dance of the interference fringes in the photodetector tells how gravity causes the spacecraft to speed up and slow down relative to each other.
In most respects, GRACE-FO’s LRI is simpler than many of its Earth-based analogues. LIGO and its cousins require building very long vacuum chambers and suspending their mirrors on triple pendulums to isolate them from nongravitational vibrations. By operating in the near-vacuum 500 km above Earth, LRI needs neither of those, nor does it need to compensate for the curvature of the planet, which in turn is one practical limitation on the size of ground-based gravitational wave observatories.
At the same time, GRACE-FO must deal with its own set of error-causing phenomena. Earth’s residual atmosphere—its exosphere—creates some drag even at 500 km, while pressure from solar radiation also generates fluctuating accelerations. More regularly, gravitational tugs from the Sun and Moon have an effect on the spacecraft that varies over the course of months and years. The onboard accelerometers allow GRACE-FO researchers to separate sources of noise from scientifically important data.
However, as Klipstein points out, the spacecraft themselves are also buffeted slightly by the same forces, changing their orientations by a measurable amount. That isn’t a major problem for the microwave ranging apparatus, which send out widely dispersing beams, but it does create trouble for the LRI. The lasers must follow a straight line from one spacecraft to the other, so the LRI equipment includes a mirror which the onboard computer steers automatically as guided by differential wavefront sensing to make sure the beams end up where they need to go.
“We have to scan in two degrees of freedom on both spacecraft, and then scan the laser frequency to get a signal on the detector,” Klipstein says. “Once we lock onto it, then the tracking becomes simple.”
The GRACE-FO team had been most worried about that potential failure point after launch, but the lasers found the other spacecraft almost immediately, to everyone’s relief. In fact, the LRI has performed so well that similar missions in the future will use laser interferometers as primary instruments.
However, the primary reason for LRI was as a testbed for the long-anticipated Laser Interferometer Space Antenna (LISA), a joint project of the US National Aeronautics and Space Administration (NASA) and the European Space Agency (ESA). This gravitational wave observatory will consist of three small spacecraft flying in an equilateral triangle formation 2.5 million km on each edge. It will carry laser interferometers, built to detect many kinds of gravitational waves that LIGO and other ground-based observatories cannot. If all goes as planned, the spacecraft will launch in the early 2030s.
Credit: Airbus DS GmbH/A. Ruttloff
The first GRACE mission was, like the first Starship Enterprise, intended to last five years, but unlike the television show it was so wildly successful that it was extended to 15 years. By then, researchers had already scheduled GRACE-FO to minimize the gap between missions. Combined, the nearly 20-year set of observations benefit scientists like King, who need data on both short- and long-term trends to understand the Greenland ice sheet.
“I’m interested in measuring total mass change of the ice sheet, but at high temporal resolution,” she says. Specifically, in a stable climate, ice sheets would grow in winter and shrink in summer but maintain their basic size on average. “By having monthly estimates of mass change, we can start to pick apart the dominant processes of that change in that month. If we are able to see which month we observed the greatest mass loss, we can then relate that to what was going on atmospherically at that time.”
King applies GRACE-FO data in combination with other observations to a theoretical input-output model that combines things like snowfall, glacier movement, melting, and iceberg calving. Gravitational data are especially powerful for her work not just because they reveal hidden mass changes, but because they provide an independent measure of ice movement in Greenland.
“That’s super important for understanding just how the ice sheet mechanically works and which processes are dependent on each other,” she says. “Then we can start to understand how sensitive the ice sheet will be to future changes. There are methods that get detailed estimates of mass change, but they’re really small-scale studies. We’re at the point with climate change that we need to be able to observe ice-sheet-wide processes.”
Thanks to the successes of the LRI, the next generation of GRACE-like Earth observation spacecraft will have even greater precision. Though these projects are still very much in the planning stages (to the point where they don’t have official names—it’s unlikely they will be GRACE-FO-FO), NASA and ESA are committed to building them.
Many physicists consider studies of gravity a somewhat impractical pursuit, even when, like LIGO, they reveal new information about the evolution of stars and galaxies. However, the experimental LRI placed aboard GRACE-FO as a test for LISA lets scientists grapple with climate change, the most pressing issue of this era.
Matthew R. Francis is a gravitational physicist, science writer, and frequent wearer of jaunty hats. His website is BowlerHatScience.org.
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