Feature | September 14, 2017

Lumps, bumps and gravity: ‘Space scales’ weigh planet Earth

An animation of Earth’s gravity field. Water has mass, and GRACE tells us where that mass is changing, or where Earth’s water is going. Areas with less mass (less gravity), such as ocean basins, show up as blue, while mountains such as the Andes are red, representing the greater pull of gravity. Credit: NASA/Goddard Space Flight Center Scientific Visualization Studio.

By Holly Shaftel,
NASA's Jet Propulsion Laboratory

The twin Gravity Recovery and Climate Experiment (GRACE) satellites, a collaboration between NASA and DLR, the German Aerospace Center, have provided a revolutionary perspective on the changing Earth beneath them for the past 15-plus years. Launched in 2002, the gravity field detectives have been telling a detailed story about our ever-changing home planet, giving us a new look at what appears to the satellites as a lumpy space rock.

Soaring in the same orbit 286 miles (460 kilometers) above Earth with 137 miles (about 220 kilometers) of distance in between, the twins, informally referred to by some as “Tom” and “Jerry,” detect gravity changes by constantly growing closer and farther apart (like some real-life siblings).

As the lead satellite passes over an area with slightly stronger gravity, it detects an increased gravitational pull and speeds up ever so slightly, thus increasing its distance from the trailing satellite.

When a mission exceeds expectations, it’s only reasonable to continue pushing the boundaries of spaceborne engineering and science.

Conversely, the leader slows down when passing over an area of slightly weaker gravity, decreasing the distance from its twin. The changes in distance are so minute — about one-tenth the width of a human hair — that the human eye could not detect them.

This odd sibling relationship, or this force of attraction, is based on Newton’s Law of Universal Gravitation.

“We measure the distance between these satellites constantly as they go around Earth, and this allows us to get a global ‘snapshot’ of Earth’s gravity field every month,” GRACE engineer David Wiese said. “Then we can see how the gravity field changes over time in every location around the world – and this gives us information about how the climate is changing.”

When a mission exceeds expectations, it’s only reasonable to continue pushing the boundaries of spaceborne engineering and science. The GRACE Follow-On (GRACE-FO) mission is scheduled to launch in early 2018 with some technological upgrades that should give scientists and the world a necessary gift: an even clearer picture of climate change.

GRACE’s bounty of discoveries about Earth and its climate system have made national headlines (see Fifteen years of watching water on Earth), but what’s less known are the unique science and ground-breaking technologies that underpin the mission.

A GRACE-ful approach to measuring gravity

Artist’s concept of GRACE
Artist’s concept of GRACE. The twin satellites use a microwave ranging system in combination with Global Positioning System (GPS) technology, star trackers and other instruments to measure gravity changes. (Distance between satellites not to scale.) Credit: NASA/JPL-Caltech.

The heart of the GRACE mission is its microwave ranging system, which measures gravity changes by generating microwave energy pulses (a highly energetic form of electromagnetic radiation) that bounce back and forth between the two satellites. The time a microwave pulse takes to travel from one satellite to the other and back determines the distance between the satellites.

This system, partnered with the same Global Positioning System (GPS) technology that we have on Earth, in our cell phones, can measure the distance between its twin satellites within one micron—about the diameter of a blood cell.

There is no other trick to it. This simple, smart system tells an unparalleled story about Earth’s constantly changing gravity field and constantly changing climate.

To make matters more interesting, the follow-on mission will test a new “ranging system” using lasers instead of microwaves, which promises to measure the distance and the angle between the twins at least 20 times more precisely. NASA’s first laser-linked spacecraft, GRACE-FO will detect gravitational differences at even smaller scales than its predecessor.

A new camera will also provide attitude “intel” to tell engineers which way the satellite is pointed, and an accelerometer will measure everything that’s not gravity, which will help them knock out “noisy” information, such as atmospheric mass.

Does this gravity make me look fat?

GRACE measures Earth’s gravity, and we get gravity from mass, or the amount of “stuff” that makes up an object. The more mass something has, the more gravity it has, and this fluctuates daily everywhere on Earth.

Ice and liquid water experience gravity changes that tell us about climate change.

A whiteboard animation about how NASA uses GRACE to measure gravity from space. Watch more whiteboard animations.

People can also weigh differently from day to day, depending where they are on Earth. You’re slightly heavier when you’re standing in the Himalayas than when you’re in Death Valley, California.

There is only roughly a 1 percent gravity change between the equator and the North Pole, enough to actually change the weight of a 200-pound (about 91 kilograms) person by roughly 2 pounds (about 0.91 kilograms), according to NASA hydrologist J.T. Reager.

Wiese notes that this weight difference for humans depends on your distance from the center of Earth and the density of the surface below you.

From rain gauges to space scales

“GRACE has changed our understanding of hydrology,” Reager said. “GRACE provides observations of what’s happening beneath the surface, which is about 80 percent of the freshwater action.”

Reager is talking about the water table, or groundwater table, and the aquifer. The water table is the highest underground level at which rocks and soil are completely wet, and below the water table, the aquifer, is nature’s filtration system of porous rock through which water easily travels.

Water table and aquifer
A couple of illustrations of the water table and the aquifer. Upper credit: Encyclopedia Britannica. Lower credit: Hans Hillewaert / Wikimedia CommonsView larger image.

Fifteen years of GRACE data show that humans are depleting one-third of the world’s largest aquifers with little to no recharge, meaning that significant segments of Earth’s population are consuming groundwater quickly without knowing when it might go dry.

We’ve come a long way from rain gauges; now we’re peeking underground from space. “It’s hard to look at what’s underneath without GRACE,” Reager said.

Ambitious efforts to quench civilization’s thirst can cause the ground to collapse. In California, the over-pumping of groundwater in response to the recent historic drought sank the land faster than ever before, nearly 2 inches (5 centimeters) per month in some locations. Such terrestrial liposuction is called “subsidence.”

The most overburdened aquifers are in the world’s driest areas, where populations draw heavily on underground water. Climate change and population growth are expected to intensify the problem, and GRACE-FO will continue to monitor changes in the water world deep beneath our feet.

Wait! Wait! There’s more:

  • GRACE maps Earth’s entire gravity field every 30 days.
  • These maps reveal important details about ice sheets, glaciers, sea level, ocean currents, Earth’s water cycle and Earth’s interior (solid Earth) over time.
  • There’s less gravitational pull if there’s less water below you, so you’d weigh a little less, especially in times of drought. Conversely, you might weigh more after it rains.
  • GRACE detected rock density changes beneath the sea floor several months after the 2004 Indian Ocean Tsunami, one of the deadliest on record.
  • When thirsty continents store more water before it hits the ocean, it offsets sea level rise, making it challenging at times to get the best observations. Thankfully, GRACE has been helping to solve those mysteries of missing sea level. (See the years 2010 to 2011 on this sea level plot, where there was a temporary sea level “drop” due to a strong La Niña event that transferred large amounts of water primarily to Australia, northern South America and Southeast Asia.)