The most reliable information about ice mass balance has been coming from the Gravity Recovery and Environment Experiment (GRACE) satellites for the last decade.

But that pair of satellites was reaching the end of its useful life, and scientists have been anxiously awaiting launch of the important GRACE FO (Follow On).

Successfully launched, will start sending useful data in 90 days.

Smithsonian:

Today, NASA successfully launched a pair of satellites collectively known as GRACE-FO (Gravity Recovery and Climate Experiment Follow-On mission) as a replacement for the two GRACE satellites currently in orbit.

Launched in 2002, GRACE helped provide a better understanding of many of Earth’s most pressing conditions, including rising sea levels, melting ice sheets and droughts. But last year, after 15 years of service, the original GRACE duo completed its mission.

The new satellites will continue GRACE’s work, but feature updated tech, including improved batteries and an extra camera. The pair will map out changes in Earth’s gravitational field, which scientists use to monitor distribution of water on the planet’s surface. And as NPR’s Christopher Joyce notes, they might even help in earthquake prediction.

As Joyce explains, the Earth’s gravitational field changes with our planet’s mass. It’s stronger over areas with lots of mass, like mountains or bodies of water, and weaker where there’s less mass.

As Alessandra Potenza writes for The Verge, to observe these tiny variations, the pair of car-sized spacecraft will zip around Earth—one trailing roughly 137 miles after the other. According to NASA, the pair will use super-sensitive “microwave ranging instruments” to continually monitor the distance between them. By measuring minute changes in this gap, they can track differences in the tug of Earth’s gravity over the planet’s many features.

By measuring these changes month after month, the satellites can monitor long term shifts of water resources on the ground—glaciers growing or shrinking, shifts in underground water storage, snow melt in the spring. ”[T]hat shift of water leaves an imprint on the gravity field, and that’s what we detect and what we’re after,” NASA Jet Propulsion Laboratory geophysicist Felix Landerer tells Joyce.

These measurements will enable researchers to improve weather models and more accurately forecast catastrophic events like floods, water shortages and droughts. “The GRACE-FO mission gives us a rich understanding of a fundamental resource on our Earth, which is water,” says Sascha Burton, systems engineer for the mission at NASA’s Jet Propulsion Laboratory, in a NASA video. “How it moves and how it’s changing and that helps us better understand our climate.”

Another thing GRACE-FO will be able to detect is movements in the Earth’s crust, which could be used to help predict earthquakes. As Joyce reports, within GRACE data, French scientists found the signatures of the catastrophic 9.0-magnitude earthquake and tsunami that hit northern Japan in 2011, killing 16,000 people. Perhaps looking for similar signatures in the new GRACE-FO data, researchers could potentially provide much more warning before other massive earthquakes.

The previous GRACE satellites made many contributions to science. Using this data, scientists could estimate aquifer depletion in Central California and monitor changes as 4,000 gigatons of Greenland’s ice melted over the course of 15 years, Joyce reports. Potenza writes that GRACE also revealed that Antarctica is losing 120 gigatons of water a year.

Frank Webb, project scientist at Jet Propulsion Laboratory, says the mission will provide necessary data over the long-term to help researchers make accurate predictions. “Even though GRACE flew for 15 years, climate takes place on decadal time scales. It’s really important that we look at these trends over longer time periods so we can establish the forces that are driving them.”

According to NASA, the $430 million GRACE-FO mission will provide data on a monthly basis, with its first collected data becoming available 90 days after the launch. The satellites will fly for at least five years.