This afternoon, SpaceX is slated to launch a batch of satellites into space, including two twin satellites for NASA that will observe how water moves around our planet. Called GRACE-FO, the mission replaces the two original GRACE satellites, which were put into orbit in 2002 and went offline last year. Those satellites gave scientists valuable insights into melting ice sheets, droughts, and sea level rise. The new probes will continue that work, helping researchers better understand how water behaves on Earth, especially now that the climate is changing.

“Water resources are vital to life on Earth and the way we operate civilization,” says Frank Webb, GRACE-FO project scientist at NASA’s Jet Propulsion Laboratory. “It’s very important to understand how those resources are changing.”

While in orbit, the two car-sized GRACE-FO satellites will constantly follow one another 137 miles (220 kilometers) apart, which is roughly the distance between Los Angeles and San Diego. This configuration is critical for measuring water changes on Earth. When the first GRACE-FO probe passes above a big area of mass, like a huge underground aquifer, its gravity will tug at the satellite and the distance with the trailing spacecraft will change. By constantly measuring the distance between the two probes, using microwave signals, the satellites can create a gravity map of the Earth every month. Changes in that map mean changes in how water is distributed.

Though other things on our planet like mountains have mass, “on that time scale, the only thing that’s moving is really water,” Webb tells The Verge.

Using this method, the first GRACE satellites were able to show that Greenland, for instance, is losing mass, not gaining it. The measurements revealed that the melting Greenland ice sheet — which is roughly three times the size of Texas — is dumping 281 gigatons of water a year into the ocean, Webb says. That’s about as much water as 112 million Olympic-size swimming pools. The GRACE satellites also showed that overall, Antarctica is losing 120 gigatons of water a year, or about as much water as 48 million swimming pools, even if areas in eastern Antarctica are gaining some mass. These melting ice sheets contribute to 0.039 inches (1 millimeter) of sea level rise a year, Webb says.

But it’s not just about ice: the GRACE satellites can also help us monitor droughts by measuring how underground aquifers change during dry periods, as well as the general distribution of water in soil, lakes, rivers, and glaciers. In 2011, for instance, sea level rise slowed down a bit although the ice sheets were still dumping water into the ocean. The GRACE probes showed that the extra water was being stored on land in Australia and South America, which were hit by heavy rains. When that water flowed into the ocean, the rate of sea level rise picked up again, Webb says.

“In order for us to understand how our climate system is evolving and predict the future course of it, we really need to understand how the different elements of it function and how they’re trending,” he says. “Water ... is one of the vital signs of how our climate is evolving.”

The new twin satellites will use the same microwave technology as the old ones so that their measurements are consistent. But the tech got a bit of an upgrade: the satellites now have better batteries as well as an additional camera. The GRACE-FO probes are also equipped with new laser beams that could replace the microwave signals in future GRACE-like missions, according to Frank Flechtner, GRACE-FO project manager at the German Research Centre for Geosciences (GFZ), a co-funder of the mission. The lasers, which are expected to make measurements that are 10 times as accurate as the microwaves, will be tested to see if they work properly.

The old GRACE satellites stopped doing science in October 2017, and the new ones are expected to start providing the first data about 90 days after launch. The whole mission, which is costing the US space agency $430 million and the GFZ 77 million euros ($90.7 million), is expected to fly for at least five years. Because the satellites make very sensitive measurements on how far away they are from each other, they’re not equipped with thrusters to adjust their orbits. That means that they won’t be in space as long as other satellites, and eventually, they’ll fall back to Earth.

“We just kind of launch them and let them decay with as little perturbation of the orbit as possible,” Webb says. The satellites are expected to fall below the altitude needed for gathering scientific data in seven to 8.5 years, he says. (The new probes won’t last as long as the old ones because of how solar activity will affect the spacecraft.)

But first, the satellites need to get to space. The GRACE-FO probes are sharing a ride with five Iridium NEXT communications satellites on a SpaceX Falcon 9 rocket. The rocket is scheduled to take off at 3:57PM ET from Vandenberg Air Force Base in California with an instantaneous launch window. SpaceX is using one of its used Falcon 9s for this flight — the same vehicle that launched the classified Zuma satellite — however, the company will not attempt to land the rocket after takeoff. Live-streaming on NASA TV begins at 3:15PM ET, so check back then to follow the launch.