SpaceX is about to launch 2 NASA satellites into orbit. Here's how to watch live.

• Update: SpaceX's Falcon 9 rocket launched seven satellites Tuesday.
• Two of the satellites are part of a NASA mission to detect slight changes in the Earth's gravitational field, which will give scientists a clearer picture of our planet's water and ice content.

Update: The rocket launched. You can check out its progress below.

SpaceX, the rocket company founded by Elon Musk, will use its Falcon 9 rocket to launch seven satellites on Tuesday: two state-of-the-art NASA satellites and five commercial communications satellites.

The NASA satellites are part of a mission called Gravity Recovery and Climate Experiment Follow-On (GRACE-FO). Once in orbit, they'll be used to analyze Earth's water and ice content by detecting small changes in the planet's gravitation field.

When water or ice fills an area — such as a coastline or river basin — the liquid slightly increases that area's mass. Objects with more mass have a stronger gravitational pull, so measuring these variations in gravity gives NASA data on phenomena like sea-level rise, glacial retreat, drought, and changes in the size of underground aquifers.  

To get these gravitational measurements, the two satellites will fly 220 kilometers (137 miles) apart, according to NASA. When one passes over a part of the planet where the gravity is abnormally strong or weak, that will affect its orbital path, changing the distance between the two satellites. The satellites will be constantly measuring the distance between them by sending microwave signals back and forth. Changes in that distance will therefore allow scientists to determine the gravity, and consequently the mass, of the area below. 

The two satellites will replace the previous GRACE mission, which operated between 2002 and 2017.

They will be launched along with five Iridium Next communications satellites on a Falcon 9 rocket from Vandenberg Air Force Base in California at 3:47 p.m. ET.

SpaceX has successfully launched 56 missions on a Falcon 9 rocket since its debut in June 2010. In 2018 so far, the Falcon 9 has flown eight times. SpaceX recently debuted a new version of its workhorse Falcon 9 rocket — Block 5 — but today's launch will use Block 4.

"I believe Falcon 9 was the most-launched rocket worldwide in 2017," Musk said on a call with reporters on May 10. "If things go well — big caveat — SpaceX will launch more rockets than any other country in 2018."

You can watch the launch here starting around 3:30 ET: