This morning, Virgin Orbit — the spinoff company of Richard Branson’s space tourism venture Virgin Galactic — dropped a small test rocket from an airplane over Southern California where it slammed into the ground. The rocket’s plunge to the earth was part of a major flight test for Virgin Orbit that will pave the way for the company’s first launch to space later this summer.

This morning’s event was known as a drop test, and it was meant to see if Virgin Orbit’s fledgling rocket system behaved as expected. For the last four and a half years, Virgin Orbit has been developing a new small rocket called LauncherOne, which is designed to put satellites the size of washing machines into low orbits around Earth. But unlike most other commercial rockets on the market, LauncherOne doesn’t take off vertically from the ground. Instead, it’s carried to a high altitude under the wing of an airplane and then dropped. It’s then meant to ignite its engine and climb to orbit.

“It’s a really major milestone for us.”

This process is a relatively rare type of space travel known as an air launch, and it’s the same launch technique that Virgin Galactic uses to get its tourist spaceplane to the edge of space. Today’s exercise was the last big test Virgin Orbit’s LauncherOne needed to pass before the vehicle ignites its engine during an actual flight for the first time. “It’s a really major milestone for us. It really sort of caps off the development program for us,” Dan Hart, Virgin Orbit’s president and CEO, tells The Verge.

Now THAT’s what I call a drop test! Video from today’s very successful drop test of our #LauncherOne small satellite launch vehicle. More photos and videos coming soon. pic.twitter.com/aOib4HYVPU — Virgin Orbit (@Virgin_Orbit) July 10, 2019

For today’s test, Virgin Orbit filled LauncherOne with water and antifreeze to simulate how heavy the rocket will be when its tanks are filled with actual propellants of liquid oxygen and kerosene. It was then carried to an altitude of about 35,000 feet by its carrier aircraft, a specially outfitted Boeing 747 named Cosmic Girl. The gutted plane has been modified to carry LauncherOne under its left wing. Once the pair reached their target altitude, Cosmic Girl’s pilot, Kelly Latimer, released the clamp holding the rocket, letting it fall away.

All of these steps will be the same leading up to LauncherOne’s first test launch, which is why today’s test was so crucial. The goal was to see if LauncherOne dropped like it’s supposed to during a normal flight and if the forces on the vehicle were more or less what the Virgin Orbit team expected. “The real data that we get from the rocket is in those first five seconds,” says Hart. Virgin Orbit had videos trained on the rocket during the test, and the dummy LauncherOne vehicle also had sensors embedded inside it to gather data about its fall. “The rocket will be telling us where it is, any motions that it’s feeling, and how it’s flying those first few seconds, which are very important for an air launch rocket especially,” says Hart.

During an actual test launch, Cosmic Girl will drop LauncherOne over water instead of land to ensure the igniting rocket doesn’t damage any people or property on the ground. In contrast, today’s drop test occurred over Edwards Air Force Base in Southern California, and the Virgin Orbit team plans to recover the now-demolished LauncherOne that crashed onto the base.

Prior to today’s drop, Virgin Orbit had been thoroughly testing the basic components of LauncherOne by doing tests of its main engine, the NewtonThree, as well as full “hot fire” tests of the vehicle, which is when the engine is ignited while attached to a full rocket. The team has also been doing “captive carry” tests with LauncherOne, which is when Cosmic Girl flies with the rocket underneath the plane’s wing without dropping the vehicle.

“The rocket will be telling us where it is, any motions that it’s feeling, and how it’s flying those first few seconds.”

Now, it’s full steam ahead to Virgin Orbit’s first test launch. Over the next few weeks, the company will continue to test the rocket as well as practice all of the procedures they’ll need to perform on the day of the flight. Sometime by the end of the summer, LauncherOne will carry an unknown test payload into orbit. “It’s really primarily an engineering flight to understand the performance of the first stage, second stage, and that the mission comes off cleanly,” says Hart.

Once that’s done, Virgin Orbit’s commercial operations will begin. Hart says the company has five completed LauncherOne rockets in the factory, and the team has started work on the sixth. The goal is to start flying customers two and a half months after the first test launch, but that timetable could change. “Obviously, it will depend upon the performance that we see,” says Hart, “and whether we have any work to do on the rocket itself, which is always a risk on first launches.”

Today’s drop test comes just a day after Virgin Orbit’s original parent company, Virgin Galactic, announced plans to go public through a reverse merger with Social Capital Hedosophia, an investment company run by former Facebook executive Chamath Palihapitiya. Since Virgin Orbit is now separate from Virgin Galactic, the merger does not affect the launch provider, and the company is still private for the time being.