Elon Musk’s out-of-this-world brainchild, SpaceX, is working on a new way for mankind to travel to Mars with its Interplanetary Transport System. The ITS, as it’s known, needs fuel to function. A lot of fuel, in fact, and on Wednesday SpaceX announced that it had taken a massive fuel tank out to sea for a successful test.

The humongous, 40-foot-wide carbon-fiber prototype fuel tank was loaded up on a big truck and transported to a barge so it could be tested out on the water last week. In a tweet, SpaceX revealed that the tank had passed, explaining that “hit it both of our pressure targets.”

“Next up will be full cryo testing,” the post continued.

In September, Musk spoke at the International Astronomical Congress (IAC), where he first revealed substantive details about the ITS. During Musk’s talk, he explained that the tank was “really the hardest part of the spaceship.” The entrepreneur said SpaceX had “a pretty good handle on,” but said the tank was “the trickiest one. So we wanted to tackle it first.”

In a Reddit thread last month, Musk said that the tank was regarded as “the big news” by those in the know when he revealed it.

“In theory, it should hold cryogenic propellant without leaking and without a sealing linker,” he wrote.

It’s vital that the final tank — which will be slightly longer than the prototype they tested — function properly, otherwise the entire ITS could explode.

Luckily, things seem to be working so far.

Photos via SpaceX