Late on Sunday night, SpaceX completed a critical cryogenic test of a Starship prototype at its launch site in South Texas. The successful test, during which chilled nitrogen was loaded into pressurized fuel tanks, was reported on Twitter by SpaceX founder Elon Musk.

The vehicle, dubbed SN4—which stands for Serial Number 4—was pressurized to 4.9 bar, or 4.9 times the atmospheric pressure at the surface of the Earth. This pressure is not as high as Starship's fuel tanks and plumbing system are designed to withstand, but it is enough for a basic flight.

This marks an important moment in the Starship program. Since November 2019, the company has lost three full-scale Starship prototypes during cryogenic and pressure tests. The most recent failure came on April 3. This is the first time a vehicle has survived pressure testing to advance to further work. Such tests are designed to ensure the integrity of a rocket's fueling system prior to lighting an engine.

SN4 passed cryo proof! pic.twitter.com/EJakThZRGF — Elon Musk (@elonmusk) April 27, 2020

Now, Musk said, SpaceX engineers will attach a single Raptor engine to this vehicle and conduct a static fire test. The company hopes to move forward with this test later in the week.

Should the static fire test be successful, Musk has said the SN4 vehicle will make a 150-meter "hop" test, much as the "Starhopper" prototype performed in August 2019. The company has yet to receive regulatory approval for this test, so it may not happen for several weeks.

SpaceX already has fabricated most of the parts for its SN5 vehicle, which may be the first prototype to attempt a higher flight. Musk said, if all goes well with SN4, the plan is to attach three Raptors to SN5 for a higher flight test later this spring.

Starship is the "upper stage" of a two-part, fully reusable launch system that SpaceX is developing. The company's goal is for the "Super Heavy" rocket to boost Starship into orbit, where this vehicle can either carry cargo to some destination or carry dozens of passengers.

At its Boca Chica site, the company has taken the build-test-fly-iterate process and put it on steroids. In a matter of months, SpaceX built what is essentially a rocket factory in tents in the dusty scrubland along the Texas-Mexico border, hard by the Rio Grande River. While the program has experienced several failures to date, the ability of engineers to learn from their failures and rapidly churn out new prototypes within a month or less appears to be allowing Musk and his coveted Starship program to advance ever closer to the stars.