Elon Musk’s closely held company announced the new date in a tweet on Monday, hours after postponing a planned inaugural test flight for the fully assembled vehicle. Minutes before scheduled liftoff on Monday, SpaceX scrubbed the flight, citing a “pressurization issue.” Officials had said they would need at least 48 hours before trying again.

Musk had flagged a “frozen” pressure valve as an issue that might delay the launch in a tweet earlier in the day. But SpaceX made no reference to the issue in a statement about the new target date for sending Starship aloft.

The new launch window will open at 8:28 a.m. local time in Boca Chica, Texas, where its launch pad is located, and closes one hour and two minutes later, SpaceX said.

The uncrewed mission is a critical step in SpaceX’s plan to send humans into deep space. The company, formally known as Space Exploration Technologies Corp., aims to send the massive rocket in a partial orbit of Earth for the first time, paving the way for later missions to the moon and Mars.

Read more: Starship’s Launch Is SpaceX’s Key First Step Toward Mars

Delays — whether because of weather, technical issues or something else — are common in space launches.

The latest postponement adds to a timeline that has already been almost two decades in the making: As early as 2005, Musk alluded to plans for a giant rocket codenamed “BFR.” More recently, the company has been testing components of the rocket system in preparation for this launch, its first attempt to reach space.

Musk’s ‘420’ History

The timing of the second attempt on April 20 appears to be a coincidence, although Musk is no stranger to the number “420,” which is associated with marijuana. He famously once claimed to have “funding secured” to take Tesla Inc. private at $420 a share. Earlier this year testifying in a lawsuit stemming from that claim, Musk said “there is some karma around 420,” prompting laughter in the courtroom.

Musk serves as chief executive officer for both SpaceX and Tesla.

Starship was conceived to bring people — including NASA astronauts — and cargo such as satellites into Earth’s orbit and beyond. The rocket is more powerful than any previous crewed spacecraft and taller than the Saturn V. The launch vehicle system also has been designed to be fully reusable, which SpaceX promises will reduce costs.

When the launch attempt happens, the Super Heavy booster rocket is expected to break away from Starship after an initial climb to space and do a controlled dive into the Gulf of Mexico. If everything goes as planned, the Starship spacecraft will continue onward and arc through space completing almost a full lap around the Earth. It will then land in the Pacific Ocean off the coast of Hawaii.

Musk tried on Sunday to temper expectations, citing a number of ways that things could go wrong, including comparing the engines to live grenades. On a Twitter audio livestream, the billionaire entrepreneur said he had one overriding goal for the attempt: “Just don’t blow up the launchpad.”

(Updates with launch window timing from fourth paragraph; Adds background.)

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