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SpaceX is preparing to launch the latest prototype of its next-generation Starship rocket on Wednesday, in a high-altitude flight that represents the company’s most ambitious test to date.

Starship prototype Serial Number 8, or SN8, will aim to fly as high as 12.5 kilometers, or about 41,000 feet. That’s significantly higher than the pair of 500-foot flight tests that SpaceX completed with prototypes SN5 and SN6 earlier this year.

The attempt comes a day after SpaceX nearly launched the rocket but was stopped short by a last second engine issue. The company has since reset for another attempt.

Notably, the goal of the SN8 flight is not necessarily to reach the maximum altitude, but rather to test several key parts of the Starship system.

“This suborbital flight is designed to test a number of objectives, from how the vehicle’s three Raptor engines perform to the overall aerodynamic entry capabilities of the vehicle (including its body flaps) to how the vehicle manages propellant transition. SN8 will also attempt to perform a landing flip maneuver, which would be a first for a vehicle of this size,” SpaceX said in a statement on its website.

Given the multiple development milestones the company is undertaking with the SN8 flight, SpaceX CEO Elon Musk gave the rocket low odds of complete success on the first try.

“Lot of things need to go right, so maybe 1/3 chance,” Musk said.

Starship SN8 is built of stainless steel, with the prototypes representing the early versions of the rocket that Musk unveiled last year. The company is developing Starship with the goal of launching cargo and as many as a 100 people at a time on missions to the Moon and Mars.

While SpaceX’s fleet of Falcon 9 and Falcon Heavy rockets are partially reusable, Musk’s goal is to make Starship fully reusable — envisioning a rocket that is more akin to a commercial airplane, with short turnaround times between flights where the only major cost is fuel.

The company is building and testing the Starship prototypes at its growing facility in Boca Chica, Texas. The facility on the coast of the Gulf of Mexico, about 20 miles east of the Texas city of Brownsville on the Mexico border.

SpaceX also noted that it has completed over 16,000 seconds – or nearly four and half cumulative hours – of tests running its Raptor series of engines, which are built to power Starship.

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