‘At least the crater is in the right place!’ SpaceX CEO Elon Musk tweeted after Tuesday’s test flight.

Space Exploration Technologies Corp.’s fourth test flight of its biggest rocket ended again in a mishap, sending smoking debris across the launch site and raising the prospect of more development hurdles for a vehicle designed to put humans on the moon and Mars.

The Starship SN-11 prototype lifted off in heavy fog at about 8 a.m. Tuesday in Texas from SpaceX’s seaside launch pad near the Mexico border. The rocket then flew to an altitude of about 10 kilometers (6.2 miles) before shutting down its three Raptor engines to begin descent, based on live SpaceX video.

On the prototype’s way down, a rumbling noise developed and the video froze just after the ship was 1 kilometer from the landing pad. Cameras operated by news website NASASpaceflight.com recorded a burst of orange and pieces of debris crashing near the launch site.

“Something significant happened shortly after landing burn start,” SpaceX founder Elon Musk tweeted. “Should know what it was once we can examine the bits later today.”

The mishap underscored the challenge facing SpaceX as it seeks to build a spacecraft capable of reaching the moon and Mars. The previous Starship test, on March 3, touched down at a slight incline and was engulfed in flames less than a minute later. Two earlier attempts ended in fireballs. No people have been near the flight tests.

Since March 12, the Federal Aviation Administration has required an on-site inspector for each test flight.

The next prototype, SN-15, will be at the launch pad in a few days with “hundreds of design improvements,” Musk tweeted. “Hopefully, one of those improvements covers this problem. If not, then retrofit will add a few more days,” he wrote.

The following craft, SN-20, will be capable of orbital flight with heat shields for re-entry and a stage-separation system, he said. The SN-20-class vehicles likely will require many flight attempts to survive the rigors of atmospheric heating and re-entry speeds at 25 times the speed of sound, Musk said.

Despite the fiery finales so far, each test flight has offered SpaceX data on the enormous rocket’s design, propulsion, navigation and other systems. The Starship will operate with six Raptor engines.

SpaceX conceived the stainless steel Starship as a versatile, fully reusable craft that can carry 100 metric tons for deep-space missions to the moon and Mars. It’s also designed to serve as a hypersonic, point-to-point vehicle to reduce travel times across Earth. Excluding a heavy booster that creates a two-stage system, Starship is 160 feet (49 meters) high with a 30-foot diameter, and able to carry as many as 100 passengers.

Musk tweeted a photo of the first Super Heavy booster on March 18, calling it a “production pathfinder” that’s helping Hawthorne, California-based SpaceX learn how to build and transport the 230-foot behemoth. The second such booster will be the one that flies, and SpaceX’s goal is to have one on the launch pad by the end of April, Musk said Tuesday on Twitter.

The full system is scheduled for a commercial flight in 2023 with Japanese entrepreneur Yusaku Maezawa, who is collecting applications for eight people to accompany him on a Starship “fun trip” around the moon.

“I’m highly confident that we will have reached orbit many times with Starship before 2023, and that it will be safe enough for human transport by 2023,” Musk said in a video Maezawa released earlier this month.

In October, Musk said he was 80% to 90% confident that Starship will be ready for an orbital flight this year. SpaceX plans to fly multiple Starship prototypes from its Texas launch site, an area the rocket maker has dubbed Starbase.

SpaceX on Tuesday also revealed plans to install a clear dome on its Crew Dragon vessel to allow for better views of space. The design change is scheduled to fly on the Inspiration4, an all-civilian mission that the company expects to launch this year.