Japan Explains How It Made an Upside-Down Moon Landing

Japan became the fifth nation to land on the moon on Saturday, but its spacecraft ended up in an awkward position, with its engine nozzle pointed up toward space.

By design, the Japanese spacecraft, known as Smart Lander for Investigating Moon, or SLIM, was supposed to land on its side, a strategy to avoid tipping over on the sloped terrain of the landing site.

But about 150 feet above the ground, one of SLIM’s two main engines appears to have failed, officials at JAXA, the Japanese space agency, said on Thursday.

With the onboard computer trying to compensate for the sudden loss of half of the thrust, the spacecraft was still able to hit the ground at a modest vertical velocity of about 3 miles per hour. But SLIM’s horizontal speed and orientation at landing were outside what it was designed to handle.

As a result, the spacecraft rolled onto its head. It escaped the fate of some other recent robotic missions, which smashed into pieces on the moon, and its systems worked, communicating with Earth. But the solar panels ended up facing west, away from the lunar morning sun, and were unable to generate electricity. With the battery mostly drained, mission controllers on Earth sent a command to shut down the spacecraft less than three hours after landing.

Despite the stumble, the mission accomplished its primary goal: a soft landing in rugged terrain on the moon, within 100 meters of a target landing site, much more precise than the uncertainty of miles that most landers aim for.

“It successfully achieved the controlled landing,” Hitoshi Kuninaka, director general of JAXA’s Institute of Space and Astronautical Science, said in Japanese at a news conference. “We confirmed that the landing position was 55 meters away from the initial target. So we concluded that we achieved the 100-meter-accuracy pinpoint landing.”

During its brief operation, an instrument on the lander took low-resolution, black-and-white images of the surrounding landscape. SLIM team members bestowed nicknames of dog breeds on rocks that caught their interest.

Two small rovers ejected from SLIM just before landing both moved around the lunar surface, and one of them snapped a photograph of the upside-down lander.

JAXA officials remain optimistic that SLIM could revive in about one week, when, during the two-week lunar afternoon, the sun will be shining from the west, illuminating the solar panels.

“We will try to establish communications as SLIM automatically starts operation when the power generation starts,” which could allow operations to resume, said Shinichiro Sakai, the project manager for SLIM, during the news conference.

If SLIM comes back to life, the lander’s instrument will make detailed measurements of the composition of the rocks and soil.

Dr. Sakai said he had “mixed feelings” about the orientation that the spacecraft ended up in. “If the solar cells happened to face down on the surface, there won’t be any chance to receive sunlight, so I feel so relieved it stayed as it is,” he said.

Dr. Sakai said photographs taken by SLIM during its descent, before and after its partial loss of thrust, indicate that one of the engine nozzles fell off. JAXA officials are investigating what went wrong.