ispace is done analyzing data from its failed Hakuto-R lunar landing, and it sounds like tricky terrain and a late change in the landing site are to blame. Apparently, Hakuto-R was able to complete the whole deceleration process in preparation of touching down on lunar soil. The spacecraft activated its descent sequence when it reached an altitude of around 100 kilometers (62 miles) and was able to slow down until it was only moving at a speed of less than 1 m/s.
However, its software had mistakenly estimated its altitude to be zero when it was still hovering around 5 kilometers (3 miles) above the ground. In other words, it thought it had already landed when it hasn’t yet, and it continued descending at a very slow speed near the surface until its propulsion system ran out of fuel. ispace wasn’t able to establish contact with the spacecraft again, but it believes it went on a free fall and ultimately crashed on the moon.
That’s the how, but what about the why? Well, the company thinks the most likely reason why Hakuto-R’s software suffered from an altitude estimation issue was because it got confused. While it was flying to its landing site, it passed over a large cliff that was determined to be the rim of a crater. The spacecraft’s onboard sensor got an altitude reading of 3 kilometers when it passed by the elevated terrain, and that was apparently larger than the estimated altitude value the Hakuto-R team set in advance.
The spacecraft’s software erroneously thought that the sensor reported an abnormal value, and it kept filtering out its altitude measurements afterward. ispace built the ability to reject abnormal altitude measurements into the lander as a safety measure in the event of a hardware issue with the sensor. However, it backfired for Mission 1 because simulations of the landing sequence failed to incorporate the lunar environment on the spacecraft’s route. ispace made the decision to change Hakuto-R’s landing site after its critical design review was already completed in 2021.
The Hakuto-R Mission 1 was poised to become the first successful moon landing by a private company and the first Japanese lunar landing overall. While it didn’t get to land on the moon, ispace will use the data from the mission to design preparatory landing sequences for Mission 2 and 3, which are scheduled for launch in 2024 and 2025, respectively.