How Japan's lander was confused by a crater on the Moon

The lander would ultimately be destroyed by crashing into the lunar surface.
Loukia Papadopoulos
An artist's impression of the moon lander.jpg
An artist's impression of the moon lander.


On April 26, the $90 million Hakuto-R Moon lander, built by the Japanese firm ispace, was descending toward the lunar surface at 120 kmh (75 mph). It should have had a gentle touchdown but it ended up crashing into the moon ultimately leading to its destruction.

Why did this happen? A report by Yahoo News published on Friday seeks to answer the question.

One explanation given was that the vehicle ran out of fuel for the engines that slowed it. This was the fault of the software used to pilot the lander and a change in destination late in the mission planning process.

Robotic Moon landers traditionally pilot themselves by using two key sources of information.

The first is an estimate of its location produced by something called an inertial measurement unit, or IMU, controlled by gyroscopes and accelerometers. The second consists of a laser rangefinder, which measures the actual distance between the robot and the moon’s surface.

A troublesome conflict

ispace engineers discovered that there was a conflict between these two sources of data: the estimated altitude over the Moon and the distance reported by the laser rangefinder recorded a difference of about 5 km (3.1 miles). 

What happened then? The moonlander ignored the rangefinder data, burning its propellant too quickly and spending two minutes in free fall.

ispace chief technology officer Ryo Ujiie noted that this fatal discrepancy came from a last minute decision to move the robot’s target landing site from a flat plain on the Moon called Lacus Somniorum to the Atlas crater. 

Ujiie told reporters on May 26 that the change in destination was prompted by the fact that the crater was considered more scientifically interesting than the plains, which can be more easily examined by telescope.

Asked if the vehicle might have landed at the original site, Ujiie said that “it is very hypothetical, [but] yes, we might have had a chance to successfully land on the Moon.” 

It is estimated that ispace lost 100 million Japanese yen ($711,000) due to the failure of this crucial mission although the firm did have the mission insured, according to Yahoo News.

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