The key goal of Japan’s Smart Lander for Investigating Moon is to demonstrate its precision landing ability.
Lunar landers try to set down within several miles of a selected landing site. For example, the landing zone for India’s Chandrayaan-3 spacecraft, which in August became the first to successfully set down in the moon’s south polar region, was seven miles wide and 34 miles long.
The SLIM spacecraft will fire its engines at 10 a.m. Eastern time to begin its descent.
Because the moon has no global positioning satellites or radio beacons, spacecraft have to figure out by themselves exactly where they are. Radar pings will tell the spacecraft how high it is and how fast it is moving. A camera taking pictures of the landscape below will help the spacecraft determine its location by matching the pattern of craters it sees with maps it has stored in its memory.
Vision-based systems on spacecraft have been limited because they have to use special computer chips that are hardened against the strong radiation of deep space. Such chips are generally one or two generations behind top-of-the-line chips, with only about one one-hundredth the processing power, JAXA said in a press kit for the SLIM mission.
For SLIM, JAXA developed image-processing algorithms that can run quickly on the slower space chips. That enables more precise maneuvering, allowing the spacecraft to aim closer to rugged terrain like craters.
During final approach, the images will allow the spacecraft to avoid hazardous rocks and other obstacles.
To land on a slope — the surface is angled 15 degrees at the landing site, close to the Shioli crater on the near side of the moon — the spacecraft will use a two-step landing method.
Just above the ground during the final moments of vertical descent, SLIM will deliberately tilt to one side, allowing the main landing gear at the base of the spacecraft near the engines to touch the ground first. Then it will tip forward onto the spacecraft’s front legs.
Simulations of this landing method showed excellent results, JAXA said.