On Sunday the 12th, the Japanese space agency (JAXA) announced that its SLIM spacecraft, in lunar orbit since December 25, had been correctly positioned in its trajectory prior to the launch attempt and added that " the current conditions of the ship are normal”. With this, it was confirmed that the descent maneuver would begin today, Friday, the 19th, from 4 p.m.
SLIM is planned to descend on the Moon through a maneuver that ensures a maximum deviation of 100 meters over the intended site (on the edge of the Shioli crater), an unprecedented level of precision for a mission of this kind. In the area that has been chosen for the descent there are rocks from the lunar layer and, therefore, of great scientific interest. The difficulty of the operation is even greater, since the place where the SLIM is planned to be placed has an inclination of about 15 degrees.
In recent months, several missions have placed the Moon in the current plane in a context of some competition. If the SLIM launch is completed successfully, Japan will become the fifth country to do so, after the United States, the former Soviet Union, China and India.
SLIM left Earth in September, and was successfully inserted into lunar orbit on Christmas Day. In its official statement, JAXA then explained that the ship was following the trajectory that had been planned, consisting of an elliptical route that, passing over the poles of the Moon, reached a height of 600 kilometers in its lowest point (called perilunium) and which separated up to 1,200 kilometers in the farthest part of its orbit (the apoluni).
Since this first orbital insertion, JAXA engineers have been correcting the ship's route to finally place it, last Sunday, in its optimal trajectory to prepare for the descent. Extract from a circular orbit 600 kilometers above the surface of the Moon. Today it will maneuver the SLIM so that it moves again in an elliptical orbit that, this time, places it only 15 kilometers high in its perilunium and moves at a speed of 1,700 meters per second. The start of the final phase of the descent will take place around 4:00 p.m. (peninsular time in Spain), with the forecast that it will touch the lunar surface after twenty minutes.
The place chosen for the landing is on one of the edges of Shioli, a crater in the southeast of the visible face of the Moon and which is about 270 meters in diameter. This place is particularly interesting from a scientific point of view, since, according to the observations made by the Selene probe in 2007, there would be minerals from the lunar layer exposed on the surface. Materials that would allow us to obtain data that complete the knowledge about the formation and subsequent evolution of the satellite.
For all of this to be possible, the ship must move away with great precision in order to be as close as possible to the rocks of interest. For this reason, the SLIM will obtain images during the descent and continuously, and will compare them with the cartography it carries on board (data captured by the Selene mission), and will execute, autonomously, the necessary corrections. The engineers of the Japanese space agency have developed special algorithms so that the probe can make these calculations with the required speed and fidelity.
In the most critical part of the maneuver, when there are only a few meters left to touch the lunar surface, the ship will use a laser fired at the surface to estimate, at every moment and with extreme accuracy, its height and be able to adjust the descent. With all this, SLIM is expected to move away with a maximum error of 100 meters on the intended point.
