Tonight at 7:42 PM EDT, the Japanese Aerospace Exploration Agency (JAXA) successfully launched its Smart Lander for Investigating Moon (SLIM) along with its new telescope the X-Ray Imaging and Spectroscopy Mission (XRISM). The launch was on the Mitsubishi Heavy Industries H-2A. You may remember the failure of the Japanese H3 launch in early March of this year. The H3's upper stage failed to ignite and since the H-2A uses a similar engine, the failure grounded the entire fleet until it was understood. It adds to the pressure to understand the engine failure when you consider the H-2A is due to be replaced by the H3 and this is one of the last H-2A missions. Once the failure analysis was completed, this mission was then scheduled for August 26th, but was delayed by weather until today.
Launch of the H-2A carrying XRISM and SLIM from Tanegashima Space Center, Japan, screen capture of coverage provided by JAXA.
XRISM separated from the second stage 14 minutes after liftoff, entering a roughly 550 x 500-kilometer orbit inclined by 31 degrees. The second stage performed an apogee-raising burn around 46 minutes after liftoff, with SLIM separating from the stage a minute later.
SLIM, also called the Moon Sniper, is heading to the moon in a low-fuel use trajectory; XRISM will stay in LEO.
SLIM won’t be taking a direct route to the moon. After a lunar transfer orbit burn, it will make a lunar flyby, heading into a wide loop away from the Earth-moon system and returning to enter lunar orbit in around four months’ time. This route allows for a smaller burn to enter lunar orbit, saving propellant and mass.
SLIM will then orbit the moon for around a month before making its 20-minute descent and landing attempt, with the aim of demonstrating a lightweight landing capability with high accuracy.
The fact that SLIM will take around five months to get to its landing raised a lot of hackles in the chat stream running alongside the YouTube JAXA video. Similarly, there are many people in positions that get quoted widely saying "we can't go to the moon anymore" comparing missions like this to the lunar probes of the 1960s or even Apollo. They miss the point that "engineering is the art of compromise" and it may be that planning missions like landings on the moon or another planet are the most extreme examples of compromise. We could launch those sorts of trajectories if we wanted to duplicate the missions. The reason for the differing trajectories is the mission planners thought their choice of a cheaper launch was a good trade off for getting a lander with the instruments they wanted on it and doing different things.
The JAXA video included a lot of information on both XRISM and SLIM, and the emphasis on SLIM was "instead of landing where we can get to, landing where we want to." To do that, SLIM will use vision systems.
SLIM, also referred to as “Moon Sniper,” aims to set down within 100 meters of its target point using vision-based navigation. The lander carries observational data from the SELENE orbiter which will be compared to features detected during the autonomous descent and landing. This will greatly reduce the landing ellipsis, or region within which a probe is expected to land based, which is usually on the order of kilometers.
The spacecraft will use a laser range finder, landing radar and navigation camera for the landing attempt, and carries two main 500N engines and 12 smaller engines.
SLIM will also deploy two separate smaller probes, Lunar Excursion Vehicles (LEV), within its last few feet of descent before landing to record the landing site and demonstrate autonomous exploration.