Wednesday, September 6, 2023

Watching Japan's XRISM/SLIM Mission Launch

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.  

As usual, more information at the source article, and this time in the video of the mission. 




  1. Getting to any point, even on this Earth, is a tradeoff between speed and mass.

    You can use a go-fast with 4 400hp outboard engines, huge fuel tanks, super slick hull, to carry a ton of cocaine, or you can use a same size boat with a 115hp to carry 6 tons of cocaine. Both get to the same point, just one arrives considerably quicker.

    In space flight, manned flights are all about speed due to constraints on life support (air, food, mental issues) and radiation hazards. The early pre-Manned Moon Landing launches were fast because partially they were a test of the launch parameters of a manned mission.

    We could have sent all our probes and tests nice and slow and huge, but then we'd have to test the whole mission with a manned flight, including landing (we learned a lot about landing from the crashes.)

    No life forms aboard? Take it nice and slow, minimum need for propellants so maximizing cargo/payload/spacecraft structure.

    It's why Starship Human Lander requires so many refueling launches. SpaceX could do it with a low-speed long-duration mission with only one refueling (or no refueling with enlarged fuel tanks and lessened cargo capacity, or even one way only) but since they eventually want to carry people directly to the Moon and back, fast requires multiple refueling.

    1. Your third paragraph is an excellent point. There's so much difference in knowledge between now and the '60s that it's a fundamental difference. They had single, huge goal and everything was based on making that goal.

      Another way of looking at the slow trajectory thing is considering we're (still) in the days of tens of thousands of $$ per pound to orbit. Imagine you're planning a mission and really want to put on another instrument; do you want to spend that money on fuel to get there faster or that instrument? What if it means have to choose a different launch vehicle if you add the instrument?

  2. As to SLIM, the landers are little roller thingies that look like toys. Which you can buy non-space hardware versions of them already. Very cool, using basically COTS to create two mini-bots.

    Though I doubt the 'bots will have long life expectancy.

  3. I wonder how my friend Kenta feels now. A rocket scientist with Mitsubishi Heavy, he was stationed at that facility for a number of years. It was so boring that he built a pizza oven to add excitement. His desired transfer out of stasis finally came through.

    Now he's on the U.S. mid-Atlantic coast with the same employer.

  4. I think the low fuel trajectory will open doors and solve several problems. In the least, the success of SLIM will force reconsideration of this mode.

    The moon is but a transition towards longer flights to distant destinations.

  5. The "faster is better" crowd needs to get on board with NERVA-based thrusters. Otherwise, we will continue to be "slow". Interplanetary travel almost *demands* higher ISPs unless you can make the consumables work in a closed-loop environment.

    1. NASA is exploring nuclear propulsion again, but with modern materials and controls. So there's that.

    2. Let's see what the watermelons do when NASA tries to go for that!

    3. At some point we just have to ignore the watermelons.