The Mitsubishi Heavy Industries H-IIA launch vehicle, as its career is being wound down in favor of the H3, launched the Smart Lander for Investigating Moon (SLIM) robotic lunar lander and the X-Ray Imaging and Spectroscopy Mission (XRISM) X-ray telescope on its 47th flight. After this flight, the second of 2023 for the H-IIA, the H-IIA will have three flights left before retirement.
The H-IIA vehicle F47 was scheduled to launch from the LA-Y1 launch pad at Tanegashima Space Center, Japan, on Monday, Aug. 28, at 00:26 UTC. After a scrub due to weather, the launch of SLIM and XRISM took place Wednesday, Sept. 6, at 23:42 UTC.
Immediately after liftoff, the H-IIA flew an eastward trajectory over the Pacific. The H-IIA’s two solid rocket boosters released near the T+1:48 mark, while the core and its LE-7A engine, using liquid hydrogen and liquid oxygen as propellants, operated until around T+6:35.
After stage separation, the second stage, equipped with a LE-5B engine and using the same propellant combination as the LE-7A, burned until approximately 15 minutes after launch. The two payloads separated sometime after the stage shuts down its engine.
The XRISM X-ray observatory was placed into a 550-kilometer circular low-Earth orbit inclined 31 degrees to the Equator. The SLIM lunar lander will also be placed in the same orbit but will use its own engines to get to the Moon.
This flight’s main payload is the XRISM — the observatory is a replacement mission started in 2016 after the failure of the Hitomi X-ray observatory weeks after reaching orbit. Hitomi was in its commissioning phase, having made some test observations when false information from a sensor and software issues caused the spacecraft to spin in orbit and break apart.
Hitomi’s failure could have left the scientific community without an orbiting X-ray observatory for a long period of time from the early 2020s to the late 2030s. JAXA began the XRISM project in June 2016, three months after Hitomi’s failure. NASA, ESA, and major universities on three continents are collaborating on the project.
X-ray astronomy has only been performed within the last sixty years, as X-rays from deep space are attenuated by the Earth’s atmosphere. Humanity has observed the heavens in visible light with its own eyes for millennia and with optical means for centuries. The advent of spaceflight has enabled observations of stars, galaxies, and the background of the universe in wavelengths inaccessible to astronomers prior to the 1960s.
The first Japanese X-ray observatory, Cygnus X-1, was launched in 1979, and Japan has successfully flown a number of X-ray telescopes. XRISM will join other space-based observatories such as the Chandra X-ray Observatory, XMM-Newton, NuSTAR, and IXPE in orbit. These spacecraft all observe the universe in the X-ray spectrum but do so in different ways which complement each other.
X-rays are generated by objects like exploding stars, black holes, radio galaxies, pulsars, and other high-energy phenomena. XRISM’s science objectives are to study clusters of galaxies, how the structure of the Universe evolves, how matter spreads through interstellar space, how energy is transported through the Universe, and how matter behaves under strong gravitational and magnetic fields that cannot be created on Earth.
To accomplish these objectives, XRISM is equipped with two instruments, both attached to a dedicated X-ray mirror assembly. The Resolve spectrometer is designed to make highly detailed measurements of an X-ray emitting object’s temperature and composition, and can make detailed Doppler measurements to determine how objects in the Universe move.
Resolve needs to be cooled to -273.1 degrees Celsius, which is just barely above absolute zero, to make its observations. This is done with a dewar filled with superfluid helium. The instrument observes “soft” X-rays, which have longer wavelengths than “hard” X-rays which spacecraft like IXPE are designed to observe.
The Xtend X-ray imager, like Resolve, is designed to observe soft X-rays. Xtend has a field of view that can capture the full Moon, and can image larger celestial objects. The instrument is similar to one that was used on Hitomi.
The XRISM spacecraft masses 2,300 kilograms and is eight meters long and three meters in diameter. In addition, the two solar panels will extend nine meters from tip to tip. After the spacecraft reaches orbit, there will be a critical operation phase where XRISM’s attitude control ability will be tested.
A commissioning phase will test the spacecraft’s subsystems, and a seven-month performance verification phase will evaluate the science instruments. Once this is finished, science observations will start. The primary mission is scheduled to last for two years, and a mission extension will be evaluated.
On the heels of the successful Chandrayaan-3 landing, Japan will seek to join the United States, the Soviet Union, China, and India in the club of nations that have landed probes successfully on the Moon. The SLIM lander will attempt to succeed where earlier Japanese landing attempts with the Hakuto-R and OMOTENASHI missions failed.
SLIM is the secondary payload on this flight. The project is an outgrowth of the SELENE-B lander proposed at the turn of the century, and SLIM was proposed in 2012. The spacecraft’s critical design review was done in 2019, and its launch date kept moving along with the XRISM payload’s flight.
The SLIM lander masses around 700 kilograms after it is fueled, and it is built around a cylindrical fuel tank over two meters long containing hypergolic propellants. The spacecraft is equipped with two main engines capable of 500 Newtons of thrust along with 12 thrusters capable of around 20 Newtons of thrust.
The spacecraft requires a slow, fuel-efficient trajectory that would take SLIM to the Moon in around four months. This is similar to the HAKUTO-R lander, and unlike larger landers like the Chang’e or Chandrayaan spacecraft which took less time to reach the Moon.
Once SLIM reaches lunar orbit, it will spend around a month there before its landing attempt. Unlike the Chandrayaan-3 mission, SLIM is not targeted for the south polar region. The landing site is in Mare Nectaris, and is at 13.3 degrees South latitude, 25.2 degrees East longitude, on the slopes by Shioli crater.
When SLIM’s deorbit burn is complete, it will use a system based on face recognition technology to autonomously navigate to its landing site. The spacecraft has an onboard map with observational data from the SELENE orbiter. Using that data, it will compare the terrain features it sees, and it is equipped with a landing radar, laser range finder, and a navigation camera to provide critical information to the integrated computer.
A major objective of SLIM is to demonstrate a precision landing to within 100 meters of its target. This capability, if achieved, would enable future landers to reach sites currently not able to be visited by spacecraft. Current lunar landing capabilities are on the order of at least several kilometers as the landing ellipse.
SLIM will transition to a horizontal position just before landing, and will use five fixed landing legs with crushable aluminum shock absorbers to touch down on the lunar surface. Thin film solar panels mounted on the side opposite the landing legs provide power, while an S-band communication system connects SLIM with Earth.
The spacecraft is equipped with a multi-band spectral camera that is designed to measure the composition of rocks surrounding the landing site. It is hoped that mineralogy measurements can help scientists piece together how the Moon formed.
A small probe known as the Lunar Exploration Vehicle-1 is to separate from SLIM just before landing and image the site. SLIM is also carrying the ball-shaped SORA-Q mini-rover, also known as Lunar Exploration Vehicle-2, that was designed by Tomy, the Japanese toy maker who invented the transformers toys.
In addition, NASA has provided a mirror reflector to enable precise measurement of the distance between Earth and the landing site, similar to the ones aboard Chandrayaan-3 and the Apollo missions.
A stretch goal for SLIM is to conduct operations until lunar sunset. Lunar daylight at a given location lasts around 14 Earth days, and once the sun sets the lunar surface can reach a temperature of minus 130 degrees Celsius.
The H-II family has been Japan’s workhorse launch vehicle for nearly 30 years. The H-II’s first flight was in 1994, while the H-IIA first flew in 2001 after the H-II was retired following a launch failure in 1999.
The H-IIB first flew in 2009 for HTV cargo ships to ISS and last flew in 2020 . The H-II family overall has launched communications and weather satellites, lunar and interplanetary probes, and military reconnaissance satellites along with other payloads.
The H-IIA is the only vehicle still active in the H-II family of rockets, and the H3 is due to replace it. However, the H3’s first flight in March of this year ended in failure, and the second stage was implicated in the failure. The H3 second stage is very similar to the H-IIA’s, so common failure modes had to be cleared before the XRISM/SLIM launch could fly.
After this flight, the H-IIA is slated to fly the GOSAT-GW greenhouse gas monitoring satellite no earlier than December 2023 and the Japanese military IGS-Optical 8 and IGS-Radar 8 reconnaissance spacecraft no earlier than April 2024. If all goes well, the H-IIA would end its career with 49 successful launches in 50 attempts, with the only failure being in 2003 due to the loss of an SRB separation system.
The H-IIA joins the Ariane 5 among the major launch vehicles being retired in 2023. JAXA is working to return the H3 to flight and the timetable for this is not currently known. Once the H3’s issues are resolved, it is set to become Japan’s main launcher for important missions to ISS, civilian and military weather, communications, and observation satellites, and future lunar and interplanetary flights.
(Lead image: HII-A launches XRISM and SLIM. Credit: JAXA)