When NASA astronauts return to the Moon under the Artemis campaign and eventually venture farther into the solar system, they will encounter conditions harsher than any humans have experienced before. Ensuring next-generation spacesuits protect astronauts requires new varieties of tests, and a one-of-a-kind chamber called CITADEL (Cryogenic Ice Testing, Acquisition Development, and Excavation Laboratory) at NASA’s Jet Propulsion Laboratory in Southern California is helping.
Built to prepare potential robotic explorers for the frosty, low-pressure conditions on ocean worlds like Jupiter’s frozen moon Europa, CITADEL also can evaluate how spacesuit gloves and boots hold up in extraordinary cold. Spearheaded by the NASA Engineering and Safety Center, a glove testing campaign in CITADEL ran from October 2023 to March 2024. Boot testing, initiated by the Extravehicular Activity and Human Surface Mobility Program at NASA’s Johnson Space Center in Houston, took place from October 2024 to January 2025.
An astronaut boot — part of a NASA lunar spacesuit prototype, the xEMU — is readied for testing in JPL’s CITADEL. A thick aluminum plate stands in for the cold surface of the lunar South Pole, where Artemis III astronauts will confront conditions more extreme than any humans have yet experienced.
In coming months, the team will adapt CITADEL to test spacesuit elbow joints to evaluate suit fabrics for longevity on the Moon. They’ll incorporate abrasion testing and introduce a simulant for lunar regolith, the loose material that makes up the Moon’s surface, into the chamber for the first time.
“We’ve built space robots at JPL that have gone across the solar system and beyond,” said Danny Green, a mechanical engineer who led the boot testing for JPL. “It’s pretty special to also use our facilities in support of returning astronauts to the Moon.”
Astronauts on the Artemis III mission will explore the Moon’s South Pole, a region of much greater extremes than the equatorial landing sites visited by Apollo-era missions. They’ll spend up to two hours at a time inside craters that may contain ice deposits potentially important to sustaining long-term human presence on the Moon. Called permanently shadowed regions, these intriguing features rank among the coldest locations in the solar system, reaching as low as minus 414 degrees Fahrenheit (minus 248 degrees Celsius). The CITADEL chamber gets close to those temperatures.
“We want to understand what the risk is to astronauts going into permanently shadowed regions, and gloves and boots are key because they make prolonged contact with cold surfaces and tools,” said Zach Fester, an engineer with the Advanced Suit Team at NASA Johnson and the technical lead for the boot testing.
Engineers from JPL and NASA Johnson set up a test of the xEMU boot inside CITADEL. Built to prepare potential robotic explorers for conditions on ocean worlds like Jupiter’s moon Europa, the chamber offers unique capabilities that have made it useful for testing spacesuit parts.
Keeping Cool
Housed in the same building as JPL’s historic 10-Foot Space Simulator, the CITADEL chamber uses compressed helium to get as low as minus 370 F (minus 223 C) — lower than most cryogenic facilities, which largely rely on liquid nitrogen. At 4 feet (1.2 meters) tall and 5 feet (1.5 meters) in diameter, the chamber is big enough for a person to climb inside.
More important, it features four load locks, drawer-like chambers through which test materials are inserted into the main chamber while maintaining a chilled vacuum state. The chamber can take several days to reach test conditions, and opening it to insert new test materials starts the process all over again. The load locks allowed engineers to make quick adjustments during boot and glove tests.
Cryocoolers chill the chamber, and aluminum blocks inside can simulate tools astronauts might grab or the cold lunar surface on which they’d walk. The chamber also features a robotic arm to interact with test materials, plus multiple visible-light and infrared cameras to record operations.
An engineer collects simulated lunar samples while wearing the Axiom Extravehicular Mobility Unit spacesuit during testing at NASA Johnson in late 2023. Recent testing of existing NASA spacesuit designs in JPL’s CITADEL chamber will ultimately support development of next-generation suits being built by Axiom Space.
Testing Extremities
The gloves tested in the chamber are the sixth version of a glove NASA began using in the 1980s, part of a spacesuit design called the Extravehicular Mobility Unit. Optimized for spacewalks at the International Space Station, the suit is so intricate it’s essentially a personal spacecraft. Testing in CITADEL at minus 352 F (minus 213 C) showed the legacy glove would not meet thermal requirements in the more challenging environment of the lunar South Pole. Results haven’t yet been fully analyzed from boot testing, which used a lunar surface suit prototype called the Exploration Extravehicular Mobility Unit. NASA’s reference design of an advanced suit architecture, this spacesuit features enhanced fit, mobility, and safety.
In addition to spotting vulnerabilities with existing suits, the CITADEL experiments will help NASA prepare criteria for standardized, repeatable, and inexpensive test methods for the next-generation lunar suit being built by Axiom Space — the Axiom Extravehicular Mobility Unit, which NASA astronauts will wear during the Artemis III mission.
“This test is looking to identify what the limits are: How long can that glove or boot be in that lunar environment?” said Shane McFarland, technology development lead for the Advanced Suit Team at NASA Johnson. “We want to quantify what our capability gap is for the current hardware so we can give that information to the Artemis suit vendor, and we also want to develop this unique test capability to assess future hardware designs.”
In the past, astronauts themselves have been part of thermal testing. For gloves, an astronaut inserted a gloved hand into a chilled “glove box,” grabbed a frigid object, and held it until their skin temperature dropped as low as 50 F (10 C). McFarland stressed that such human-in-the-loop testing remains essential to ensuring future spacesuit safety but doesn’t produce the consistent data the team is looking for with the CITADEL testing.
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To obtain objective feedback, the CITADEL testing team used a custom-built manikin hand and foot. A system of fluid loops mimicked the flow of warm blood through the appendages, while dozens of temperature and heat flux sensors provided data from inside gloves and boots.
“By using CITADEL and modern manikin technology, we can test design iterations faster and at much lower cost than traditional human-in-the-loop testing,” said Morgan Abney, NASA technical fellow for Environmental Control and Life Support, who conceived the glove testing effort. “Now we can really push the envelope on next-generation suit designs and have confidence we understand the risks. We’re one step closer to landing astronauts back on the Moon.”
Through Artemis, NASA will send astronauts to explore the Moon for scientific discovery, economic benefits, and build the foundation for the first crewed missions to Mars.