NASA JPL Space Simulators

NASA/JPL-Caltech. Public domain (NASA / US government work).

A robotic arm, rover or rotorcraft bound for Mars or deep space must survive combined vacuum, solar heating and cold that no single Earth environment produces on its own, and it must do so as flight hardware rather than as a coupon: the qualification article is usually the vehicle that flies. NASA JPL’s environmental test organization answers that requirement with two thermal vacuum chambers built decades apart and sized for different jobs. The 25-Foot Space Simulator in Building 150 is a 1961 chamber with off-axis xenon solar simulation, built to reproduce cruise heating and deep-space cold on a whole spacecraft [1][5]. The smaller 10-Foot Vertical Space Simulator adds a hydraulic-lift floor so that a tightly packed test configuration of rocks, tools and an arm stays reachable at Mars pressure and Mars temperature, at the cost of any solar simulation at all [2]. Both hold Mars pressure and Mars temperature on flight hardware that cannot drive; the driving is done outdoors at the MarsYard, which holds neither.

These are environmental test facilities rather than robotics laboratories, and the 25-Foot chamber is a NASA-owned facility available to other government agencies and to private industry [1][2].

Why the design split this way

A chamber that reproduces solar heating accurately and a chamber that gives full mechanical access to a test article turn out to be difficult to build as one machine, and JPL’s two simulators show why. The 25-Foot chamber’s solar simulator was the harder engineering problem from the start: its operator’s own 1966 account of the facility’s first two years of use, testing Ranger, Mariner and Surveyor hardware, spends most of its length on what went wrong with the light source rather than the vacuum or cold, and names it as the single largest source of trouble, above outgassed cable insulation and potting compound contaminating test items and a stopped mechanical pump that let diffusion pumps backstream oil onto a spacecraft and arc electronics [5]. A 1964 to 1965 spectral characterization of that beam, then produced by 133 mercury-xenon arc lamps, found it two to three times too strong in some visible bands and only a third of solar intensity in others, even though the integrated near-infrared matched the true solar spectrum to within half a percent [6]. That mismatch means an absorptance computed against the real solar spectrum is wrong for any coating whose absorption concentrates in the bands where the simulator is off, and it is why a thermal balance test plan for this chamber has always had to be built around the beam JPL actually has rather than the Sun. The chamber’s other governing constraint is optical: the xenon lamps feed an integrating lens unit that reflects off a large parabolic collimating mirror, and that mirror had to be pulled, stripped, re-plated and re-aluminized in place inside the chamber in a 1994 modernization, using the chamber itself as the coating vessel [7]. The 10-Foot chamber trades all of that away. It carries no solar simulator, and its distinguishing feature is mechanical instead of optical: an end bell on a hydraulic lift, lowered to first-floor level for full access to the test configuration and raised to seal against the shrouds at second-floor level [2]. That is a deliberate choice for arm and sampling work, where the test article is not a single spacecraft skin but rocks, tool tips and a manipulator that has to be reached, re-baselined and reconfigured between test points.

What changed, and what it enabled

The two chambers did not stay static tools built once and used unmodified. The 25-Foot chamber’s control system was itself GE Fanuc-era circa 1994, and when it was mid-replacement in 2016 while a Mars 2020 instrument team needed chamber time, the facility ran an entire campaign by hand: an 81-step manual pump-down procedure, analog pressure gauges and pumping schematics posted in every cabinet, and field operators on noise-canceling radios, completing 38 successful runs that way [10]. JPL’s environmental test group has since made redundant Hand/Off/Auto switches standard on every refurbished chamber control system as a result, which is the transferable lesson of that episode rather than the chamber’s stated performance envelope [10]. On the mechanical side, the Mars Exploration Rover program in 2001 to 2003 added a test technique that had not been attempted before in the 25-Foot chamber: a controlled re-pressurization from 1e-1 Torr back to 8 Torr in six minutes, to simulate telecom operation during descent, rather than the usual one-way pump-down [8]. What those changes enabled was scale and repetition. The chambers moved from qualifying single Ranger and Mariner spacecraft in the 1960s to running whole rover systems (MER, MSL, Mars 2020) and a subsystem as delicate as a Mars helicopter rotor through the same infrastructure, and the 10-Foot chamber’s Mars 2020 sampling and caching campaign ran for two years and dozens of hardware configurations rather than a single test article [2].

Laboratory data

Parameter	Value
Operator	NASA JPL environmental test organization [1]
Location	Pasadena, California, United States
Commissioned	25-Foot Space Simulator, 1961; 10-Foot chamber date not published
Type	Thermal vacuum chambers, one with off-axis solar simulation [1][2]
Floor area	Not published. The 25-Foot chamber stands 25.9 m tall in Building 150
Capabilities	25-Foot and 10-Foot Vertical Space Simulators
Simulant or terrain	No simulant bed. Geoanalogue rocks placed on keyed baseplates [2]
Instrumentation	600 thermocouple channels and 40 RTD channels to the article [1]
Ground truth	Kendall cavity radiometer on the solar beam; six ion gauges
Fidelity limits	Chamber conditions are hard to vary between tests and are held fixed [2]
Access	Available to other government agencies and to private industry [1]
Cited by	Curiosity [3], Phoenix arm [4]

Capabilities

25-Foot Space Simulator

Parameter	Value
Working volume	6.1 m diameter x 7.6 m high with the solar simulator; about 21.3 m high without
Test article limits	Wall hard points rated 10,000 lb each; door 4.6 x 7.6 m
Vacuum	5 x 10^-5 torr in about 3 hours, into the 1 x 10^-6 torr range on cryopumps
Temperature	-185 C on liquid nitrogen; -125 to +100 C on gaseous nitrogen
Illumination	2.0 to 11.0 solar constants, beam 8.5 to 18.5 ft, 37 xenon arc lamps
Slope	Not applicable
Gravity offload	Not applicable. Articles are hard mounted or suspended at 1 g
Instrumentation	600 thermocouple, 40 RTD and 60 heater control channels to the article

Source: [1].

The chamber is 25.9 m high and 8.2 m in diameter, and its useful test volume depends on the mode: 6.1 m diameter by 7.6 m high with the solar simulator running, or about 21.3 m of height without it [1]. Access is at ground level through a 4.6 by 7.6 m door, and articles enter on a monorail crane from the high bay. Structural support is either hard mounting to stanchions that pass through the floor shroud onto columns bearing on an isolated seismic mass below the end bell, or suspension from wall-mounted hard points rated to 10,000 lb each [1]. An earlier full facility description gives close but not identical figures, a 7.6 m diameter by 25.7 m tall chamber reaching the 1e-6 Torr range and a 5.8 m collimated solar beam to 2.3 solar constants at plus or minus 4 percent uniformity, and adds the horizontal thermal-vacuum chambers and the electrodynamic shakers and acoustic chamber that sit alongside it in the same facility [8].

Pumping runs in four stages and reaches 5 x 10^-5 torr in about three hours, with the cryopumps and turbopumps taking it into the 1 x 10^-6 torr range [1]. A 2016 account of the same chamber gives 1e-6 Torr reached in about 90 minutes [10], a faster figure than the facility description above, consistent with pumpdown time depending on chamber loading and outgassing rather than being a fixed constant. The shroud system sets the thermal boundary: louvered aluminum panels, black on every surface facing the test volume, at -185 C on liquid nitrogen or anywhere from -125 C to +100 C on temperature controlled gaseous nitrogen. Mars surface conditions are produced as a gas fill rather than as vacuum: approximately 8 torr of nitrogen for the MSL rover system thermal test [3], and a similar 8 to 10 torr nitrogen fill with shrouds between 20 C and -130 C for the MER rover surface system thermal test, run in the smaller 10-Foot chamber rather than the 25-Foot one [8][9].

Solar simulation is off-axis. Thirty-seven xenon arc lamps feed an integrating lens unit that mixes their output into a uniform beam, which passes a fused quartz window and reflects from a 23 ft collimating mirror at the top of the chamber [1]. Swapping between two integrating lens units and two collimating mirrors gives four beam diameter and intensity combinations spanning 8.5 to 18.5 ft and 2.0 to 11.0 solar constants. The best-characterized combination, the 18.5 ft beam at 2.0 solar constants, has a collimation half angle of 1.1 degrees with 96.2 percent of energy inside 1 degree and uniformity of +0.6 to -1.4 percent; the 8.5 ft beam at 11.0 solar constants is looser, at a 2 degree half angle and plus or minus 5 percent [1]. That collimation is far tighter than the beam was in the 1960s, when the real Sun’s angular divergence of plus or minus 0.267 degrees against a measured simulator divergence of plus or minus 5.3 degrees forced test engineers to redesign sun shades and shadowing hardware to compensate [5], and it depends on a collimating mirror surface that has had to be periodically restored: the mirror was re-plated to 381 micrometers of electroless nickel, polished to 64 percent reflectivity and re-aluminized in a 1994 in-chamber recoating that raised it to about 89 percent reflectivity after conditioning [7].

10-Foot Vertical Space Simulator

Parameter	Value
Vacuum	Approximately 7 torr for Mars surface work; base pressure not published
Temperature	Nominal cold testing at -65 C, with stress cases to -115 C
Illumination	No solar simulator
Simulant or terrain	Geoanalogue rocks on keyed standard baseplates at fixed positions

Source: [2].

The distinguishing feature of the smaller chamber is its layout. The floor, the end bell, rides a hydraulic lift: lowered, it sits at first-floor level and the entire test configuration is accessible; raised, it mates to the shrouds at second-floor level and the chamber seals. That arrangement gives full access to a tightly packed test configuration in a small footprint, which is why it was chosen for the Mars 2020 sampling and caching campaign [2]. An earlier facility description of the same chamber gives its dimensions as 3.0 m by 13.7 m [8].

Instrumentation

The 25-Foot Space Simulator’s data system provides 600 thermocouple channels of type E or T and 40 RTD channels to the test article, with 60 heater control supplies of 75 W each [1]. Facility instrumentation is separate: 140 thermocouple channels on the shrouds, mirror and structure, six ion gauges, a Kendall cavity radiometer measuring the solar beam, and three temperature-controlled quartz crystal microbalances watching contamination [1]. Photogrammetry has also been run inside the chamber under full solar and cryo-vacuum load: a remotely actuated camera on a mobility cart measured thermo-elastic deformation of a 5 m deployable reflector to 0.025 mm RMS over the aperture during a 36-hour solar cycle test, a fidelity the program credited with raising the reflector’s deformation analysis from TRL 3 to TRL 6 [13].

For arm and drill work in the smaller chamber the instrumentation is indirect. Target rocks are scanned into a 3D point cloud and loaded into the Rover Sequencing and Visualization Program Hyperdrive simulator, and arm and drilling operations are always validated in simulation before execution to prevent hardware damage [2].

What it does not reproduce

Gravity. Neither chamber offloads. Articles are hard mounted, suspended or stood on the floor at Earth weight [1][2].

Driving. Neither chamber has a terrain bed a vehicle can traverse. Mobility work goes to the MarsYard, which cannot hold Mars pressure or temperature.

Chamber conditions as a per-test variable. In the Mars 2020 sampling campaign, rock and hardware configurations were fixed for a given chamber pumpdown, and chamber conditions were difficult to vary from test to test, so temperature and pressure for a given configuration were generally held unchanged. The test plan was built around that constraint as a design of experiments rather than against it [2].

Camera geometry. The limited space in the vacuum chamber meant flight-like cameras could not be positioned where they sit on the physical rover, so the campaign built its 3D environment model from a fixed CAD model of the static chamber plus rock samples on keyed standard baseplates at predetermined locations and orientations, instead of from rover imagery [2].

Solar flux and Mars pressure and a moving tool, together. The 25-Foot Space Simulator has xenon solar simulation and has run Mars surface conditions as an 8 torr nitrogen fill [1][3]; the 10-Foot chamber has run approximately 7 torr and -65 C with real rock targets but no solar simulator [2]. The Phoenix icy-soil delivery failure needed all of it at once. Insolation on ice-bearing regolith held in the aluminum scoop warmed the sample enough that it adhered to the scoop wall and would not fall through the TEGA inlet on the first two attempts; a third sample obtained by scraping, with less ice, did not congeal and was delivered [4]. Reproducing that on the ground requires Mars pressure, Mars-relevant sample temperature and a representative solar flux on the tool at the same time, sustained through a transfer sequence of realistic duration. The Phoenix Payload Interoperability Testbed, at the University of Arizona, reproduced the delivery geometry closely enough that overlaid Robotic Arm Camera images from Mars and from the testbed were used to correct scoop positioning over an instrument inlet [4], but not that thermal combination. The Mars Helicopter chamber tests worked around the same gap from the other direction: its 2018 flight readiness reference states plainly that even JPL’s largest thermal-vacuum chamber cannot combine Mars air density, Mars-relevant temperature and true 1/3 g at once, so its free-flight tests ran at Mars density and ambient Earth temperature with a separate gravity offload rig standing in for reduced gravity, and its thermal qualification ran the density and temperature cases separately from the flight dynamics cases [11][12].

Campaigns run there

Ranger, Mariner and Surveyor testing, 25-Foot Space Simulator, 1963 to 1964. The chamber’s first two years of continuous use, run through a standard progression from solar panel to temperature control model to proof test model to flight spacecraft, established the operating record on which later campaigns still draw: contamination from outgassed cabling, a backstreaming incident that arced a spacecraft’s electronics, and a beam map showing 184 W/ft2 nominal with a 6 percent spread over the central 4 ft of the beam [5].

MSL rover system thermal test, 25-Foot Space Simulator, 2011. The 900 kg Curiosity rover was run for 16 days in simulated cruise conditions under vacuum and in simulated Mars surface conditions under 8 torr nitrogen, at mission-extreme hot and cold boundaries [3]. A xenon lamp solar simulator imposed solar loads during a bounding hot case and a simulated Mars diurnal case. All thermal hardware performed nominally and the Rover Heat Rejection System, the liquid loop that moves heat in and out of the chassis electronics boxes, performed better than predicted. Steady state and transient data were used to correlate the thermal models that then predicted Gale Crater performance, with critical hardware predicted to stay within allowable limits across the 669 sol surface mission [3].

MER cruise and surface system thermal tests, 25-Foot and 10-Foot Space Simulators, November 2002 to April 2003. Both Mars Exploration Rover flight spacecraft went through cruise thermal-vacuum testing in the 25-Foot chamber, suspended on steel cables inside the aeroshell under a combination of the xenon solar simulator on the cruise array and separate quartz lamp arrays on off-sun surfaces, across eleven thermal cases per spacecraft including an entry-descent-landing case with the Heat Rejection System off [9]. Each rover then went separately to the 10-Foot chamber for an 8 torr nitrogen surface thermal test with no solar simulation, standing up and deploying inside the chamber through hot and cold diurnal cycles; margins held in every case, with the tightest being a 3.8 C battery cell-to-cell gradient against a 5 C requirement [9]. The campaign’s one new test technique was a controlled re-pressurization of the 25-Foot chamber from 1e-1 Torr to 8 Torr in six minutes to simulate telecom operation during descent, which the test team reported had never been attempted in that chamber before [8].

Mars Helicopter thermal vacuum tests, 25-Foot Space Simulator, engineering and flight models. The Ingenuity flight model went through a 40 C bakeout, steady state thermal balance in vacuum and again at Mars pressure with gaseous-nitrogen backfill, a -90 C cold cycle, and cold functional testing at -50 C shroud including blade spin-ups at 50 and 1800 rpm [11]. Gas conduction across internal gaps, not radiation, turned out to be the largest driver of survival energy at Mars pressure, and the fix was geometric, enlarging the fuselage by 25 mm in one direction and 15 mm in another to open the gaps, plus a low emissivity inner blanket; the tested article also carried ground support cabling that added 50 to 60 percent more heat loss than flight, so the survival energy budget closes only in the correlated thermal model, not in the article as tested [11].

Mars 2020 system thermal vacuum test, 25-Foot Space Simulator, May 2019. A six-day test, cut short from its planned schedule by an anomaly, run on a surrogate rover chassis and a qualification MMRTG unit in place of the unavailable flight rover [14]. It validated cruise thermal design, confirmed heat rejection system leak tightness and pump-swap behavior, and found two workmanship defects, reversed thermostat wiring on one instrument and reversed catalyst-bed PRT wiring on landing engines 3 through 8. Comparing the results against the MSL system thermal test on the same chamber, a build-to-print rebuild of an MSL-heritage design still shifted zone temperature ranges by 2 to 13 degrees C, evidence that hardware inheritance does not transfer test results between programs [14].

Mars 2020 sampling and caching qualification, 10-Foot Space Simulator, 2019 to 2021. A two year series at approximately 7 torr and -70 C, with cold testing nominally at -65 C and stress cases to -115 C, on a chamber floor filled with geoanalogue rocks including deliberately selected challenge rocks [2]. Hardware was phased in as fidelity became available, ending with a qualification model adaptive caching assembly, corer, gas dust removal tool and facility contact sensor, and an engineering model robotic arm. Totals across the series: 6 qualification coring bits took 66 cores, 1 abrading bit made 34 abrasion patches, 1 regolith bit collected regolith twice, and 17 seals were activated [2]. Sample tubes and seals were reused after cleaning, unlike flight. Flight software itself was deployed into the test environment rather than a test-specific build, which was new for the project [2].

2016 campaign run under manual control. A Mars 2020 instrument test in the 25-Foot chamber landed midway through a control-system replacement, so JPL’s environmental test staff ran the entire pump-down, cooldown and CO2 backfill by hand against an 81-step written procedure, adding analog gauges to every cabinet and putting field operators on noise-canceling radios; the campaign completed 38 runs that way [10]. Not a science or qualification result, but the clearest documented case of what running this facility actually requires when its automation is unavailable.

Facilities of this kind elsewhere

The 25-Foot Space Simulator is one of a small set of large solar-thermal-vacuum chambers built for whole-spacecraft qualification rather than component testing; NASA’s own Johnson Space Center Space Environment Simulation Laboratory Chambers A and B, built for Apollo and Gemini, were designed and commissioned on a comparable scale and timeline, and their design requirements process, cost estimates and lunar-plane load specifications survive in contemporaneous program records, a facility for crewed spacecraft rather than robotic missions [15]. The environmental envelope the JPL chambers are built to simulate, extreme temperature swings, hard vacuum, and terramechanics limits set by regolith friction angle rather than by the chamber itself, is common to any planetary surface rover design, not particular to Mars; a survey of flown and proposed lunar and Mars rover concepts compiles that envelope independently of any one test facility [16]. JPL’s own earlier telerobotics and rover research overviews from the 1990s describe several of the tools these chambers later qualified, including precursor rover technology programs run after the Pathfinder mission [17].

What is not established

The spectral mismatch measured in the 25-Foot chamber in 1964 to 1965, roughly two to three times too strong in parts of the visible band and only a third of solar intensity in others, has not been shown superseded by a later remeasurement in this record; the beam’s color has been adjusted in intensity since (dimmed toward Mars solar constants without a reported change in relative spectral emission) but not requalified against the true solar spectrum band by band [6]. Floor area and base pressure for the 10-Foot chamber are not published in any source held here, only the approximately 7 torr Mars-surface operating point used in the sampling and caching campaign [2]. No source in this record states a combined test that holds Mars pressure, Mars-relevant temperature and representative solar flux simultaneously on a moving tool; every campaign found here trades at least one of those away, which is the gap the Phoenix icy-soil delivery anomaly fell into on the actual mission [4][11].