LDEF (Long Duration Exposure Facility)
In the early days of the spaceage, researchers recognized the potential of the planned Space Shuttle to deliver a payload to space, leave it there, and on a separate mission, retrieve the payload and return it to Earth for measurements. In particular, there was great interest in getting a better understanding of radiation effects and space debris impacts in the near-Earth environment on spacecraft and instrument surfaces and materials. There were simply so many uncertainties in the design of components due to the lack of models and analysis tools.
LDEF is a NASA/LaRC freeflying structure designed to take advantage of the two-way, transportation capability of the Space Shuttle. It accommodates technology, science, and applications experiments on long-term exposure to the space environment, with the objective to collect small meteoroids and space debris in LEO (Low Earth Orbit) for post-mission impact analysis of surfaces and materials that have been exposed to the space environment. Directional resolution of the flux of meteoroids and space debris particles. 1) 2) 3) 4) 5) 6)
Background: The LDEF concept evolved from a spacecraft proposed by NASA/LaRC (Langley Research Center) in 1970 to study the meteoroid environment. The project was initially known as MEM (Meteoroid and Exposure Module). The MEM project was approved by NASA in 1974, renamed to LDEF, and managed by LaRC for OAST (Office of Aeronautics and Space Technology) of NASA. In particular, retrievability was the prime requirement of the LDEF spacecraft to obtain experimental evidence to space exposure for evaluation in postflight analysis. While meteoroid research was initially seen as the primary mission requirement, eventually LDEF became a vehicle to study the following topics as well:
- Changes in materials properties over time in the space environment
- Performance tests of spacecraft systems
- Evaluations of components used in powering spacecraft
- Experiments in the growth of crystals in low gravity
- Scientific investigations in space physics and related fields.
Figure 1: Illustration of the LDEF freeflyer spacecraft on orbit (image credit: NASA) 7)
The LDEF spacecraft is an open-grid, 12-sided, cylindrical structure (plus two end faces) on which a series of rectangular trays used for mounting experiment hardware were attached. Length of cylinder = 9.1 m, diameter = 4.3 m. LDEF was a gravity-gradient stabilized spacecraft, the longitudinal axis was pointing toward the center of the Earth. Surface elements were fixed relative to LDEF's velocity vector. Magnetic actuators control the rotation around the longitudinal axis. The LDEF structure was configured with 72 equal-size rectangular openings on the sides and 14 openings on the ends (six on the Earth-facing end, and eight on the space-facing end) for mounting experiment trays. Total mass of spacecraft = 9,710 kg.
While the LDEF had no central power or data system, it did, however, provide initiation and termination signals at the start and end of the mission. Any required power and/or data systems were included by the experimenter in his respective tray.
Figure 2: View of the LDEF structure on a ground transporter (image credit: NASA/LaRC)
Launch: The LDEF was launched on Shuttle STS-41-C (Challenger) on April 6, 1984 from Cape Canaveral, FL, USA.
Figure 3: Prior to deployment the LDEF is lifted over Challenger's payload bay on mission STS 41C (image credit: NASA)
Orbit: almost circular orbit, average altitude = 477 km, inclination = 28.5º. At the time of retrieval the orbital altitude had decreased to 335 km.
Deployment and retrieval of LDEF:
LDEF was deployed on April 7, 1984 on Shuttle flight STS-41-C (Challenger). The retrieval was planned after 10 months. The newly developed RMS (Remote Manipulator System) of CSA (Canadian Space Agency) on Shuttle, also known as Canadarm-1, provided the on-orbit servicing functions of LDEF deployment and retrieval.
Due to delays caused by the Challenger accident (STS-25), it was finally recovered 69 months (5.8 years!!) after launch on January 12, 1990 (STS-32) on Space Shuttle Columbia. The Shuttle approached LDEF in such a way as to minimize possible contamination to LDEF from thruster exhaust. While LDEF was still attached to the RMS arm, an extensive 4.5 hour photo survey was performed that included photographs of each individual experiment tray, as well as larger areas. Special efforts were undertaken to ensure protection against contamination of the payload.
As a consequence of the delayed retrieval from orbit, much more data had been gathered than planned. Post mission deintegration in SAEF-II (Spacecraft Assembly and Encapsulation Facility-II) at NASA/KSC.
Figure 4: Canadarm retrieval of the LDEF on Jan. 12, 1990 (image credit: NASA/LaRC)
Some mission results:
LDEF carried over 10,000 material specimens, as well as a multitude of hardware elements. Some specimens were exposed to the space environment on the outer surface of LDEF, while others were positioned internally or shielded. The harvest was rich. 8) 9)
• The LDEF results have become the baseline for understanding long-term exposure to the LEO environment for the following reasons:
1) The stability of the LDEF orientation during flight allowed a precise definition of environments around the spacecraft and an indication of performance of specific materials as a function of exposure level
2) The spacecraft was exposed to a variety of space environments over an extended period of time
3) A very large quantity and variety of materials were examined upon LDEF's return
4) Even though LDEF was launched in 1984, many materials flown are still essential for use on spacecraft, and evaluation of their performance is technologically significant
5) LDEF results confirmed the good performance of a number of materials, components, and systems
6) The collection of on-orbit and post-flight photographs are extensive and are an extremely valuable archive
7) LDEF experimental results verified the models used to predict atomic oxygen exposure levels
8) A collection of particulate debris was observed trailing LDEF as the Shuttle approached for retrieval. Vapor-deposited aluminum backing from failed thermal-control blankets on the leading edge of the spacecraft was the apparent source of the particles. The particles appeared to be spinning as individual particles periodically reflected sunlight into the camera.
In general, the results from the LDEF spacecraft have provided much useful information on material sensitivity in the LEO environment. This is particularly true for selected materials such as thermal control coatings, composites, polymers, fasteners and solar cells. However, LDEF material sensitivity data for other materials like glasses, glass coatings, lubricants, adhesives and seal materials were limited.
A total of 57 science and technology experiments involving government and university investigators from the United States, Canada, Denmark, France, Germany, Ireland, the Netherlands, Switzerland, and the United Kingdom, flew on the LDEF mission.
All LDEF experiments are self-contained in trays that are clamped to the facility structure. The LDEF has 72 peripheral and 14 end experiment trays. The 12 sides of the LDEF structure are numbered rows 1 through 12 in a clockwise direction when facing the end with the support beam (the Earth-facing end in orbit). The six longitudinal locations are identified alphabetically as Bay A through Bay F, starting at the end with the support beam. A tray location is designated by Bay and Row: A-1, B-5, F-8, etc. The Earth-facing end is designated by a G identifier; the locations have even-number clock-position identifications (G-12, G-2, G-4, G-6, G-8, and G-10). The space-facing end is designated by an H identifier, the locations also following a clock-position convention (H-12, H-1, H-3, H-5, H-6, H-7, H-9, and H-11).
Figure 5: LDEF experiment integration model (image credit: NASA)
Figure 6: Illustration of a typical experiment tray (image credit: NASA)
Table 1: Summary of LDEF experiment complement
Exposure of Spacecraft Coatings (S0010); PI: W. S. Slemp, NASA/LaRC. The objectives of this experiment are to determine the effects of both the Shuttle-induced environment and the space radiation environment on selected sets of spacecraft thermal control coatings. The experiment utilizes a tray of 15.2 cm in depth and an EECC (Experiment Exposure Control Canister). The EECC provides protection for some of the samples against exposure to the launch and reentry environments.
Figure 7: Exposure of spacecraft coatings experiment S0010 (image credit: NASA)
HEPP (Heat Pipe Experiment Package), S1001; PI: R. McIntosh, NASA/GSFC. The objectives of the experiment are to determine zero-g start-up performance for conventional and diode low-temperature (< 190 K) heat pipes, to evaluate heat pipe performance in zero-g for an extended period of time, to determine zero-g transport capability of each heat pipe, and to determine diode operation, including forward conductance, turndown ratio, and transient behavior. Two heat pipes, a fixed-conductance transporter heat pipe and a thermal-diode heat pipe, are coupled with a radiant cooler system. HEPP is a completely self-contained and thermally isolated package designed to fit in a deep peripheral tray (30.5 cm).
Figure 8: Low temperature heat pipe experiment shown during LDEF compatibility test (image credit: NASA)
THERM (Thermal Measurement System), P0003, PI: R. F. Greene, NASA/LaRC. The objectives of this experiment are to determine the history of the interior average temperatures of the LDEF for the total orbital mission and to measure the temperatures of selected components and thermal boundary conditions.
THERM consists of six copper-constantan thermocouples (T/C's), two thermistor reference measurements, an electronic system, one 7.5 V battery, and an interface harness with the HEPP experiment. Data are recorded on dedicated channels of a shared EPDS tape recorder in the low-temperature heat pipe experiment package (HEPP).
Operationally, THERM is activated by LDEF just prior to LDEF deployment into orbit. Routine scans of data are being taken about 12 times daily; however, on occasions during the mission the HEPP logic will trigger the EPDS to record data in the high-frequency data recording mode for periods up to 15 days. During the high-frequency mode, data scans are being taken every 5 minutes to provide temperature profiles throughout typical orbits. The THERM data provides both long-term and transient temperatures. Total system accuracy is within plus or minus 10ºF for all measurements over a range from -30ºF to 170ºF.
The THERM data, other experiment temperature data, and LDEF attitude information are being reduced and analyzed postflight to provide each experimenter with an improved time history of the experiment boundary conditions encountered during the LDEF mission.
Figure 9: Location of THERM hardware on LDEF (image credit: NASA)
LET (Linear Energy Transfer Spectrum Measurement Experiment), P0006; PI's: E. V. Benton, University of San Francisco, T. A. Parnell, NASA/MSFC. The objective is to measure the LET spectrum behind different shielding configurations. The shielding is being increased in increments of approximately 1 g/cm2 up to a maximum shielding of 16 g/cm2. In addition to providing critical information to future spacecraft designers, these measurements provide also data that are extremely valuable to other experiments on LDEF.
A combination of thermal luminescence and track type detectors are being used to measure the LET. Aluminum is being used for the shielding. The passive detectors and shielding material are being placed in the canister (Figure 10).
Figure 10: LET spectrum measurement canister (image credit: NASA)
Frecopa (French Cooperative Payload), A0138-1; PI: J. C. Mandeville, CERT/ONERA and others. Objective: determination of the number of impacts, and the size and chemical composition of the impacting cosmic dust and debris. A collection area of about 2000 cm2 is exposed to the space environment (multilayer thin foil detectors). In addition a large variety of materials placed on the same tray (8500 cm2) is exposed.
Figure 11: Layout of the Frecopa experiments (image credit: NASA)
Chemical and Isotopic Measurements of Micrometeoroids by Secondary Ion Mass Spectrometry), A0187-2; PI's: E. Zinner Washington University, St. Louis, E. K. Jessberger, MPI Heidelberg, and others. Objective: chemical and isotropic measurements of micrometeoroids by secondary ion mass spectrometry. The dust particles have a mass of > 10-10 g for most of the major elements.
The experiment utilizes a passive Ge target which is covered with a thin metallized plastic foil. The foil is coated on the outer (i.e., space facing) surface with a Au-Pd film for thermal control and to protect the foil from erosion by atomic oxygen present in the residual atmosphere. The inner surface of the foil is coated with tantalum, which was selected in order to optimize the analysis of positive secondary ions by secondary ion mass spectroscopy (SIMS).
The experiment occupies a peripheral tray of 7.5 cm in depth near the LDEF leading edge, one-third of a 15 cm deep peripheral tray near the LDEF trailing edge, and two-thirds of a 15 cm deep peripheral tray on the LDEF trailing edge. Figure 12 shows the one-third-tray experiment hardware and illustrates the micrometeoroid detection principle. An incoming meteoroid penetrates the foil before striking the target plate.
Figure 12: Layout of SIMS experiment (image credit: NASA)
MAP (Multiple-Foil Micro-Abrasion Package), A0023; PI: J. A. M. McDonnell, University of Kent, UK. Objective: measurements of impactor velocity, density, angle of incidence, and chemical composition. A capture cell experiment. Each detector consists of two foils and a pure, polished stop plate to catch fragments of impacting particles. Deployment of MAP detectors on leading and trailing surfaces, and the surfaces normal to them, of LDEF.
Figure 13: Illustration of impact events and scope of investigation (image credit: NASA)
The experiment approach utilizes the well-established technique of thin-foil hypervelocity penetration supported by extensive investigations and calibrated in laboratory simulation to a high precision. The detector design utilizes rolled aluminum foil down to a thickness of 1.5 µm.
UHCRE (Ultra-Heavy Cosmic Ray Experiment), A0178, a high resolution study; PI: D. O'Sullivan, Dublin Institute for Advanced Studies, Ireland, and others. The objective of the experiment is a detailed study of the charge spectra of ultraheavy cosmic-ray nuclei from zinc (Z = 30) to uranium (Z = 92) and beyond using solid-state track detectors. Further goals are the study of the cosmic-ray transition spectrum and a search for the postulated long-lived superheavy (SH) nuclei (Z ≥ 110), such as 110SH294, in the contemporary cosmic radiation.
Cosmic rays constitute a unique sample of material from distant parts of our galaxy which still bears the imprint of the source region. The ultraheavy cosmic-ray composition will provide a great deal of information about the evolution of matter in the universe. This question is closely related to understanding the origins of the elements in the solar system.
The experiment uses solid-state track detectors to identify charged cosmic-ray particles. The basic detector component is a thin sheet of polymer plastic (typically 250 µm thick). The determination of charge and velocity depends on the mechanism by which cosmic-ray nuclei that penetrate the plastic sheets produce radiation damage along the particle trajectories. A total area of 18 m2 of thermal blankets are being used to collect a large number of meteoroid and debris impact records of various sizes.
Figure 14: Cosmic-ray experiment configuration showing location of detector stacks (image credit: NASA)
The nuclear track detectors, with lead foil energy degraders, are being assembled in stacks that are mounted in aluminum cylinders designed to fit into 30.5 cm deep peripheral trays. Three cylinders, each containing four stacks, are being placed parallel to the x-axis of each tray (Figure 14). The cylinders are approximately 1.17 m long and approximately 25 cm in diameter, and have a wall thickness of approximately 0.5 g cm2. The trays are thermally decoupled from the LDEF frame and carry thermal covers flush with their outer rims. Sixteen trays are being employed.
Passive Exposure of Earth Radiation Budget Experiment Components, A0147; PIs: J. R. Hickey, F. J. Griffin, The Eppley Laboratory Inc., Newport, RI. The objective is to determine the accuracies needed for ERB (Earth Radiation Budget) instruments to measure the solar and Earth radiation flux properly. The ERB instrument was operational on Nimbus-6 (launch June 12, 1975) and is operational on Nimbus-7 (launch Oct. 24, 1978). However, the in-flight calibration of the instrument was rather difficult for the solar and Earth flux channels.
The goal of of this experiment is to expose ERB channel components to the space environment and then retrieve them and resubmit them to radiometric calibration after exposure. Subsequently, corrections may be applied to ERB results and information will be obtained to aid in the selection of components for future operational solar and Earth radiation budget experiments.
The passive exposure of solar and Earth flux channel components of the ERB radiometer is the basis of the approach taken. Three Earth flux channel types of ERB are being mounted in one-fourth of a 7.5 cm deep end center tray on the Earth-viewing end of the LDEF (Figure 15). The channels underwent complete radiometric and spectrophotometric examination prior to delivery.
Figure 15: Illustration of Earth flux channel components (image credit: NASA)
The solar channel components are being mounted in one-sixth of a 7.5 cm deep peripheral tray near the leading edge of the LDEF (in the direction of the velocity vector) to view the sun in the manner most like the ERB instrument on Nimbus. The solar channel components include thermopiles, interference filters, and fused silica optical windows (Figure 16). Additionally, some state-of-the-art vacuum-deposited interference filters have been included to examine space environment effects on these components. The two thermopiles have different black paint on the receivers. The cavity unit is similar to that proposed for future solar constant measurement missions.
Figure 16: Layout of solar channel components (image credit: NASA)
1) A. S. Levine (editor), “LDEF - 69 Months in Space, First Post-Retrieval Symposium,” NASA Conference Publication 3134 (Part 1 and Part 2), Proceedings of a symposium sponsored by NASA at Kissimmee, Florida, June 2-8, 1991
2) “The Long Duration Exposure Facility (LDEF), Mission 1 Experiments,” 1984. NASA SP-473, edited by L. G. Clark, W. H. Kinard, D. J. Carter Jr., J. L. Jones Jr., URL: http://ia600507.us.archive.org/1/items/nasa_techdoc_19840016564/19840016564.pdf
5) J. W. Watts, T. W. Armstrong, B. L. Colborn, “Prediction of LDEF exposure to the ionizing radiation environment,” Radiation Measurements, Vol. 26, Issue 6, November 1996, pp. 893-899
9) W. H. Kinard, R. L. O'Neal, “Long Duration Exposure Facility (LDEF) Results,” AIAA 29th Aerospace Sciences Meeting, Reno, NV, Jan 7-10, 1991.
10) G. B. Rauch, R. D. Sudduth, “Design application and development of spacecraft in LEO utilizing LDEF results,” NASA/MSFC publication, `LDEF Materials Results for Spacecraft Applications,' pp. 511-523 (SEE N94-31012 09-12)
11) W. Flury, “Europe's Contribution to the Long Duration Exposure Facility (LDEF) Meteoroid and Debris Impact Analysis,” ESA Bulletin, Number 76, November 1993, pp. 112-118
12) Thomas H. See, Martha K. Allbrooks, Dale R. Atkinson, Clyde A. Sapp, Charles G. Simon, Mike E. Zolensky, “Meteoroid & Debris Special Investigation Group, Data Acquisition Procedures,” URL: http://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.77.9971&rep=rep1&type=pdf
The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: ”Observation of the Earth and Its Environment: Survey of Missions and Sensors” (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates.