Skylab’s major nonmedical scientific project was the Apollo telescope mount, which became a part of the program in 1966. The most complex and expensive of the scientific programs and the most demanding in terms of technical requirements, the ATM had been allotted one of the three AAP missions (pp. 55, 74). When the decision was made to launch the solar observatory along with the rest of the cluster, its peculiar requirements dictated many features of the orbital assembly and the missions.
With its four solar arrays extended like the sails of a medieval windmill, the Apollo telescope mount was the most striking feature of the orbiting Skylab. At the hub of the arrays was the canister carrying the six major instruments making up the solar observatory. (App. D tabulates information on all experiments.) Five of these measured radiation in the high-energy ultraviolet and x-ray regions of the spectrum—radiation that does not reach the earth’s surface because it is absorbed by the atmosphere. The sixth photographed the sun’s corona, a tenuous body of gas whose faint light is blotted out for observers on earth by the brilliant light of the solar disk, scattered by the atmosphere.*
From a study of the wavelength and intensity of x-rays and ultraviolet radiation, scientists could deduce the composition, density, and temperature of the region under study. Photographs of the corona would provide information about its motion, physical state, and magnetic environment and would relate changes in the corona to events at the sun’s surface. This information, scientists believed, would help them understand the processes by which energy is transferred from the sun’s interior out into space. To get that information, solar physicists needed instruments with high resolution, pointing accuracy, and stability. Such characteristics had to be designed into the telescopes and their supporting systems from the start.1
The Apollo telescope mount as depicted on a 1972 briefing chart. MSFC-72-PM 7200-115A.
Though initially conceived for use on the Advanced Orbiting Solar Observatory, the ATM instruments were general-purpose telescopes; with suitable modifications, they could be used on other missions. The major change made when the instruments were moved to the ATM was to convert them to photographic recording (all except Harvard’s ultraviolet spectrometer). Film gave better spectral and spatial resolution* than photoelectric recording, but photoelectric instruments could record a wider range of intensities and had a shorter response time. Since film had to be replenished during the mission, this decision made all the experiments except Harvard’s dependent on the astronauts, who would recover exposed film and reload the cameras during extravehicular activity. It was a bold step to take in 1966, when working outside the spacecraft was still a questionable area of manned spaceflight and when early experience in Gemini had not been encouraging.2
Changing film at the Apollo telescope mount, a briefing slide from late 1970. The viewing ports for the various instruments are on the raised center of the white circle. S-71-48024.
To assist the human operator of the solar telescopes, several accessory instruments were added to the ATM in the later stages of design. A monitor measured the total x-ray output of the sun, a useful index of overall solar activity. It was connected to an audible alarm, set to go off when a predetermined high level of x-radiation was exceeded, alerting the crew that a solar flare might be imminent and that the control panel should be manned. Another monitor displayed an image of the sun in ultraviolet radiation and similarly served as a means of locating active solar regions. In 1968 two pointing-control telescopes were added to the instrument package. Equipped with filters to pass a single wavelength, the red-orange light in the spectrum of incandescent hydrogen, these hydrogen-alpha or H-alpha† telescopes revealed much of the fine granular structure of the sun’s surface, which they displayed on a television monitor at the control panel. Both had variable focal length (zoom) lenses and cross-hairs to enable precise pointing of the other instruments, with which they were aligned. Cameras provided a permanent record of where the H-alpha telescopes were pointed when observations were taken.3
The experiments and their supporting systems were designed to be nearly independent of the carrier vehicle—until 1969, a modified lunar module, whose ascent stage provided a pressurized cabin with room for two crewmen and a control and display console for the instruments. When the lunar module was discarded in the change to the dry workshop, some changes to the ATM were required. It was moved onto a supporting structure above the multiple docking adapter, its pointing system was modified to control the entire workshop, and the control panel was moved into that now-vacant module. The instruments were scarcely affected by this change, and their development, which was well under way by the end of 1968, was hardly perturbed.
The solar telescopes were massive—the entire collection weighed over a ton and some of them were three meters long—but they were still precision optical devices, requiring elaborate support systems. Primary among the requirements was the ability to aim the instruments precisely at any desired point on the sun and hold them there in spite of disturbances to the orbital assembly. Another was strict control of temperature. To preserve the alignment of its optical elements, the Naval Research Laboratory’s telescope had to be kept within 1.5°C of the temperature at which it had been calibrated, and the temperature could not change more than 0.005°C per minute—all of this while the instrument canister was exposed to the full blast of unattenuated solar radiation. Then there was the matter of using the human operator effectively, automating as many operations as possible while still allowing him to use judgment and make on-the-spot decisions about what should be observed and which instruments used. Finally, systems such as electrical power and data management, if not as challenging as some of the others, were no less essential.4
Responsibility for these supporting systems, which with the structure that supported the canister made up the Apollo telescope mount, fell to Marshall (p. 75). Apollo Applications Program Manager Leland Belew established an ATM Project Office in July 1966, with Rein Ise as project manager. Ise, whose tenure dated back to pre-NASA days with the Army Ballistic Missile Agency, was one of several engineers who came to AAP from the defunct Saturn IB-Centaur program. Marshall’s Astrionics Laboratory would build parts of the mount, contract some of the systems out to industry, and assemble the test, prototype, and flight articles.5
Besides the test and flight hardware, engineering simulators and training mockups were required for design work and crew training. By mid-1968, Marshall had built a control and display simulator on which engineers worked out the switches, controls, and computer logic. Later, this simulator was upgraded to provide computer-generated displays simulating the observations that would be made in flight; it was then used by crews and engineers to develop and verify inflight operating procedures. Simulators for the power, attitude-control, and pointing-control systems were also being built in 1968. Training hardware included a one-g trainer, a full-scale mockup of the entire mount (except for the solar power arrays) with functional work stations, and a mechanically functional control and display console. There was also a zero-g trainer, consisting of mockups of the work stations that could be flown in a KC-135.6
Zero-g testing was critical to the ATM design. Film cameras attached to the telescopes contained all of the experimental data (except for that from the Harvard instrument), and they had to be retrieved by the crew working outside the vehicle. This requirement produced close collaboration between Houston and Huntsville; astronauts frequently conferred with engineers and tested designs of the work stations where film cameras were removed and replaced.
For this kind of design work the 20–30 seconds of zero gravity obtainable in aircraft were inadequate. The best alternative w as working under water, with the subject’s arms, legs, and body carefully weighted until they were neutrally buoyant, neither sinking nor floating. This technique had been used in preparing for Gemini, and in the early days of the Apollo Applications Program Marshall had done some neutral buoyancy design work in a water tank once used for explosive forming. In 1968 the center was putting the finishing touches on a new Neutral Buoyancy Facility expressly designed for the purpose—a tank 22.8 meters in diameter and 12 meters deep, in which full-size mock-ups of cluster components could be immersed. The new tank was built primarily as a design aid for Marshall engineers, but later in the program it also became an important crew training facility. Underwater simulation of zero g was not perfect, but astronauts found that anything they could do in the tank could generally be done in orbit. Better still, underwater simulations were conservative; they required more effort than the same task required in space and therefore did not lead to underestimating the difficulty of a task.7
Instruments with the capabilities of the ATM solar telescopes had never been flown on manned spacecraft, and their requirements placed severe demands on systems in the cluster. Pointing accuracy requirements were unprecedented; the instruments had to be pointed within 2.5 arc seconds of the desired spot and held there without drifting more than 2.5 arc seconds in 15 minutes’ time. (A quarter, viewed from a distance of a kilometer, is about 2.5 arc seconds in diameter.) Conventional thruster engines for attitude control could not be used; they were insufficiently delicate, they required too much fuel for long missions, and their exhaust gases would interfere with optical observations. From 1966 onward, the attitude control system for the solar observatory was based on control moment gyroscopes.
A control moment gyroscope (CMG) is, as the name implies, a gyroscope large enough to impart controlling moments or torques directly to a spacecraft.* Engineers often called them “momentum exchange” or “momentum storage” devices, meaning that the turning motion produced by external forces acting on the spacecraft could be transferred to the gyroscopes rather than moving the spacecraft itself. Three CMGs, each with a 53-centimeter rotor weighing 65.5 kilograms and turning at about 9000 rpm, were mounted on the ATM support structure. Any two could control the cluster; the third provided the required redundancy. Each was mounted in two gimbal rings that could rotate around two axes.8
Practicing the retrieval of film from the telescope mount in Marshall’s Neutral Buoyancy Facility, left. MSFC 027034. The film magazine is on the end of the boom; the white flotation collar near it provided neutral buoyancy. Below, the task being performed in space. 73-H-979.
Control moments were generated by exerting a force on the gimbals. Any attempt to turn the gimbal of a spinning gyroscope produces a seemingly anomalous motion: force applied to the outer gimbal results in motion of the inner one, the gyro rotor moving at right angles to the applied force and its own axis. At the same time a reactive force opposes the force applied at the outer gimbal, and since the gimbal is attached to the spacecraft framework, this reactive force turns the cluster around one of its axes.
Coupled with the CMGs was a set of sensors that determined the cluster’s attitude with respect to the sun and the horizon, as well as the cluster’s rate of rotation. A sun sensor and a star tracker determined attitude errors, while rate-sensing gyroscopes determined how fast the vehicle was turning in each of three directions. Signals from these sensors went to the ATM’s onboard digital computer, which calculated the necessary changes in attitude and sent corrective commands to the CMGs. Torque motors on the gimbals applied a precisely controllable twisting force, the gyro rotors moved to new positions, and the spacecraft remained in the desired orientation. The net effect was to transfer the rotational motion of the workshop (its angular momentum) to the gyroscopes.9
For observations with the solar telescopes it was necessary to point the instrument canister directly at the sun and hold it there as long as possible. When the spacecraft came out of the earth’s shadow, the attitude control system pointed the canister at the sun, with the vehicle’s long axis in the orbital plane, holding it there until the workshop was about to enter the dark side of its orbit again. This “solar inertial” attitude was the one in which the spacecraft would spend most of its time, and the electrical power and temperature control systems were designed on that basis.
There were several sources of unwanted motion for the orbital assembly. Crew motion within the vehicle would produce small random forces; aerodynamic drag, though small at orbital altitude, would still be appreciable. The largest perturbation, however, was produced by gravity, which acted unevenly on a large unsymmetrical structure like Skylab. While the spacecraft’s center of mass faithfully followed the prescribed orbit, the heavier end was pulled toward the earth more strongly than the lighter. This gravity-gradient torque caused the cluster to turn slowly around its center of mass. Part of this torque could be eliminated by properly positioning the spacecraft in the plane of its orbit, but that solution was limited by the necessity to point the telescopes at the sun. The residual gravity-gradient torque and aerodynamic drag produced a net rotation that the CMGs had to absorb.
The CMG system was capable only of coarse pointing—within 6 arc minutes (0.1 degree), two orders of magnitude larger than the instruments required. Mechanical constraints limited the travel of the CMG gimbals; and after a long period of absorbing unwanted torques, the CMG rotors reached a position of saturation, an alignment in which no further correction could be produced. When all of the CMGs became saturated they could no longer control the spacecraft until the rotors were returned to their original position. A way had to be provided to “desaturate” the gyroscopes during periods when the solar instruments were not in use.10
For this purpose engineers used the same force that caused saturation in the first place: gravity-gradient torque. As the orbital assembly entered the dark side of its orbit, the ATM digital computer—the most sophisticated ever put on a manned spacecraft—determined the degree of saturation and commanded a maneuver into an attitude such that the gravity-gradient torque would return the gyros to their original position in time for the next sunlit portion of the orbit. Maneuvering for this procedure (called momentum dumping) was accomplished by the thruster attitude-control system.11
For the fine-pointing control required by the telescopes, the spar on which the instruments were mounted was suspended inside gimbals. The gimbal rings could be moved two degrees up or down and left or right; they were mounted inside a roll ring to enable rotation around the long axis of the canister (the sun line). The entire fine-pointing assembly was suspended by frictionless flexible pivots capable of damping out small disturbances. Each degree of freedom was controlled by a fine sun-sensor and rate gyroscopes that normally pointed the instruments within 2.5 arc seconds of the sun’s center. On the control panel was a joystick—much like an airplane’s control stick—which activated an optical device in the fine sun-sensor, permitting accurate offset pointing of the canister to any point within 24 arc minutes of the sun’s center.*12
The wet workshop was to have depended on an auxiliary attitude-control system powered by chemical fuels for use on the first three manned missions and CMGs for the fourth, the ATM flight. For the dry workshop, engineers adopted a thruster attitude-control system powered by compressed nitrogen. It was simpler than chemically powered systems and did not contaminate the space around the solar telescopes, but it was heavier—a penalty that was accepted in view of the system’s advantages. Twenty-two spherical tanks around the S-IVB’s thrust structure fed gas to six thrusters (two in each axis) in the stage’s aft skirt. These thrusters provided the force required in docking and maneuvered the spacecraft when the CMGs could not manage the task.13
The attitude and pointing control systems were Marshall’s responsibility, but MSC astronauts would have to operate them, so early in 1967 an intercenter task team was formed to work on the crew’s interface with the solar experiments, among other problems. The Houston members were dissatisfied with Marshall’s proposals for the ATM control panel; it looked more like a system for an unmanned spacecraft than for a manned one. MSC wanted more information provided to the ATM operator and more participation by him in the control of spacecraft systems. Houston particularly objected to Marshall’s digital-address system, whereby control commands were entered into a computer by punching 5-digit numbers into a keyboard. If man was as important to the solar observatory as everybody insisted, Houston believed he should do more than relay numbers to a computer and monitor a few status indicators. In August 1967, Bob Thompson collected MSC’s comments on the control-panel design and forwarded them to Lee Belew, recommending a number of changes and spelling out MSC’s philosophy for panel design.14
Five months later, however, a preliminary requirements review showed that the two centers were still not in agreement. Though an MSC representative had been working closely with Marshall designers, the Houston review team strongly disagreed with several concepts—once more concentrating on the digital-address system.* A complete redesign of the control and display system seemed necessary, and a working group was established to resolve the differences. By mid-May, working with astronauts and the contractor, the group had a concept that reasonably satisfied everyone, and detailed design work could go on.15
The control and display panel that resulted was probably the most complicated ever put into a spacecraft. It had three times as many controls as the Apollo command module; one ATM experimenter thought it was “at least as complicated as one for a large aircraft.” Painstaking design work, however, produced a control center that was highly functional and not inordinately difficult to operate. The exact status of each instrument was displayed to the operator, along with information on the workshop’s attitude and orbital position and the condition of the ATM power system. Two television screens displayed the sun as seen through the H-alpha telescopes; another displayed the coronagraph’s field of view, and a fourth the x-ray monitor. The logical design put all instrument controls, attitude and pointing controls, and telemetry within arm’s length of the seated operator.16
With experiment development reasonably well in hand, the ATM scientists turned their attention to other matters in 1969. Operational procedures—when and how the instruments would be operated, how much observing time was allowed, how rigid the flight plan would be—were of primary concern during the next two years. No one had any experience with missions quite like Skylab. The astronomers, experienced with sounding rockets and unmanned’missions devoted to a single set of experiments, now found themselves competing with medical experiments for operational time. Flight controllers who were used to having total control over communications with the spacecraft now had to deal with experimenters who insisted on talking directly with the astronauts. Not only that, the experimenters wanted to be able to change the flight plan every day—even oftener, if the capricious sun unexpectedly spouted flares. The flight controllers’ ideal mission—never realized, to be sure—was nicely predictable and offered just enough of the unusual to challenge their ingenuity. The scientists, on the other hand, needed to be able to adjust their observing program to unpredictable events, to change the mode of operation of each instrument as conditions indicated, or to drop everything else and watch the sun for several orbits if something really interesting was happening. Moreover, the scientists never hesitated to complain to NASA’s top management when things did not go to suit them.
Astronaut Edward Gibson at the control and display console for the Apollo telescope mount, above. S-74-17306. His hand is on the joy stick that aimed the solar instruments, as indicated by cross hairs in the hydrogen alpha telescope, right. 108 KSC-73P-434. Gibson was scientist-pilot on the third crew; this solar flare was photographed by the first crew.
Operational questions were a major subject at a principal investigators’ meeting in April 1969. E. M. Reeves, representing the Harvard College Observatory, wanted to know how priorities would be assigned to the telescopes and who would resolve any conflicts that arose. Marshall’s experiments manager assured him that Martin Marietta was devising a computer program to distribute observing time equitably, and Martin would brief the astronomers. Reeves then expressed concern that MSC was not giving Skylab operations enough attention. Assured that Skylab would get higher priority once MSC had landed the first men on the moon—three months away, if all went well—Reeves then urged that planners provide direct communication between principal investigator and astronaut at least once a day. Such free-ranging conversations were not normally allowed on manned flights. When Houston’s representative invited the astronomers to visit the Mission Control Center during one of the upcoming Apollo missions, to see how manned flight operations were conducted, Reeves rejoined with an invitation to flight controllers to Cambridge, where they could learn how scientific missions were run.17
Late in 1968 Harvard proposed to change the cluster control system so that its instrument, the only one that produced data in real time, could be operated from the ground during unmanned periods. In effect, they wanted to add all the capabilities of unmanned instruments. When Marshall’s preliminary tradeoff studies showed a large cost and schedule impact, Program Director Bill Schneider demurred, but Harvard’s principal investigators persisted, seeking support from other ATM investigators. Three of the other four project scientists indicated that they, too, would like some unmanned operating time; and despite Marshall’s insistence that the proposals were not feasible, additional studies were ordered. Again Marshall showed that large cost increases and long schedule delays would result. They convinced the program manager from the Office of Space Science and Applications, but not the scientists, who were certain the studies were (perhaps intentionally) too pessimistic. In mid-December, however, the Harvard astronomers finally agreed to accept substantially less than they had originally asked for, and Schneider agreed to preserve the option of unmanned operation; he stipulated, however, that there must be no hardware changes costing more than $50 000 and no schedule delay, and that both the Space Science and Applications Steering Committee and the Manned Space Flight Experiments Board must approve any change before he would accept it.18
Already unhappy over the loss of the second ATM flight that George Mueller had promised them in 1967 (p. 90), the solar scientists were annoyed in July 1969 by the dry-workshop decision. Both Leo Goldberg and Gordon Newkirk complained to Mueller that they had been given no chance to evaluate the effect of that change on their scientific programs. Mueller tried to placate them by explaining the advantages of the dry workshop, including a higher probability of success for the ATM mission and considerably more observing time; but the failure to consult rankled, all the same.19
Another surprise was in store for the ATM scientists later in the year, when they learned that Headquarters was about to add a group of earth-sensing experiments to Skylab—another example, to astronomers, of Mueller’s tendency to make major changes without consulting those whose experiments would be affected. Not only would these new experiments compete with ATM for crew time; they would require holding the cluster in an attitude that precluded solar observations. The film and tape they used would add to the load in an already overloaded command module. This new disturbance, coupled with the fact that observing-time allotments for the dry-workshop missions were still unsettled, prompted the astronomers to request immediate attention to operational procedures.20
At a meeting in late September 1970, ATM experimenters and MSC officials discussed Skylab operations, which Houston intended to conduct in much the same way it had run its previous missions. Experimenters would specify the observations they wanted carried out and the time they wanted spent on them; the flight operations office would impose the many operational constraints; and after the usual reiterated tradeoffs, a flight plan acceptable to both the scientists and mission controllers would be laid down. During the missions, changes to this agreed plan would be passed through a long chain of command and relayed to the spacecraft by the CapCom. While this might have worked for many types of experiments, it was unsuited to studying the sun—mainly because the sun was unpredictable, but also because experimenters wanted to base later observations on the results of earlier ones. When OSSA’s representative pointed out that a few really good photographs were worth more than a lot of uninteresting ones, Houston promised to work with the astronomers to assure success on the scientists’ terms.21
While this early encounter with operations personnel was encouraging to the scientists, their first look at the computerized time allotments produced by Martin Marietta was not. Nobody was totally satisfied with the program; Richard Tousey of NRL found it unacceptable. In response to Tousey’s protests, Marshall’s experiments manager acknowledged the program’s shortcomings, but assured NRL’s principal investigator that further refinements by Martin Marietta’s experts would improve it. The principal investigators, however, decided to take matters into their own hands. Without informing NASA officials, the investigators devised a time-sharing plan that would make best use of their instruments. After listing the most important problems in solar physics, they selected those to which the ATM instruments were expected to contribute significantly. From this analysis a set of procedures was developed that would make use of every instrument during all the time allotted to solar observations. At first they called this the Program Oriented Observing Program, but when the humor in the acronym grew stale they changed it to the Joint Observing Program. In time there were 13 programs (table 1), each with a set of defined objectives, a list of the data required to satisfy the objectives, and a list of building blocks—sequences of instrument operation—that would gather the necessary data.22 (Joint Observing Program 2, Active Regions, is reprinted as app. G.)
When the scientists presented their plan at a meeting late in March 1971, it was NASA’s turn to react indignantly to an unexpected change proposal. KSC’s representative pointed out that adopting the scientists’ proposal would mean scrapping the work that Martin Marietta had already done, and perhaps even rewriting the mission-requirements document—both, apparently, unthinkable at that stage. The astronomers, believing no one could plan better use of their instruments than they themselves, stuck by their proposal. With the help of some engineers in mission planning at Houston, the scientists got their observing programs adopted as the basic mode of operation for the solar instruments.23
Table 1. Joint Observing Programs (August 1971)
1. Study of the chromospheric network and its coronal extension
2. Active regions
A. Rapidly developing active region
B. Long-term evolution of an active region
G. Structure of an active region
D. Sunspots
E. Chromospheric velocities
3. Flares
A. Flare-centered pointing
B. Non-slewing flares
C. Limb flare
4. Prominences and filaments
A. Evolution of filaments and prominences
B. Structure of a prominence
G. Structure of a filament
5. The sun’s atmosphere as derived from center to limb variation
6. Synoptic observations of the sun
7. Atmospheric extension
8. Coronal transients
9. Solar wind
10. Lunar librations
11. Instrumental calibration
12. Solar eclipse
13. Stellar observations
Work on the ATM instruments and supporting systems progressed smoothly during 1969. Critical design reviews were completed on all of the instruments, on the solar-cell wing assemblies, the ATM control computer, and the star tracker. Qualification tests on a number of experiments and support systems were completed, and by the end of the year Houston’s thermal vacuum chamber was being prepared for tests on the ATM. An assessment of the project in January 1970 showed that, except for the prototype instruments, the solar observatory was generally on schedule. The situation was helped by postponement of launch date from July to November 1972, which allowed for hardware delivery to the Cape in November 1971. Another review on 11 March 1970 showed no problems that could delay the schedule, though some subsystems required close attention to keep them on track.24
A solar wing for the telescope mount being assembled. At right, a completed wing is stored in its launch configuration. ML71-7321.
The telescope mount, constructed at Marshall Space Flight Center, undergoing thermal vacuum testing at Manned Spacecraft Center in July 1972. 72-H-1040.
By the middle of the year, however, an accumulation of niggling problems was having an effect. After an ATM review 11–12 August, KSC’s representative was pessimistic about the project, noting that the wet-to-dry change and the scientists’ predilection for tinkering with their instruments had produced “near chaos” in the configuration control system. Already, delivery of the flight unit had slipped 18 months, and the addition of the unmanned capability could be expected to delay the ATM even more.25
If the KSC official was unduly pessimistic, the reason probably reflected that center’s enormous work load in checking out the entire cluster. Furthermore, checkout problems always looked more serious from Florida than they did from Alabama. Generally, the experiments were doing well in 1970; by the end of June, four of the five instrument prototypes had been delivered and were in storage at Huntsville. Persistent problems did exist, however; S082B had trouble with its electronic assemblies and film cameras, and the zoom lenses for the H-alpha telescopes would not always focus properly. Then in July, testing of S082A revealed serious deterioration of its spectrograph grating; replacement would take time. In spite of some schedule relief at the end of August, when the target launch date (July 1972) was officially dropped, all of the ATM instruments were having problems. NRL’s two S082 instruments would be delivered late, and Marshall was having trouble finding money to complete the test program.26
Much of the pressure on the ATM was relieved in January 1971, when Headquarters postponed the last Apollo flight (Apollo 17) to ensure that it would carry an optimum load of experiments. Skylab was put off again, this time for four and a half months; when the last launch-readiness schedule was published on 13 April, the new launch date was 30 April 1973. Experiment problems were no longer a threat to the schedule, but they continued to demand attention.27
In May, after passing its acceptance review, experiment S082B showed serious deterioration in its response to short wavelength radiation. Examination of its optical components revealed that its main diffraction grating was afflicted with “purple plague,” a condition resulting from an unexpected chemical reaction between the gold coating of the grating and the aluminum coat applied over that. The grating had to be replaced, causing an eight-week delay that took all of the cushion out of the ATM delivery schedule.28
At the end of 1971 a midterm review of the entire Skylab program gave grounds for cautious optimism. The ATM posed no serious problems, but the project manager’s overall assessment was that no time remained to take care of major problems. Everything had to go right from then on. The flight unit could be delivered to Kennedy Space Center by 1 October 1972, as scheduled, but it was going to take constant hard work to make it. In that respect the ATM was in much the same shape as the rest of the cluster.29
Only one serious anomaly showed up in ATM testing, and that one had to some extent been anticipated. Thermal vacuum testing at MSC in August and September resulted in failure of one of the control moment gyros, caused by inadequate lubrication. This defect had been suspected earlier, and backup units with better lubricating systems were substituted. The ATM flight unit was flown from Houston to KSC on 22 September 1972, the same day that the orbital workshop arrived by barge from California. Final checkout and mating with the other cluster components were ready to begin.30
* The corona can be studied from the earth during total solar eclipses, or by use of coronagraphs at high altitudes, where the atmosphere is thinner.
* Resolution refers to an instrument’s ability to separate closely spaced lines in the spectrum, to separate the images of adjacent points on the sun, or to respond to two separate events closely spaced in time.
† This red-orange light (wavelength 656.3 nanometers) is the first (longest wavelength) line of the Balmer series in the hydrogen spectrum, hence the designation.
* The gyroscopes ordinarily used in guidance and navigation systems are much smaller; they function by generating electrical signals that in turn activate attitude control systems.
* The sun’s angular radius is about 18 minutes of arc, hence the instruments could be pointed at regions as far out as 1.3 solar radii without using the CMGs.
* Gilruth told von Braun that “the old test pilot types … are just more in favor of mode selector switches than the more scientifically inclined scientist-astronauts.” Von Braun’s handwritten annotation on Haeussermann’s “Notes” of 29 Jan. 1968.