Encouraged by the accomplishments of the first two manned periods, and especially the adaptability of the second crew to long periods in the weightless environment, NASA decided to extend the third manned period to 84 days, if crew and space station conditions would permit. In addition, many tasks were added for the crew to perform during its scheduled 12 weeks in space.
Preparations for Launch
Even before the flight began, trouble arose.
Ground crews, making routine preflight inspections of the Saturn launch vehicle, discovered hairline cracks in the bases of the rocket's eight huge stabilizing fins. The cracks were small, but ground-support engineers took no chances. New fins were installed.
During another check, technicians discovered similar cracks in seven of eight support beams in the structure connecting the first and second stages of the rocket. They "beefed up" the beams with heavy aluminum plates.
Then, two of the launch vehicle fuel tanks buckled slightly while fuel was being drained. This was soon corrected by refueling the tanks under pressure, which forced the dome-shaped tops back into shape. Metallurgists inspected the tanks and ruled that they were safe for launch.
On the morning of November 16, 1973, everything was in readiness for the launch. Strapped into the astronauts' couches in the Apollo command and service module was a rookie crew, soon to set new space records. Although neither Gerald Carr, Edward Gibson, nor William Pogue had ever flown in space, each had compiled a wealth of experience in preparation for the venture.
The command module was loaded with supplies and additional equipment needed for the third crew's lengthy mission. It also carried additional items needed for reservicing or replacing equipment, replacing items with improved designs, providing for changes in planned activities, replenishing supplies because of higher-than-planned usage rates, replacing lost items, providing additional crew comfort, and providing for improvements in communications, television transmission, and photography. And additional food for the longer mission-some 160 pounds-was aboard, along with replacement film and new cameras. To make extra room, engineers had removed the vibration padding which normally was used for packing equipment and had replaced it with clothing.
The Third Manned Period Begins
Seven hours after launch, Carr and his crew spotted the Skylab.
"She looks pretty as a picture," Carr called out. Expertly, he maneuvered the command and service module to join Skylab, and a short while later, the two craft were docked.
Because the previous crew had experienced motion sickness early in their manned period, Carr  and his fellow astronauts took preventive medication. Ironically, Pogue, a former member of the Thunderbirds, the Air Force acrobatic team, became ill. The crew spent the first sleep period in the command module. Both Pogue and Carr experienced symptoms resembling motion sickness during the first 3 days in space, but this soon passed.
The crew also had problems during activation of the workshop that earlier crews had not faced. One of its first tasks was to unload and stow within the spacecraft thousands of items needed for their lengthy manned period. The schedule for the activation sequence dictated lengthy work periods with a large variety of tasks to be performed. The crew soon found themselves tired and behind schedule.
As the activation period progressed, the astronauts complained of being pushed too hard. Ground crews disagreed; they felt that the flight crew was not working long enough or hard enough.
In the last weeks preceding the third manned period, many tasks had been added without additional time for training of the crewmen to execute them. New medical tests, new scientific experiments, and extra equipment and supplies which....
 ....had to be stowed, all contributed to a feeling on the part of the crew of being overwhelmed. In addition, the crew found that many of the items in the workshop used previously were not where they had expected them to be.
Struggling to keep up with the heavy workload, the crew made errors on several experiments. The astronauts found that lack of training on some of their new medical experiments was frustrating. "We've never seen some of these before," they complained.
Faced with a problem of lagging schedules and mistakes, ground and flight crews resorted to a time-honored means of resolving the problem: they talked, openly and frankly. Work schedules were adjusted, and the crew was given more time to relax. Both crew performance and communication with Mission Control improved almost immediately. And by the end of the manned period, the third crew had completed even more work than had been planned.
Commander Carr summed up the crew's feelings in a discussion with ground-support crews: "A guy needs some quiet time to just unwind if we're going to keep him healthy and alert up here. There are two tonics for our morale-having time to look out the window and the attitude you guys take and your cheery words."
Thanksgiving in Outer Space
Thanksgiving Day, Gibson and Pogue spent 6 1/2 hours in extravehicular activity. The first part of their spacewalk was devoted to replacing film in the solar observatory. The remainder of the time was used to repair a malfunctioning antenna. Utilized to view the Earth as a part of the Earth resources experiment package, the antenna was mounted outside the Skylab on the airlock truss.
During the second manned period, antenna motion had become erratic. Ground engineers determined that the condition was most probably caused by a short in the electronic device controlling either pitch or roll. They developed a procedure to isolate the problem and correct it, and fabricated a repair kit, which included a jumper box for isolation of the short, a tool pouch with appropriate repair tools, and astronaut restraints for use during the extravehicular activity. Prior to flight, the third flight crew carefully rehearsed the repair procedure in the neutral buoyancy tank.
Using the checklist they had developed during simulated operations, Gibson and Pogue shared the work outside. Carr remained in Skylab guiding their operations, performing fault isolation, and making sure that Gibson and Pogue followed the prescribed routine.
Removing three flight cable connections and installing the jumper box, Pogue tested the circuit while Gibson held his feet and steadied him. A malfunction test showed that the trouble was a short in the pitch axis only. The procedure then called for them to remove a linchpin and install a manual gimbal lock.
To free the launch lock, they had to tap it several times. This two-man operation required alining holes in the pitch gimbal housing with a hole in the pitch gimbal shaft by rotating the antenna and locking it in that position, then putting a disabling plug on the launch lock circuit.
With their chores completed and the antenna functioning, they sat down to an ample Thanksgiving dinner. Carr selected prime rib for his main course, and Pogue chose chicken and gravy. Gibson adhered to tradition; he ate turkey and gravy.
As earlier crews had done, the third crew reported that the food was very good, but slightly bland. Although condiments usually enhanced the taste, the crews could not use them as much as they would have preferred. The amount of salt they could use, for example, was restricted for medical purposes. And the quantity and type of food consumed was rigidly controlled because of their strict diet.
The crew found that they could not go as long without eating in zero gravity as on Earth, especially during the early part of the mission. They became hungry faster and felt the adverse effects of hunger more quickly.
Generally, they found the packaging of the foods very good. But some of the packages made of material with elastic properties sometimes catapulted bits of food into the laboratory.
Problems With the Control-Moment Gyroscope
Seven days into the manned period, a serious problem developed in the control-moment gyroscope system, which threatened early termination of the mission. This system was the principal means of maneuvering and controlling the attitude of the space station. The thruster attitude control....
....system had been used often during Skylab's initial 10 days. As a result, its propellant had been depleted to the extent that it could not be depended upon to control the Skylab attitude throughout the remainder of the third manned period. Consequently, any problem in the control-moment gyroscopes was very serious.
The system depended upon three large gyroscopes, sized so that any two of them could provide sufficient torque to control and maneuver Skylab as desired. The third added to the control capability and acted as a backup in the event of failure of one of the others.
As Skylab neared contact with the Bermuda tracking station, ground observers noted a severe rise in the bearing temperature of one of the gyroscopes. At the same time, they saw a decrease in wheel speed and an increase in wheel spin motor current. As soon as they detected the irregularity, ground control shut down this gyroscope and the computer automatically switched to two-gyroscope control.
The failure of this gyroscope was attributed to insufficient lubrication. Somewhat later in the period, a second gyroscope gave similar indications, but special temperature control and load reduction procedures kept the second one operating, and no further problems occurred.
The star tracker continued to give problems as it had during the first and second manned periods. The most frequent problem experienced with it was its tracking of contaminant particles which entered its field of view. If such a particle reflected light with sufficient intensity, the particle was....
...tracked as a target star. Contaminant particles, generated by the sloughing of paint, dust, outgassing, or venting from Skylab, often distracted the star tracker.
During the second manned period, the shutter of the sensor had stuck in the open position on five occasions. This was attributed to mechanical binding of the shutter drive mechanism. On each occasion, the shutter would recover, usually within several hours. But bright light from the Earth's albedo apparently reached the photomultiplier tube while the shutter was open, degrading the tube response and lowering its sensitivity. As a result, one target star, Alpha Crux, could no longer be tracked. The astronauts selected the star Rigel Kent as an alternate and tracked it successfully.
Halfway through the third manned period, the star tracker malfunctioned once more, and far more seriously. Failure of its outer gimbal position encoder rendered the star tracker useless, and it was lost for the remainder of the mission. After this time, ground crews computed equivalent information from telemetered data. The crew then used a sextant to measure star positions.
Crew Alerted by Warning Signals
Problems which developed aboard Skylab were usually detected immediately, either by the crew or by ground-support engineers through a caution and warning system which provided visual displays and audio warnings when problems developed.
This system monitored selected functional systems characteristics, and alerted the crew to imminent hazards or out-of-limit conditions which could result in jeopardizing the crew, compromising mission objectives, or losing a critical subsystem. Characteristics monitored were categorized as either emergency, warning, or caution. When any of the characteristics reached predetermined out-of-tolerance level, appropriate visual and acoustical alarms were set off.
The emergency category included any condition which could cause crew injury or threat to life and which required immediate corrective action, including predetermined crew response.
The warning category included any condition or malfunction of a Skylab system that would adversely affect crew safety or compromise mission objectives and which required immediate action by the crew. The caution category included any out-of-limit condition or malfunction of a Skylab system that affected primary mission objectives or could result in loss of a system if not responded to in time, and which required crew action, although not immediately.
Twenty-two fire sensors were located throughout the pressurized compartments of the space station. Each consisted of an ultraviolet detector and a quick-release adapter plate for ease of replacement. There were 2 fire sensors in the docking adapter, 8 in the airlock, and 12 in the workshop. Each sensor was a self-contained unit with a 120-degree field of view. A fire signal from any of the sensors resulted in the generation of emergency alarms.
Other sensors in Skylab detected rapid losses in pressure. A pressure decrease within the space station of 0.1 pound per square inch per minute, or greater, would have immediately set off an emergency alarm.
Signals set off by these sensors were converted into audio tones-a siren for fire and a loud buzzer for rapid loss of pressure.
The caution and warning system monitored 76 system characteristics including flow rates, volt....
....ages, and coolant temperatures. Thus, a serious malfunction was detected immediately, and corrective action could be taken promptly.
Most of the alerts during the first mission resulted from recycle of a system function; troubleshooting, attempted repair, or "workarounds," by the crew or the ground; or when the crew went outside the space station, and the hatch was initially opened. These alarms were all expected.
The system operated normally throughout the Skylab mission. successfully monitoring all 76 system characteristics. Some of the 220 system alerts were false alarms, however, all of which were associated with fire sensors. These false fire alarms were attributed to high temperatures associated with the loss of the meteoroid shield prior to deployment of the thermal parasol, high radiation levels as the Skylab passed through the South Atlantic anomaly, and exposure to ultraviolet radiation which entered the vehicle as direct sunlight or as reflected light.
Repairs Reflected Ingenuity
Skylab's third crew made repairs to correct some problems that had developed earlier. Much of their maintenance activity was similar in nature to that carried out by homeowners to retain necessary appliances and household fixtures in operating  condition, but involved much more sophisticated equipment. The astronauts replaced the solar observatory television monitor in the control and display panel, repaired the laboratory tape recorders, replaced an electronic unit in the videotape recorder, and replaced a defective seal in the washcloth squeezer.
One of the repair jobs performed by the third crew had been anticipated in advance and a special kit prepared to service the leaking primary airlock coolant loop-the same problem that had been identified during the second mission.
Since these coolant loops had not been designed for onboard servicing, repair required innovation. Ground crews had to find a means of reservicing the loops, develop the necessary equipment and tools, and test them.
The solution was indicative of the ingenuity displayed consistently by ground-support engineers and flight crews. The procedure called for stripping insulation from a coolant line in the airlock, piercing the line with a saddle valve assembly which had a quick-disconnect attachment for attaching a service hose, and forcing coolant into the line from a small storage tank.
The third crew had taken the necessary parts and tools with them in the command module. The saddle valve fitted over the coolant line and provided a tight seal so that when the built-in cutter pierced the line, no coolant could escape. With the valve open, the crewman forced coolant into the line by applying pressure to a bellows in the supply tank. After sufficient fluid had been added to it, the primary loop was reactivated. It operated satisfactorily for the remainder of the mission, and no further addition of coolant was necessary.
Life in Zero-gravity Environment
Once over their initial bout with motion sickness and with their work schedule problems resolved, the crew found their environment stimulating.
"There is very definitely an adjustment period at the beginning of orbital operations," they observed. They compared it to the period of adaptation one experiences when he moves from a sea-level environment to a high-altitude environment, or vice versa, except that the physiological changes involved are accentuated when going to zero gravity. They found that there was a definite degradation in personal physical reserves or stamina while the body tries to make its adjustment.
Exercise was essential during the spaceflight, and the hour and a half exercise period each day kept the crewmen in excellent physical condition. Each crewman reported feeling better after heavy exercise.
They used a variety of exercise devices. The bicycle ergometer was exceptionally good for cardiovascular, pulmonary, and large leg-muscle conditioning. The Mark I exerciser, a floor-mounted unit with a rope and handle, was used for stressing the upper torso and arms. A treadmill device was especially beneficial for conditioning the calves and putting a heavy compression load on the total skeleton, particularly the spine, feet, ankles, and knee joints. Walking, running, and toe-rise exercises maintained the smaller muscle groups necessary for balance.
As the manned period progressed, the astronauts each grew an inch or more in height, and became slimmer, the result of stretching of the vertebrae in the absence of gravity and from the shifting of body fluids from the lower to the upper extremities. Shortly after their return to Earth, they resumed their earthly size.
The crew was in good health and high spirits as Christmas neared. On Christmas Eve, their families gathered in Mission Control, and the crew trained their television cameras on a tree they had fashioned from food containers. Christmas messages were relayed from ground to orbit by means of the teleprinter. In the messages were clues sent by their families which sent them on a search through the space station for presents from their wives.
A Long Look at the Blue Planet
Like the two crews before them, the third crew spent many hours looking at the Earth. Carr and Pogue alternately manned the controls, operating the sensing devices which measured and photographed selected features on the Earth's surface. And, when not otherwise occupied, they watched through the workshop window as the Earth rolled steadily beneath them.
In the 12 weeks they spent in orbit, the crew watched in fascination as vegetation changed color with the seasons, as busy rivers froze and lay...
....dormant in winter's icy grip, as puffy clouds floated away to reveal vast expanses of the planet they called home.
One awed astronaut could only exclaim, "Holy cow! " as he watched the lights of Acapulco, Guadalajara, and then Mexico City brilliantly greet them through the clear, cold December sky. Then, as they passed over the Texas coast, they could see clearly from Brownsville to Port Arthur, then New Orleans and, finally, the entire eastern United States, with lights aglow from the Great Lakes to the Gulf. "It's like a spider web with water droplets on it," Carr said.
The crew expressed delight in the opportunity to watch the Earth below them. Much of its appeal, they said, rested in the fact that they were permitted to exercise their judgment in selecting sites and times for observation. In contrast, they expressed dissatisfaction with not being allowed the same flexibility for observations with the Earth resources experiment. Using hand-held cameras, they photographed many features of their own choosing, gathering some 20 000 photographs and recorded data on 19 miles of magnetic tape.
Oceanographers, in close communication with the flight crews, followed their observations of the shimmering light-green Falkland Current and concluded that they were watching a vast river of plankton. The crew reported swirling pools, believed to be cold water, in the warm Gulf Stream that runs from the Caribbean along the southeast U.S. coast, and then eastward to Europe.
From their orbital vantage point, the astronauts recorded the scars left by strip mining, studied the sandy wastes of the Sahara and Gobi Deserts, and assessed potential new energy sources.
Patience a Virtue in Solar Observation
Solar observations continued, with about 75 000 new telescopic images of the Sun recorded, in virtually all its phases. Images taken in the X-ray, ultraviolet, and visible portions of the spectrum added vast new knowledge about our most important celestial body. The new data strengthened the conviction that the solar corona is more dynamic and complex than astronomers had previously believed.
Throughout all three manned periods, the crews eagerly watched for signs of a solar flare. There is as yet no way to predict when a flare will occur,  and scientists were eager to study the processes that take place as the flare is born. It is then that the transfer from magnetic energy into heat energy takes place. Unlocking the secret of this energy transfer, some scientists believe, might offer a way to obtain inexpensive energy for use on Earth.
As the end of the manned period drew near, Gibson continued his patient watch of the solar surface. Day after day, he sat before the monitors of the solar observatory console, watching and waiting. Then, on January 21, an active region on the Sun's surface gave birth to a bright spot which intensified and grew. Gibson quickly began filming the sequence, as the bright spot erupted. His...
....patience had been rewarded. He had filmed the birth of a solar flare from space, the first such recording in history.
Following the discovery of the Comet Kohoutek, scientists prepared an electronographic camera with image converter tube, which was the backup camera used on the Moon by the astronauts of Apollo 16. The third crew took it with them in the command module. This camera could be operated either from inside the workshop, through the scientific airlock with its articulated mirror, or on the outside during extravehicular activities of the crew.
On December 13, the crew sighted Kohoutek and trained the solar observatory and hand-held cameras on it. They continued to photograph it as it approached the Sun. And on December 30, as it swept out from behind the Sun, Carr and Gibson spotted it as they were performing an extravehicular activity. "It looks yellow and orange, just like a flame," Carr remarked. "Mostly yellow," said Gibson, who described it as "one of the most beautiful sights in creation I've ever seen."
Goodby to an Orbiting Home
"It's been a good home," Gibson observed, as the Apollo module carefully maneuvered away from the Skylab. "I hate to think we're the last guys to use it."
Astronaut Robert L. Crippen, in Mission Control, asked the flight crew to "say goodby for us. She's been a good bird."
In spite of difficulties which seemed at times almost insurmountable, Skylab had met or exceeded every requirement placed on it. Every phase of the work planned-solar observations, Earth studies, student experiments, materials processing, medical tests-had resulted in spectacular success. The three flight crews had spent 171 days 13 hours and 14 minutes in orbit, had made a total of 2476  revolutions of the Earth and had traveled 70 500 000 miles in space. They had spent 41 hours and 46 minutes outside their spacecraft in extravehicular activity, had recorded 182 842 observations of the Sun and had made 40 286 photographs of Earth.
James C. Fletcher, NASA's Administrator, said, "We will be living with Skylab's achievements for a long, long time." With the vast amount of data collected or transmitted to Earth by instruments aboard Skylab or through crew participation, scientists will be many years completing their studies.