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SL-4: Third and final crew on Skylab

Anatoly Zak, Spaceflight Correspondent
Aug 14, 2016, 22:08 UTC

Sen—With two record-breaking expeditions aboard Skylab under its belt, NASA had little time before another push into the unknown. The launch of the third trio of astronauts to Skylab, designated SL-4, was scheduled just a month and a half after their predecessors had returned to Earth. All three astronauts on the SL-4 crew—Gerald Carr, Edward Gibson and William Pogue—would be making their first spaceflight.

The main goal of the third expedition was to set another flight duration record—this time 84 days, almost three months in space. The mission was also timed to enable observations of the newly discovered Comet Kohoutek, which was expected to reach its closest point to the Sun on Dec. 28, 1973, and produce an impressive celestial show as the solar radiation would evaporate its contents. The crew would have an opportunity to train the station's powerful telescopes at the comet, recording the rare event without distortions of the Earth's atmosphere. As a bonus, the third Skylab crew would also become the first mission greeting the new year in space.

The original launch date of November 9 for the SL-4 crew had to be scrapped when the launch personnel discovered cracks on the aerodynamic stabilizing fins of the Saturn-1b rocket, which had to lift the mission into orbit. They could cause the fins to break off during the ascent through the dense atmosphere, leaving the rocket unstable at the critical point of the launch. NASA decided to replace all eight fins, postponing the launch for five days. Fortunately, the laborious and dangerous repair on the launch pad was successful and the vehicle was cleared for flight.

The SL-4 mission lifted off from Pad 39B at Kennedy Space Center on Nov. 16, 1973, at 9:01 a.m. local time. Despite all the previous concerns, the Saturn-1b performed by the book, delivering the command and service module of the Apollo spacecraft into its prescribed orbit. The crew then successfully guided the vehicle to a successful rendezvous and docking at the Skylab station.

Once inside Skylab, the astronauts discovered what appeared to be three other crew members already occupying various positions inside the outpost, including its toilet. These turned out to be crude effigies fashioned by the previous expedition as a surprise for the new arrivals. Much less funny for the SL-4 crew were worsening symptoms of motion sickness. Ironically, it was Pogue, a former aerobatic pilot, who suffered the most. Still, confident that the problem would go away by itself, the crew conspired to hide the fact of Pogue's heavy vomiting. However, their plotting was overheard by mission control, resulting in a public reprimand from NASA officials.

Fortunately, the crew was back in good shape after a couple of days and on November 22, made its first spacewalk lasting six hours 33 minutes. Besides scientific photography and the routine maintenance of the telescope module, astronauts ventured to the Earth-facing side of the station in an attempt to repair a remote-sensing antenna, which had stuck in one position due to a short circuit on September 14. The improvised repair was mostly successful and the crew returned back to the orbital home for a happy Thanksgiving dinner.

However, next day, one of three gyros used to point the station in right direction in space broke down, threatening the outpost's scientific capabilities. The second such contraption also tittered on the brink of failure, giving mission controllers frequent headaches. Still, the crew continued its busy program of astronomical and remote-sensing observations, while following a strict diet on the use of propellant for the movements of the station.

On December 24, the crew celebrated Christmas greatly improved by a hint from mission control about where to find a hidden plastic tree.

On Christmas Day itself the astronauts spent seven hours outside the station, installing specialized cameras to photograph the comet. Two more spacewalks were performed during the mission: A three-hour 28-minute venture on December 29 and the final five-hour 19-minute sortie on February 3.

When all said and done, the SL-4 crew captured 75,000 astronomical images and 17,000 photos of the Earth's surface.

After a busy three months on the station, Carr, Gibson and Pogue boarded their Apollo spacecraft and undocked from Skylab on Feb. 8, 1974. The command module splashed down in the Pacific Ocean, off the coast of San Diego at 11:17 a.m. EDT. Rescue helicopters then afely delivered the crew to a nearby navy vessel.

The SL-4 mission lasted 84 days, one hour and 16 minutes. The crew logged 1,214 orbits and flew a distance of 34.5 million miles. The total time spent outside the station in four spacewalks reached 22 hours 21 minutes.

The SL-4 expedition would remain the longest U.S. spaceflight for more than two decades. The next time NASA had an opportunity to exceed this duration was during the Shuttle-Mir program in the second half of the 1990s. The USSR beat the SL-4 flight-duration record in 1978.

In the meantime, Skylab was left in orbit, which was at the time considered to be high enough to keep the station aloft until the beginning of the 1980s. By that time, NASA hoped to have the Space Shuttle ready to deliver a propulsion system to the station. It could be employed to boost Skylab into a higher orbit for future use or to dispose of it safely over the designated area of the ocean. The 90-ton behemoth was large enough to guarantee that its massive debris would reach the Earth's surface during the reentry. However at the end of the 1970s, the increased solar activity caused the Earth's atmosphere to "bulge," in turn, increasing the atmospheric drag on all low-orbiting satellites. At the same time, the Space Shuttle development fell behind schedule, making the rescue mission impossible. As a result, Skylab made an uncontrolled, but fortunately mostly harmless fall back to Earth in 1979, raining its debris over the Australian coast.