For most of us, the Covid-19 pandemic will define the spring and summer of 2020. But 41 years ago, the impending crash of the Skylab space station defined the summer of 1979 for people across much of the southern hemisphere. The largest spacecraft ever to fall back to Earth was about to do so – but no one knew exactly when or where.
Many people were genuinely frightened; no one has ever been killed by falling space debris, but there’s a first time for everything, especially in 1979. Others made bets about when and where the falling space station would re-enter Earth’s atmosphere, and newspapers offered prizes for finding pieces of debris. Some enterprising sorts even sold Skylab-themed hard hats. It was, to say the least, a strange time; most of us in 2020 can probably relate.
On July 11, 1979, Skylab scattered debris across a sparsely populated 150 km (90 mile) wide section of Western Australia. But it was never supposed to happen that way.
When astronauts left Skylab in February 1974, they expected another crew to take over. Skylab still carried enough oxygen, water, and food to support another three-person crew for several more months, and NASA had tentative plans for a fourth mission to the United States’ first orbiting space station. As of 1974, the agency was just waiting for its new fleet of space shuttles to be ready to fly; one of the shuttles’ first missions would be to boost Skylab into a higher orbit and ferry astronauts and supplies up to do more science.
So before climbing into their Apollo command capsule to head back to Earth, the SL-4 crew packed a bag of welcome supplies for the next crew. They even left the station’s hatch unlocked on their way out. Five years later, as Skylab fell, the unopened welcome bag and unlocked hatch still waited aboard the burning station.
NASA thought it had more time.
Before undocking from Skylab, SL-4 used the thrusters on their Apollo capsule to give Skylab a little boost. With its orbit now 11km (6.8 miles) higher, NASA predicted that Skylab could keep circling the planet for another 9 years before atmospheric drag slowed it down enough to lose the battle with gravity.
Atmospheric re-entry is a bit of a misnomer, after all. The atmosphere doesn’t stop cleanly at an invisible boundary line in space; it fades away gradually, so that even in low Earth orbit, there are still a few scattered air molecules out there. It’s not enough to save you if you decide to take your helmet off mid-spacewalk, but it’s enough to gradually slow down orbiting spacecraft, so that they slip closer and closer to Earth, until eventually the downward pull of gravity overcomes the horizontal pull of speed, and an orbit becomes a fall.
A thruster burn, like the one SL-4’s capsule helpfully provided, can compensate for that drag, giving the spacecraft a little burst of speed, which helps move it into a higher orbit. But Skylab had no thrusters of its own, so the empty station had to wait for help. And with 9 years to go, NASA expected to have the space shuttle ready in plenty of time.
They didn’t exactly plan on the Sun interfering. Solar activity fluctuates over time, and in the late 1970s, our Sun radiated much more energy than NASA had predicted back in 1974. That solar energy heated the outer layers of Earth’s atmosphere, which made the air molecules a little less sparse and a little more energetic. That increased the amount of drag on Skylab.
In 1977, NORAD predicted that the station’s time would run out by mid-1979. By 1978, NASA still hoped to get a space shuttle to Skylab in time (other rescue missions were discussed and eventually rejected). But within a few months, everyone realized no one was ever going to pick up that welcome bag; it was going to burn up in the upper atmosphere with the rest of the station.
A NASA ground crew managed to re-establish contact with the computers aboard Skylab in 1978, and in mid-July 1979, they sent a last set of commands to the abandoned station. By changing the station’s orientation as it hit the atmosphere, they could change its path and aim Skylab where its debris would do the least damage. That turned out to be a patch of ocean 1,300 km (800 miles) south of Cape Town, South Africa.
Things didn’t go according to plan.
Most objects that plunge into Earth’s atmosphere get torn apart by the heat and friction of their high-speed passage through the air. But Skylab broke up just a bit slower than NASA’s models predicted; the station was mostly intact until about 16 km (10 miles) from the ground, based on the way the debris scattered.
In the end, Skylab burst apart over the Indian Ocean, heading for Western Australia. It up the sky with streaks of fire, scattered debris across fields and small towns, and it became a key part of Australia’s space history.
“In its re-entry […] the disembodied spacecraft became tangible, visible, and collectible, in the form of its widely scattered and charred remains. Anyone could own a piece of space if they wanted; the debris was both space junk and a precious artifact,” wrote space archaeologist Alice Gorman in a 2011 paper. “The social significance of Skylab came to outweigh its historic significance and it passed into popular consciousness as a rare Australian space icon.” (Gorman also recently posted an excellent Twitter thread full of Skylab information and imagery.)
The first space shuttle mission, STS-1, launched in April 1981. In an alternate universe, that might still have been in time to save Skylab.
But perhaps the most interesting thing about the Skylab story is that this sort of thing doesn’t happen more often. Space debris scattered across a landscape, even a sparsely populated one, is a rare event. Most de-orbiting spacecraft end up in a 5,000-km-wide stretch of the Southern Ocean called the Spaceship Cemetery (or, much less evocatively, the South Pacific Ocean Uninhabited Area).
That’s where Russia’s Mir space station came to rest in 2001. And as I wrote in 2015, “It’s dark here, because no sunlight penetrates water this deep. […] Temperatures hover between 2⁰ and 4⁰C. For ships used to the cold and darkness of space, it just might feel like home.”