The growing problem of space junk
More than 3 million years ago, members of an unknown hominin species sat on a river bank at the site of Lomekwi, in what is now Kenya, and made a set of stone tools for their daily tasks. Only a trained eye can distinguish the detritus they left behind from naturally broken rocks. In the intervening millennia, human trash has grown in complexity and quantity, introducing novel materials like plastics and metal alloys. What humans discard is fodder for archaeologists, but it’s also an environmental problem that is becoming interplanetary.
The Soviet satellite Sputnik 1, which launched on October 4, 1957, was the first human-made object in space. It kick-started the space race and inspired dreams of holidays on the moon and Martian colonies. But the satellite’s orbit decayed just three months later, and it burned up as it reentered Earth’s atmosphere. It was the first piece of space trash.
Nothing survived of the basketball-sized aluminum sphere with distinctive antennas. That’s not likely to be the case for the Long March 5B rocket, which is expected to fall back to Earth this weekend after delivering the Tianhe module of the new Chinese space station to orbit in April. It’s one of the largest uncontrolled space objects to fall out of orbit. The rocket uses cryogenic fuel, so its fuel tanks are extremely robust to contain liquid oxygen and hydrogen under high pressure. Based on my observations, fuel system components are the most common rocket element to make it back to Earth.
Most concerns about the uncontrolled reentry of the 22-ton rocket are about how much will remain intact and the potential damage it might inflict on life and property on Earth. But we shouldn’t just focus on what makes its way back to the ground.
Old satellites, rocket bodies, fragments and particles make up an estimated 9,000 tons of material circling Earth, from a few hundred kilometers to more than 35,000 kilometers in altitude. Most of it is in low-Earth orbit, and pieces of space junk can lose altitude over time and incinerate in the atmosphere. Space junk reenters the atmosphere on a daily basis, although it mostly goes unnoticed because it burns up long before it can hit the ground.
Reentry is considered the most desirable outcome as it removes the space junk from orbit where it can collide with functioning satellites, create more junk, and threaten human life when it comes to crewed spacecraft. But very little work has been done on the effects of reentry on the upper atmosphere and the incineration that happens creates alumina particles that can have an environmental impact.
Studies of fuel exhaust from rocket launches have shown that particles of soot and alumina remain trapped in the stratosphere and can deplete the ozone layer. The ozone layer protects life on Earth from the savage effects of ultraviolet radiation by absorbing it. It’s been under threat before, from chlorofluorocarbons or CFCs, which were once commonly used in aerosols and as coolants in refrigerators (the ozone is recovering after international action under the 1987 Montreal Protocols).
As the number of rocket launches increases, the more space junk there will be — and it’s unclear what the long-term effects on the atmosphere may be. The quantity of human materials in orbit is only increasing more rapidly with the launch of “megaconstellations” of communication satellites, like Starlink, SpaceX’s plan to provide low-cost satellite Internet access. While the effects may be small-scale at the moment, the number of satellites could increase from about 6,000 to 15,000 in the coming decade. This means the number of reentries will also increase.
Scientists are now looking at novel materials for spacecraft manufacture that will not create alumina particles as a by-product of combustion. Silica-based ceramics, like the tiles used for heat shields on the US space shuttles, are durable and light and already widely used in aerospace. A Japanese company has been developing satellites encased in wood with a view to reducing harmful particles. Antennas and electronics are protected from the space environment inside the shell, and no alumina particulates would be created on reentry. It sounds unusual but the creative use of new materials has great potential.
Earth’s atmosphere has become a liminal zone that marks a zombie spacecraft’s transition to true death. It’s now effectively the equivalent of landfill for the space industry. Humans have been discarding junk on Earth for millions of years and the Industrial Revolution brought on a dramatic increase of emissions into the atmosphere. Just like the international waters of the ocean, the atmosphere is considered a global commons. As space activity accelerates over the coming decade, events like the Long March 5b reentry remind us to take nothing on Earth for granted.