Although there are many challenges to the continued well-being of our species, some of which we have created ourselves, we have a particular fascination with the idea of death from space. Knowing that a massive asteroid wiped out the dinosaurs and most of the rest of life on Earth proves that such celestial collisions can happen, and we’ve imagined what it would be like if it happened again in movies like I’m Looking for a Friend Until the End ‘ of the world and do not look up.
Fortunately, the likelihood of such an impact is low, and we have significant government programs that keep a watchful eye on the skies for potential impacts. The risk of a smaller apocalypse affecting an individual or a group of people is not that unlikely. In a new study, Michael Byers of the Department of Political Science at the University of British Columbia and colleagues calculated the risk of injury or death from re-entering rocket stages and found that it is more likely than we would like. Their findings are published in the journal Nature Astronomy.
For a long time, and even to this day, the risk of injury or death from a spacecraft part re-entry was thought to be incredibly low. Most of the world is covered in water, and even the surface of the world is largely uninhabited. In a space game of Russian roulette, it’s unlikely that any piece of space junk will hit the target, but each additional piece is another camera spin and the risk increases.
“The risk is certainly increasing over time, partly because there are still a lot of old missile bodies out there, many of which were launched during the Cold War, and partly because the frequency of launches has increased quite dramatically recently. There are counterfactors that make predictions difficult, including SpaceX’s development of first stages that can be landed and used repeatedly,” Byers told SYFY WIRE.
The problem arises largely from rocket stages that are abandoned in orbit. In simple terms, orbits are a function of velocity. Maintaining a stable orbit requires maintaining this speed. Low orbits, however, involve interaction with the atmosphere, which creates drag, slowing the rocket stages and ultimately degrading their orbits. Over a long enough period of time, they are destined to bounce back, and a certain percentage of them will pass through the atmosphere intact and reach the ground.
The risk of re-entry is not without precedent. The study notes that in 2016, the second stage of a SpaceX rocket re-entered with two fuel tanks the size of refrigerators reaching the ground. In May 2020, the main stage of a Long March 5B rocket disintegrated in the atmosphere and fragments, some of them quite large, hit two villages in Côte d’Ivoire. A year later, another stage of the same type of missile re-entered, but landed in the Indian Ocean. Most recently, an abandoned rocket stage hit the moon.
“We would like to see a controlled return regime, mandated by a multilateral treaty, that would cause all rocket stages to return to Earth and land, ideally, or crash in remote parts of the ocean.” There would have to be a transition period, of course, along the lines of the model used by the United States and then the International Maritime Organization to move from single-hull to double-hull oil tankers,” Byers said.
Generally accepted guidelines for an individual launch state that the risk of re-entry should remain below 1 in 10,000, although this requirement is often waived if it is considered difficult or expensive to do so, according to the study. While the risk that any single launch poses is statistically low, all abandoned rocket stages in orbit combined are potentially more dangerous.
Using publicly available data, the team found that over the past 30 years, more than 1,500 rocket bodies have re-entered the atmosphere, with approximately 70% of them uncontrolled. The cumulative accident risk for these rocket bodies was calculated to be 14%, significantly higher than 1 in 10,000.
The risk of injury or death was calculated by taking the orbital path, assuming an equally likely probability that the stage would re-enter at any point on that path and collide with the population data along that line. The team also gave preference to rocket bodies with a perigee of less than 600 kilometers, as they are thought to be the most likely to go out of orbit in the coming decades. Finally, they took the trend over the past 30 years mentioned above and used it to calculate a 6% to 10% risk of injury or death over the next decade. Without action, these risks are likely to increase as a result of both the growing human population and the increasing number of abandoned missile bodies in orbit. Moreover, this risk may be exacerbated if the impact occurs in densely populated areas.
“One next step in our research concerns low-probability/high-consequence events, for example, a civilian aircraft that is struck by part of a missile body that has re-entered in an uncontrolled manner.” Again, we haven’t done this research yet, so we don’t know what the risk is. But there is certainly some risk, and most of it can be avoided in the future through a controlled re-entry regime,” Byers said.
Space exploration is an important part of human endeavor, and we have, perhaps unwittingly, created a scenario for potential disaster. The best time to solve this problem was 65 years ago, the second best time is now.
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