In a recent publication in Nature Astronomy (https://www.nature.com/articles/s41550-022-01718-8) Byers point out an obvious hazard that seems to be increasing in frequency: all those big rockets tend to eventually come down, somewhere, and the return is generally uncontrolled. Modest-sized bits of debris meet a fiery end, burning up in the atmosphere, but larger pieces hit the surface and the kinetic energy makes comparison of them to an oversized bullet or cannon-ball make the latter seem relatively harmless. In May, 2020, wreckage from the 18 tonne core of a Chinese Long March 5B rocket hit two villages in the Ivory Coast, damaging buildings. In July 2022, suspected wreckage from a SpaceX Crew-1 capsule landed on farmland in Australia, Another Long March 5B landed just south of the Philippines. In 1979, NASA’s Skylab fell back to Earth, scattering debris across Western Australia. So far, nobody has been injured, but it is something of a matter of luck.
According to Physics World the US has an Orbital Debris Mitigation Standard Practices stipulation that all launches should have a risk of casualty from uncontrolled re-entry of less than one in 10,000, but the USAF, and even NASA have flouted this rule on numerous occasions. Many countries may have no regulations. As far as I am aware my own country (New Zealand) has none yet New Zealand launches space vehicles. The first stage always falls back into the Pacific, which is a large expanse of water, but what happens after that is less clear.
In the past thirty years, more than 1500 vehicles have fallen out of orbit, and about three quarters of these have been uncontrolled. According to Byers, there was a 14% chance someone could have been killed.
So what can be done? The simplest is to provide each rocket with extra fuel. Each time it is time to end its orbit, the descent can be controlled to the extent it lands at the point in the Pacific that is farthest from land. So far, this has not been done because of the extra cost. A further technique would be to de-orbit rocket bodies immediately following satellite deployment. That still requires additional fuel. In principle, with proper design, the rocket bodies could be recovered and reused. Rather perversely, it appears the greatest risk is for countries in the Southern hemisphere. The safest places are those at greater inclination than the launch site.
Meanwhile, never mind the risk to those left behind; you want to go into space, right? Well, you may have heard of bone density loss. This effect has finally had numbers put on it (https://www.nature.com/articles/s41598-022-13461-1) Basically, after six months in space, the loss of bone density corresponded to 20 years of ongoing osteoporosis, particularly in load bearing (on Earth) bones, such as the tibia. Worse, these only partially recovered, even after one year on Earth, and the lasting effect was equivalent to ten years of aging. The effect, of course, is due to microgravity, which is why, in my SF novels, I have always insisted on ships either having a rotating ring to create a centrifugal “artificial gravity”. On the other hand, the effect can vary between people. Apparently the worst cases can hardly walk on return for some time, while other apparently continue on more or less as usual and ride bikes to work rather than drive cars. And as if bone loss was not bad enough, there is a further adverse possibility: accelerated neurodegenerations. (https://jamanetwork.com/journals/jamaneurology/article-abstract/2784623). By tracking the concentration of brains specific proteins before and after a space mission it was concluded that long-term spaceflight presents a slight but lasting threat to neurological health. However, this study concluded three weeks after landing, so it is unclear whether long-term repair is possible. Again, it is assumed that it is weightlessness that is responsible. On top of that, apparently there are long-lasting changes in the brain’s white matter volume and the shape of the pituitary gland. Apparently more than half of astronauts developed far-sightedness and mild headaches. Seemingly, this could be because in microgravity the blood no longer concentrated in your legs.
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