Ecology is the study of the relationship between organisms and their environment. For humans, it is a complex subject. Humans have changed Earth’s environment in ways that are well studied but often poorly understood. But it is becoming increasingly clear that humanity’s influence extends even further.
In the last 50 years or so, humans have begun to explore near-Earth space, a zone that extends about 1 million kilometers (621,000 miles) from Earth. This activity is changing the environment. Debris is piling up in this region, which humanity has polluted with radioactive elements and blasted with electromagnetic radiation. And today Elena Nikoghosyan at the Byurakan Astrophysical Observatory in Armenia outlines the factors that the emerging science of near-Earth ecology needs to get to grips with.
The pristine environment of near-Earth space is dominated by energy from the sun. Earth’s atmosphere absorbs this energy, particularly at wavelengths corresponding to the presence of molecules such as water, carbon dioxide, and oxygen. Indeed, ozone absorbs almost all the radiation with a wavelength between 200 and 320 nanometers—so-called ultraviolet-B radiation.
The atmosphere itself varies drastically in character as it extends away from Earth. About 90% of its mass sits in the zone just 12 kilometers (7.5 miles) from the surface, the troposphere. The atmospheric density is highest here too, at about 1019 particles per cubic centimeter.
This density drops dramatically at higher altitudes. In the ionosphere, which extends from 30 to 1,000 kilometers from the surface, the density drops from 1013 particle per cubic centimeter at 100 kilometers to 109 at 300 kilometers.
Even at these low densities, these particles play an important protective role. Earth is constantly showered with dust and rock, which decelerates as it enters the ionosphere and burns up.
What is clear is that near-Earth space is a vibrant, active environment that is home to a rich variety of complex processes. And humans are beginning to influence it.
The most obvious impact of humanity’s exploration of near-Earth space is the amount of debris it has left behind. In 1957, the Soviet Union launched Sputnik into a pristine environment. Today, near-Earth space contains more than 17,000 objects large enough to be tracked from the ground, and orders of magnitude more below this size.
Of this stuff, just 6% is in the form of active satellites. The rest is made up of dead satellites, expired rocket bodies, and just debris, much of it from the explosion of rocket bodies when their unused fuel ignites. There have even been deliberate explosions in space.
The largest creation of space debris occurred on January 11, 2007, when China destroyed a defunct weather satellite using a kinetic kill vehicle launched from the ground. The impact created a cloud of more than 150,000 debris particles that spread around the Earth at an altitude of 850 kilometers. Much of the stuff is still there.
The big fear is that this debris could trigger a Kessler event (so-called after the NASA scientists who first imagined it). Kessler’s idea is that one particle of debris could hit another, creating more debris that goes on to destroy other satellites in a chain reaction that could make near-Earth space completely unviable for satellites.
This kind of event was the subject of the film Gravity. But it is by no means fictional—the risk is real. Indeed, a Kessler-type event becomes increasingly likely as the density of space debris increases.
A natural flushing system keeps low-Earth orbits, below 400 kilometers, relatively clear: the higher density of particles in this zone decelerates anything in orbit, causing it to plunge to Earth. And the sun heats the atmosphere and causes it to expand in an 11-year cycle that clears higher orbits, too.
But the lower density of particles at higher altitudes means the flushing process is much slower. Indeed, the nuclear tests in the upper atmosphere in the 1960s blasted radioactive material into the ionosphere that took decades to flush away.
These regions also contain unspent rocket fuel and exhaust, although its volume is tiny compared with that of greenhouse gases emitted on Earth’s surface. It also gets flushed away relatively quickly. However, it is likely to become a significant problem on the moon, the only body to sit continuously in near-Earth space. The moon’s unique and delicate environment is easily damaged by rocket landings.
The moon has a thin atmosphere produced by the solar wind hitting and vaporizing the lunar surface. The content of this atmosphere is poorly understood, but it is thought to consist of about 100 tons of gas in total.
However, each Apollo landing injected some 20 tons of exhaust gases into the lunar atmosphere. This exhaust would not easily disperse. And the ultimate impact of the landings on the lunar atmosphere is unknown. But interest in the moon is growing, and it is easy to imagine how the pristine lunar atmosphere could be quickly replaced with a thick smog of exhaust gases.
One final aspect of near-Earth space is the electromagnetic environment. Humanity pumps electromagnetic waves into space at an unprecedented rate. And satellites send them back. But this activity fills the environment with noise that swamps the natural signals at these frequencies. So human activity makes it harder to see what’s out there.
All this suggests an interesting approach from Nikoghosyan, who says with some understatement: “The technical achievements of our civilization are accompanied by certain negative consequences.”
The first step in mitigating these consequences is understanding and cataloguing them. And for that, a new scientific discipline is desperately needed. The ecology of near-Earth space will play an important role (although it desperately needs a new name—astroecology, perhaps?).
But those who practice it must be willing to go a step further and hold humanity to account for the impact it has on this pristine environment.
Ref: arxiv.org/abs/1812.10478 : Ecology of Near-Earth Space