23 November 2006 – Satellites in a low Earth orbit may remain aloft for longer as a result of global warming. Unfortunately, the same applies to space junk.
Researchers predicted in 1989 that rising greenhouse gas concentrations in the atmosphere would affect the climate in the upper reaches of the atmosphere. Now, Jan Laštovièka, at the Institute of Atmospheric Physics in Prague, Czech Republic, and his colleagues say that there is mounting evidence for this. Their review of recent findings appears in journal Science.
Rising levels of carbon dioxide in the lower atmosphere is causing it to warm due to the greenhouse effect. Over the past three decades, the temperature at the Earth’s surface has increased by up to 0.4 °C.
This heat is travelling upward to the upper atmosphere where it causes CO2 there to emit infrared radiation. Because the atmosphere is very thin at these altitudes, only some of the radiation is re-absorbed by other CO2 molecules. The net result is a cooling of the upper atmosphere, which the researchers say has been confirmed by several measurements over the past decade.
Cooling gases contract and the researchers say that when this happens in the upper stratosphere, (around 40 km above the Earth) it causes the higher levels of the atmosphere (around 80 km and above) to become less dense.
This, says Laštovièka, will affect the life expectancy and trajectory of satellites orbiting the Earth. On the upside, less drag on satellites will give them a longer lease of life, but on the downside, space junk will take longer to fall towards Earth and burn up.
“Perhaps fewer and smaller adjustments to the orbits of low Earth satellites, such as the International Space Station, would be needed to keep them in their assigned orbits,” says Richard Langley of the University of New Brunswick in Canada.
But while Laštovièka says that the thinning of the upper atmosphere could affect GPS satellites, Langley disagrees, pointing out that GPS satellites fly much higher. “They orbit at heights above the Earth’s surface of about 20,200 km, so their orbit changes are not measurably affected by atmospheric drag,” he says.
Another of Laštovièka’s concerns is that a thinner upper atmosphere will offer satellite solar panels less protection from high-energy particles which that could degrade the satellites’ primary power source.