Orbital Tracking Reveals Thinning Upper Atmosphere

By Maggie McKee The orbits of 27 free-floating objects in Earth's uppermost atmosphere have not dropped as much as expected over the last 30 years, according to a study published on Thursday.

The finding suggests that the thermosphere, the part of the atmosphere rising from 90 to 1000 kilometres above Earth, is cooling and decreasing in density by two to five per cent per decade.

The researchers attribute the change to the same greenhouse gases thought to cause global warming near the Earth's surface. If the trend continues, the thermosphere may lose half its density by 2100, prolonging the orbital lifetimes of both satellites and space junk.

Three decades of ground-based radar measurements by the US Air Force were used to track the paths of 27 satellites, expended rocket bodies, and space debris. All the orbits drifted lower, due to the friction of the atmosphere, but not as low as they would have if the atmospheric density was constant.

Carbon dioxide in the atmosphere has increased from 316 parts per million in 1959 to 371 ppm in 2001. The researchers believe this and increases in other greenhouse gases can explain the dropping density of the thermosphere.

Below the thermosphere, these gases are dense enough to trap the radiation from the Sun and that reflected from the Earth, which warms the planet. But at higher altitudes, the lower density makes the greenhouse gases efficient at siphoning heat into space.

"In the upper atmosphere, carbon dioxide acts as an air conditioner, whereas in the lower atmosphere, it acts as an insulator," says John Emmert, lead author of the study and a scientist at the Naval Research Laboratory in Washington, DC, US.

As the temperature of the thermosphere cools, it contracts to lower altitudes. This makes it a useful gauge of global climate change occurring in the lower atmosphere, says Thomas Woods, an atmospheric scientist at the University of Colorado, US. "As the Earth's atmosphere changes, the biggest changes are seen higher up in the thermosphere," Woods told New Scientist . "It's like the tail on a dog - when the dog gets excited the tail wiggles the most." Theoretical simulations and previous measurements of atmospheric temperatures and the orbits of a handful of space objects have supported the idea that the upper atmosphere is cooling.

But radar measurements of the height of electrons in the ionosphere, which lies above the thermosphere, have produced conflicting results. "It's a controversial topic right now," says Woods.

The 11-year solar cycle actually changes the thermosphere's density by a factor of five - a much larger effect than that of greenhouse-gas-induced cooling.

But engineers will have to take the phenomenon into account when calculating satellites' orbits, and space junk will take longer to burn up in the atmosphere. The cooling could also cause unforeseen changes in how the thermosphere reacts to space weather events, suggests Emmert.

Journal reference:

Journal of Geophysical Research - Space Physics (vol 109, article no. A02301)

Orbital Tracking Reveals Thinning Upper Atmosphere 1

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