CO2 less of a drag on orbiting space junk
Shrinking atmosphere Increasing levels of carbon dioxide in the upper atmosphere are allowing satellites, and space junk, to stay in orbit for longer, according to a new study.
The research, reported in the journal Nature Geoscience, helps explain why the upper atmosphere has contracted, reducing orbital decay rates.
The paper's lead author Dr John Emmert from the Naval Research Laboratory in Washington DC says levels of carbon dioxide and carbon monoxide have reached 200 parts per million in the thermosphere - 100 kilometres above the Earth's surface - a third more than predicted by atmospheric models.
Current CO2 levels are just below 400 parts per million near ground level.
"We've seen a decrease in satellite drag and the suspected primary culprit for that was CO2 increases," says Emmert.
"This is the first direct evidence we have that CO2 is definitely increasing in the thermosphere, it's pretty straight forward."
CO2's cooling effect
In the lower atmosphere, carbon dioxide acts as an insulating blanket, preventing heat from the surface escaping.
"In the upper atmosphere, CO2 radiates thermal energy back into space, causing a net loss of heat," says Emmert. "That makes CO2 the primary cooling agent of the thermosphere."
"So the upper atmosphere shrinks, which reduces the drag on spacecraft, and it takes longer for their orbits to decay."
Emmert and colleagues used data gathered by the Canadian Space Agency's Atmospheric Chemistry Experiment (ACE) satellite which measured the temperature, pressure and density of atmospheric chemicals.
They examined changes in carbon dioxide concentrations at altitudes up to 110 kilometres between 2004 and 2012. Prior to this study concentrations had only been measured to altitudes of 35 kilometres.
Percolating from below
They speculate the larger than expected upper atmospheric trend may be caused by changes in upper atmospheric circulation and mixing.
"Changes in the lower atmosphere percolate up to the higher atmosphere," says Emmert.
"We expect to see an increase in some of the dynamics there, and that could possibly explain why the CO2 might be drawn up. But this is the first time we've ever directly detected this increase."
Studies are now underway to see how these changes will affect the long-term presence of space junk.
"Atmospheric drag is the only method we have for removing orbital debris," says Emmert.