By Jonathan Amos
Science reporter, BBC News, San Francisco
Satellites experience drag as they skim the outer atmosphere
US scientists say increased levels of carbon dioxide (CO2) are changing the outer atmosphere, making it easier for orbiting objects to stay aloft.
Dr Stan Solomon and colleagues say the region known at the thermosphere, which extends above 85km, is slowly cooling and contracting.
Their modelling suggests emissions of CO2 will produce a 3% reduction in the density of the thermosphere by 2017.
With less drag on them, satellites and space junk will stay in orbit longer.
So, while space agencies can count on using reduced fuel loads to keep their missions going in future, the risks their spacecraft face from being hit by debris will increase.
"In both cases it makes them last longer; it's a bit of a two-edged sword," said Dr Solomon, a senior scientist at the National Center for Atmospheric Research.
The issue was discussed at this week's American Geophysical Union Fall Meeting.
It is one of the peculiarities of atmospheric chemistry that the presence of carbon dioxide at low altitudes has a warming effect, whereas at high altitudes, it has a cooling effect.
This paradox occurs because the air thins with height.
In the lowest parts of the atmosphere, CO2 traps infrared radiation very efficiently and causes the well-known "greenhouse" warming effect.
In higher parts of the atmosphere, however, infrequent collisions between gas molecules mean CO2 can more readily radiate heat out to space, thereby causing a cooling effect.
As it cools, the thermosphere settles, so that the density at a given height is also reduced.
"The densities we're talking about are [a billionth of the densities] at the surface," explained Professor Robert Dickinson, from Georgia Institute of Technology, Atlanta.
"This is a much higher vacuum than the vacuums people create in laboratories. However, there are still molecules up there and it can still be treated, on a very large scale, as an atmosphere."
The cooling and thinning, which has already been observed, is projected to continue under the new model as CO2 rises.
A reduction in density of 3% is expected towards the end of the next decade under present trends.
This is significant for many satellites that move just a few hundred kilometres above the Earth.
The International Space Station would not need boosting so often, said Dr Solomon.
Space is a hazardous environment at the best of times
"But at the same time that we are trying to keep important objects in orbit, we still have thousands of defunct objects in orbit, so called space junk," he added.
"So, as this thermospheric cooling and reduction in density occurs, this space junk will have a longer lifetime and it will become more important in the future to track it, predict it, and to hopefully prevent it."
Fifty years of space activity have left a huge debris field above our heads: spent rocket stages, lost astronaut tools, exploded fragments and the like - even flecks of paint.
There are thought to be 100,000 items the size of a centimetre or bigger swirling around the Earth at thousands of kilometres per hour.
If any one of them was to hit an operational spacecraft or astronaut, the consequences would be catastrophic.
"As a technological society, we're putting more objects up there," said Dr Kent Tobiska, of Space Environment Technologies in Pacific Palisades, California.
"For humans and the high-value assets like imaging satellites, it's a more complex, more difficult and a more dangerous area for them."
Dr Tobiska said efforts were being made at the level of the UN to try to reduce the amount of redundant material left in orbit.