A new study published in the journal Nature Geoscience indicates that increasing levels of carbon dioxide and carbon monoxide in the thermosphere are decreasing the density of that region of the upper atmosphere. The decreased density is expected to produce less atmospheric drag on passing spacecraft, thereby increasing the amount of time defunct low Earth orbit satellites can remain in crowded orbits.
“The observed CO2 increase is expected to gradually result in a cooler, more contracted upper atmosphere and a consequent reduction in the atmospheric drag experienced by satellites,” said a statement from the Naval Research Laboratory, which took part in the study. Space expert Hugh Lewis explained to AFP the potential result of this change: “Consequently, space junk will accumulate at a faster rate and we will see more collisions between space objects as a result.” Lewis also indicated that near-miss collisions are likely to to increase.
The study results answer a conundrum faced by thermosphere researchers just two years ago, when they found that contraction rates were much greater than could be accounted for by solar activity, the traditional culprit in thermospheric variation. The researchers began wondering whether increasing levels of greenhouse gases might have somthing to do with the phenomenon. As it turns out, they were only too right.
Increasing atmospheric greenhouse gas concentrations are usually associated with warming temperatures. But that’s only on Earth’s surface. The same insulation effect that prevents infrared radiation from leaving the surface also prevents it from reaching the upper atmosphere. As a result, the upper atmosphere cools. Temperature and density are directly proportional, so decreasing the temperature of the upper atmosphere likewise decreases its density. While this phenomenon is not new, the latest research indicates that the extent of the impact is greater that expected, and changes are occurring much more quickly than predicted by existing models.
Atmospheric drag is the principle mechanism for removing space debris objects from space. Drag gradually reduces the objects’ altitudes, resulting in their eventual destructive reentry into Earth’s atmosphere. The most internationally accepted approach to controlling space debris proliferation in low Earth orbit is to require satellites to reduce their altitudes at end of operations such that, within 25 years, atmospheric drag will pull the bodies out of orbit entirely. However, if the thermosphere continues to thin, altitudes that might effectively deorbit satellites today could be entirely inadequate to that purpose in the coming years.
There is an upside to this change. Currently, low Earth orbit spacecraft fight against atmospheric drag throughout their operations, repeatedly using propulsive capabilities to reboost and avoid being pulled into lower altitudes. With decreased drag, spacecraft such as the International Space Station will require fewer reboosts and hence need less fuel. It could be possible that fuel savings would allow for larger deorbiting maneuvers at end of life, ensuring that spacecraft eventual leave room for their successors.
Of course, active debris removal could circumvent these concerns altogether, but that technology is yet in the future. For the moment, atmospheric drag is the best space debris weapon around, and quickly losing efficacy.
Below, ATV-3 reboosts the International Space Station in April 2012 to compensate for atmospheric drag: