Climate Change Increases Space Debris Longevity

3

Profile of Earth’s atmosphere (Credits: John Emmert/Naval Research Laboratory).

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:

 

3 Responses

  1. Andrew

    I’m not a science major, but I do seem to remember from physics classes that the majority of materials become less dense as they heat up (they expand when heated). I know there are some exceptions to this (such as water in a certain temperature range around freezing). This article is stating that the upper atmosphere will cool, causing the gas to be LESS dense. Also, it seems like this magazine article is making far reaching ASSUMPTIONS based on the study it is speaking about, even making assertions that are either not in the original study, or at least misinterpreting them.

    Reply
    • Merryl Azriel
      Merryl Azriel

      Thanks for your comments, Andrew. The literature on this issue is reasonably extensive and generally in agreement as to the thermospheric density issue. I would invite you to highlight any assumptions you believe to be unsubstantiated and I can point out the relevant sources.
      You ask a good question regarding the inversion of the usual relationship between temperature and density. While it’s not my area of expertise, I would make an educated guess that the density in the thermosphere is different because the density is so low to begin with – the thermospheric is really space and is all-but vacuum. So instead of the tightly packed matter we are used to on the surface and in the lower atmosphere, when the thermosphere heats up, individual molecules can take up a little bit more room, but still don’t manage to fill it all. When the thermosphere cools down, the individual molecules take up even less room and contract – still maintaining a near-vacuum environment. The density is a result of the relative change in space taken up by rapidly moving molecules and the amount of space between molecules. What this tells us is that the reduction of space between molecules outweighs the reduction in molecular movement in this case. There are also composition effects in play due to molecular stratification that also result in density changes, but that’s not strictly the phenomenon under discussion.
      I would invite any of our knowledgable readers to chime in on this question.

      Reply
      • Hugh

        My understanding is that thermal energy is radiated away so atmospheric molecules have less energy and susequently can’t reach the same altitude. Consequently, at a given altitude, there are fewer molecules and hence the density is lower.

        Reply

Leave a Reply

Your email address will not be published. Required fields are marked *