On April 2, for the fifth time in less than three years, the International Space Station fired its engines to dodge a piece of orbital debris that appeared on a collision path. Other spacecraft also regularly scoot out of the way of rocket and satellite debris. Such evasive action will be needed increasingly frequently, a new study finds.
Friction between the atmosphere and materials passing through it, known as drag force, offers the only natural means for culling detritus left in orbit by space launches. But the thermosphere — a large region of the upper atmosphere — is cooling. A resulting drop in its density is also cutting its drag force, thereby increasing the lifetime of orbiting trash (including pieces in that heavily populated band at 800 to 1,000 kilometers).
Space agencies around the world have been discussing a need to actively remove aerospace debris. One reason: The number of pieces has been steadily rising, driven in part by collisions between orbiting pieces of trash or trash and spacecraft. Among the biggest debris multipliers: a spectacular 2009 crash between the dead Russian Kosmos 2251 spacecraft and the U.S. Iridium-33 telecommunications satellite.
Two years ago, aerospace engineer Hugh Lewis of the University of Southampton, England, and his colleagues calculated that within a few decades, space agencies would have to begin culling perhaps five major pieces of debris annually to slow this collision-enhanced growth in the number of orbiting trash particles. But in a paper in the Journal of Geophysical Research, posted online Aug. 10, the Southampton team now doubles that number, pointing out that the thermosphere’s falling density renders the old trash-pickup requirements obsolete.
The thermosphere does not behave as a gas, explains Lewis. Molecules originating on or near Earth’s surface are propelled upward based on their energy, he observes. With cooling, fewer of them reach satellite (and associated debris) heights.
Growing emissions of carbon dioxide, a greenhouse gas, contribute to the thermosphere’s cooling, the Southampton team points out. The mechanism, Lewis says, appears to be collisions between CO2 and atomic oxygen at high altitudes. Those collisions release heat in the form of infrared energy, which radiates out into space — removing warmth from Earth’s atmosphere.
A drop in the sun’s activity will also cool the thermosphere. Although the new JGR analysis assumed that solar cycles during the next 70 years would roughly match those seen over the past 30, this may prove an overly conservative assumption, Lewis acknowledges. This spring and summer, scientists have been reporting that the current solar cycle is particularly anemic. And solar activity might remain lackluster for the indefinite future.
Upper atmospheric increases in carbon dioxide “is the primary cooling agent of the thermosphere,” observes thermosphere climate scientist John Emmert of the Naval Research Laboratory in Washington, D.C. The Southampton team’s new analyses, he says, “demonstrate for the first time that space climate change has significant consequences for orbital debris proliferation and for debris mitigation strategies.”
Trash collection realities
Although actively removing space trash from orbit “is absolutely desirable,” focusing on how many pieces to remove annually “is sort of a moot point, since we don’t know how to clean up even one,” says Nicholas Johnson, chief scientist of NASA’s Orbital Debris Program Office, at the Johnson Space Center in Houston.
There’s also the issue of relative risk, he says. “Although there is a sort of sandblasting going on in space all of the time, both from man-made and natural debris, we’ve only had two operational spacecraft ever hit by man-made debris (that we know of) that sustained any major damage.” One was the Iridium-33 catastrophe, the other a French satellite hit in 1996 which was temporarily disabled. While not wishing to dismiss the risk of a possible catastrophic impact, Johnson notes that the risk of a spacecraft-killing collision remains rare — and that “even two times a small number is still a small number.”
But even if space engineers were given the go ahead to develop a waste-collection service for space, succeeding would likely take a very long time. “There is nothing on the horizon that either DOD or NASA believes can do the job [space-trash removal] from either a technical standpoint or from a financial one,” Johnson notes. Still, that won’t stop U.S. researchers from formally brainstorming solutions — and on Uncle Sam’s dime.
Johnson notes that the President’s new national space policy, announced last year, for the first time directs NASA and the Defense Department to develop technologies for removing threatening debris. Their challenge is complicated by the fact that no one has decided which trash to target first. And the issue isn’t as simple as it might at first seem.
There is debris in low Earth orbit — between 400 km and perhaps 1,000 km — where the Hubble Space Telescope, International Space Station and some other satellites reside. Then there’s the geosynchronous Earth orbit regime at altitudes of perhaps 36,000 km. Protecting craft orbiting at such vastly different altitudes will require different strategies.
Engineers also will have to decide whether to focus on protecting today’s operational spacecraft over the next decade or two or protecting craft that may orbit a century from now.
If the focus is going to be on protecting future generations, Johnson says, then the priority should be ridding the skies of big pieces of trash — perhaps the car-size multi-ton behemoths that can break up into hundreds (if not thousands) of shards. Shifting the emphasis to current-generation spacecraft, he says, will argue for getting rid of small debris. “If we’re going to lose spacecraft in the next two decades,” he explains, “statistically, we’re going to lose them to small things we can’t track.”
Government agencies are already tracking thousands of large debris particles in low Earth orbit. Another half million smaller ones, between 1 and 10 centimeters, also pose threats. Uncertainties in their paths currently prompt satellite managers to be overly conservative, maneuvering spacecraft to new paths more frequently than is truly necessary, Johnson notes. The only way to limit that, he says, is to improve the tracking of trajectories for small, but potentially spacecraft-killing debris.