John Emmert first got involved in studying orbital debris while studying climate change in the upper atmosphere. For the purposes of his research, the pieces of debris in low Earth orbit are less a problem than an opportunity. To him, they’re a collection of atmospheric probes.
Emmert, a research physicist with NRL’s Space Science Division, has been using tracking observations of changes in their orbits to infer the density of the atmosphere. He has been studying the long-term trends and the density derived from the orbital debris tracking observations in the Space Surveillance Network data since he first joined NRL 15 years ago as a National Research Council postdoctoral fellow.
What he’s found is that increasing levels of carbon dioxide are exerting a gradual but dramatic effect on the upper atmosphere. More carbon dioxide in the atmosphere is causing the atmosphere to contract, he says. That’s because, while the atmosphere nearest the Earth’s surface is warming because of climate change, the atmosphere above the troposphere is cooling.
“The day-to-day orbital changes tell you how much [atmospheric] density the orbital debris went through, and that density seems to be going down relative to what we expected it to do,” Emmert said.
According to a 2004 study conducted by Emmert and fellow NRL researchers, which corroborates the work of European and American researchers, the atmosphere about 400 kilometers above the Earth is thinning by two percent a decade. If carbon dioxide levels double as projected, the density at this altitude could decrease by as much as 50 percent by the end of the century.
Above the troposphere, carbon dioxide is a major cooling source, the main way the upper atmosphere sheds its energy from the sun, Emmert said. Acting as a cooling agent, increasing levels of carbon dioxide are tipping the balance toward colder temperatures, lowering the density of the upper layers of the atmosphere that normally produce drag on the 15,000 or so objects in low Earth orbit.
And that means that orbital debris—between 80 and 2,000 kilometers above the Earth—could stay in orbit even longer than it otherwise would, while future space operations ensure that more and more of it continues to accumulate.
“Think of this as of long-term interest to the space debris problem,” Emmert said. “Satellites that are still active can change their trajectory to reenter on their own. But once something’s dead up there, there’s nothing to cause it to deorbit except atmospheric drag. What’s going to happen to the amount of debris in the future? One possibility is that, because the drag is less, it slows the reentry process of debris, so less of it reenters the atmosphere as a result.”
During a recent project, which Emmert named Presage, he worked with Alan Segerman, head of NRL’s Mathematics and Orbit Dynamics Section, to characterize atmospheric density as a source of uncertainty in tracking objects in space. When it comes to orbit determination, knowing the uncertainty is almost as important as knowing where the object is going to be, Emmert said.
“There is always uncertainty, especially when you have so many thousands of objects,” he said. “Understanding
that uncertainty of where these things are is a big issue called ‘covariance realism’ that’s now recognized in the community—making sure we have an accurate estimation of the error, which sounds oxymoronic.”
Emmert calls atmospheric density the biggest source of error when it comes to predicting where an object in low Earth orbit will travel over the timespan of three to seven days, the window during which satellite operators generally want to know whether another object has the potential to enter the path of an operational satellite.
As part of the project, which wrapped up in September 2017, Emmert and Segerman produced a model predicting varying levels of uncertainty that takes into account not just atmospheric density but also solar activity, which varies day to day as well as according to an 11-year sunspot cycle.
During solar maximum (the period of greatest solar activity during this cycle) solar energy causes the Earth’s thermosphere to expand, which causes additional drag on debris in low-Earth orbit, deorbiting some of it. The sun is currently headed toward solar minimum, its period of least activity.
“The solar part [of the research] was a major part of it, because a lot of the uncertainty of the density is coming from our not being able to predict what the sun is going to do,” Emmert said. “We wanted to have a quick way to say, if this is your given density uncertainty, this will be what you can expect your position uncertainty for a given debris object or satellite. And we came up with a way to do that. It’s actually a very interesting mathematical problem.”
According to Emmert, neglecting to account for these factors of atmospheric density and solar activity when performing orbit determination can lead to levels of uncertainty that are unrealistically small.
“So if you think your uncertainty is smaller than it is—if you’re handling the atmospheric uncertainties in sort of an ad hoc way—you might think that something is not a threat and have a false negative of your assessment of the threat,” he said.
“Astrodynamicists who are working for NASA or DoD and just navigating their spacecraft through all this debris— they know this.”