IN FEBRUARY 2017, a SpaceX Falcon 9 rocket lifted through low clouds, pushing a Dragon capsule toward orbit. Among the spare parts and food, an important piece of scientific cargo, called SAGE III, rumbled upward. Once installed on the International Space Station, SAGE would peer back and measure ozone molecules and aerosols in Earth’s atmosphere. Its older siblings (SAGEs I and II) had revealed both the growth of the gaping ozone hole and, after humans decided to stop spraying Freon everywhere, its subsequent recovery.
This third kid, then, had a lot to live up to. Like its environmentally conscious predecessors, SAGE III is super sensitive. Because it needs unpolluted conditions to operate optimally, it includes contamination sensors that keep an eye on whether and how its environment might be messing up its measurements. Those sensors soon came in handy: When the next three Dragons docked at the Space Station, over the following months, SAGE experienced unexplained spikes in contamination. Something on these Dragons was outgassing—releasing molecules beyond the expected, and perhaps the acceptable, levels. And those molecules were sticking to SAGE.
Outgassing, in earthly terms, is what makes a new car smell like a new car. “There are volatile chemicals in those new materials that migrate through the material to the surface,” says Alan Tribble, author of Fundamentals of Contamination Control. You’re smelling escaped seat ingredients, in other words.
Outgassing also builds up as a greasy film on the inside of your new car’s windows—or the outside of your space station. This grime is mostly a problem for instruments that measure light, but it can also reduce solar panels’ efficiency and can make surfaces hotter than they’re supposed to be. To avoid all that, engineers build Space Station additions and satellites in clean rooms, use only prequalified materials, bake out contaminants before launch, and set strict limits on how much proverbial new-car smell a craft can release. “It’s an intense process and considered extremely critical,” says Meg Abraham of the Aerospace Corporation, which consults on a number of space projects. “Everyone thinks about this.”
NASA has dealt with such dirt for decades—but the dirt has so far largely come from the agency’s own creations. Dragon, though, is different. It belongs to SpaceX. Today, the company plans to launch another Dragon capsule full of cargo, and perhaps outgassing contaminants, to the Space Station. This launch will be the company’s sixteenth commercial resupply mission.
As NASA offloads operations to private companies like SpaceX, Orbital ATK (now part of Northrop Grumman), and Boeing, it’s having to react when their children misbehave. And that’s a big deal: New ISS instruments tell us about how our planet and the universe around it work, at prices in the tens of millions, with years-long development timelines. With those stakes, the last thing scientists, engineers, and citizens want is for a private company’s capsule to muck up measurements.
These crystals are not of the healing variety (spoiler alert: none are) but are, instead, “thermoelectric quartz crystal microbalances.” Each of SAGE’s eight such sensors has two twin crystals, and working together they make two “contamination monitoring packages.” These crystals oscillate at a certain frequency, which matches up with their mass. If a crystal gains mass—like, say, if a spacecraft sends new-car-smell its way—its frequency changes.
When the twin crystals start out on their missions, they’re precisely the same. But on the Space Station, one of the two is exposed to the outside environment, while the other stays sheltered. It’s kind of like twin studies in psychology. By measuring the difference in the twins’ frequencies, scientists can determine how much contamination was deposited onto the exposed one, although it doesn’t reveal the layers’ nature.
After the eleventh Dragon arrived, one contamination-monitoring package’s frequency steadily shifted, according to a presentation posted on September 1 to NASA’s Technical Reports Server, a database of documents created or funded by the agency. The data was noteworthy because NASA sets contamination limits for sensitive surfaces, which include some of the more delicate parts of the ISS. The crystals in this case were serving as canaries, warning of potential harm to those exquisite instruments.