The International Space Station is a notoriously noisy place. With experiments and life-sustaining equipment running 24/7, it can take some adjusting for new astronauts to adapt – and sleep – in the midst of all that ruckus. All of which makes one particular project out of NASA sound a bit non-intuitive: researchers at NASA’s Langley and Johnson centers are working on a way to rapidly detect high pitched sounds produced by a leaking hull aboard ISS in the Ultrasonic Background Noise Test (UBNT).
“If a leak does occur, it’s one of those things where you may not have a lot of time,” says UBNT principal investigator Eric Madaras. “These guys can always go sit in the Soyuz capsule and close the door and go home. They’ve got that capability. But no one wants to just abandon ship, so there’s always this desire to deal with it.”
The first sign of a leak on the station is usually a loss of pressure. But while a drop in pressure will tell the crew that there is a leak, it is not very useful in locating that leak. The next best option is to listen for the familiar hissing sound a leak produces. Such a situation actually occurred in January 2004, when a 0.04 psi leak – about 0.25% – was recorded during Expedition 8. With just two crew members on board – NASA’s Michael Foale and Roscosmos’ Alexander Kaleri – it took three weeks to locate the leak using a handheld ultrasonic probe. The two experienced great difficulty differentiating between potential leaks and ambient noises. In fact, they repeatedly checked the culprit, a Destiny window vacuum jumper, days earlier, but were convinced the sound they heard was from an adjacent science instrument.
Luckily, that leak was quite small and with compensatory measures the crew was able to take the time they needed. Of course, for Foale, who had a front row view to the Spektr module’s rapid depressurization on Mir, the incident probably wasn’t severe enough to get the adrenaline pumping. Even so, the expedition came quite close to having to seal off modules to isolate the undiscovered leak.
ISS is not completely air-tight. Since it was assembled in space, the station did not have access to Earth-based manufacturing and testing techniques applied to most spacecraft. As a result, a leak rate of about 0.05% is to be expected and is factored into available stockpiles of oxygen. As a side effect, this low, steady leak rate can make it more difficult to identify and locate small off-nominal leaks. This is exactly the situation UBNT could help address.
UBNT, which kicked off at the end of 2012, involves installation of 13 ultrasonic detectors along ISS pressure walls. The detectors monitor ambient noise during normal operations, searching for frequencies at which a leak might be audible that are not otherwise occupied by existing equipment. Ultimately, the investigators hope that the detectors can be replaced with a permanent, systemic leak detection system, so that next time a window comes ajar, the crew can detect and correct the problem within minutes instead of weeks.
“The idea of giving them more time, trying to help them out and get that part done so they can get to the leak, and now they have the tools to fix the leak,” Madaras says. “That, to me, would be a good deal.”
While ISS is undoubtedly a noisy place to live, most of that noise is quite predictable from day to day. With sensors monitoring routine frequency usage, listening for leaks may become a much more effective safety procedure.