Listening for leaks aboard the International Space Station with ultrasonics
In space, no one can hear you scream, but you can hear an air leak. In old science fiction movies, air leaks on spaceships and stations reveal themselves as convenient holes to slap a patch on, but on the complex International Space Station (ISS), it isn't that simple. NASA is working on a new system for detecting the ultrasonic noise of an air leak quickly before it turns into a dangerous race against time.
In 2004, a drop in air pressure pressure was detected aboard the ISS, indicating a leak. Two astronauts took three weeks to locate it using a handheld ultrasonic probe to track down the high-frequency shriek of escaping air, which was eventually identified as a faulty cable in a porthole. If this hadn't worked, the alternative would have been to seal off the station module by module until the leak was isolated, with all the effort and loss of mission time that implies.
The crew was lucky. A larger hole would have drastically reduced the chances of finding it before the astronauts were faced with the choice of sealing off a station section or retreating to the Soyuz capsule kept aboard as a lifeboat. With this in mind, an alternative is being sought by NASA.
The problem is, the ISS is a very noisy place full of fans, pumps, running gases and liquids, humming electronics, and all the other noises associated with people living in a string of tin cans. The usual procedure is that a drop in pressure sounds an alarm and the astronauts start hunting for the cause, which may be a hull puncture or equipment failure. Even with an ultrasonic listening device, hunting down a leak is a slow process made harder by the fact that equipment or structural components may be covering the leak. On top of it all, it doesn't help that the station leaks anyway no matter how well it’s sealed.
Eric Madaras, an aerospace technologist at NASA's Langley Research Center, is the principal investigator for the Ultrasonic Background Noise Test (UBNT) and he leads a team working on a new way to pinpoint air leaks quickly.
"If a leak does occur, it's one of those things where you may not have a lot of time," Madaras says. "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 UBNT approach is not to just hunt for a leak, but to deal with all the other high-frequency noises in the ISS as part of the development of an automated leak locator system. This will involve installing 14 distributed impact detection system (DIDS) units on the inside of the pressure hulls of the Destiny and Tranquility modules on the station. Each DIDS has four pressure-sensitive transducers, which work like the pickup coils on an electric guitar. This will allow the system to listen for the leak through the metal of the hull itself, and the signals to the different units can help triangulate on the leak.
But the clever bit is that Madaras wants to identify and characterize routine background noises, so it will be possible to filter them out when a leak occurs. "One way to look at that is to think about a cocktail party," he says. "It's sometimes very hard to hear people, even near you, because of all the background noises. That's the same phenomenon that this has got to deal with. How do you get that cleaned up so that you can hear specifically the type of noise – the signal – that you're looking for?"
The purpose of the system would be to give astronauts more time to find and repair leaks before so much air is lost that the only alternatives are abandoning the station or sealing off a module. "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."