But it's also a poison. And if it leaks, the astronauts will need to know quickly. Ammonia becomes dangerous at a concentration of a few parts per million (ppm). Humans, though, can't sense it until it reaches about 50 ppm.

Ammonia is just one of about forty or fifty compounds necessary on the shuttle and space station, which cannot be allowed to accumulate in a closed environment.

And then there's fire. Before an electrical fire breaks out, increasing heat releases a variety of signature molecules. Humans can't sense them either until concentrations become high.

Astronauts need better noses!

That's why NASA is developing the Electronic Nose, or ENose for short. It's a device that can learn to recognize almost any compound or combination of compounds. It can even be trained to distinguish between Pepsi and Coke. Like a human nose, the ENose is amazingly versatile, yet it's much more sensitive.

"ENose can detect an electronic change of 1 part per million," says Dr. Amy Ryan who heads the project at JPL. She and her colleagues are teaching the ENose to recognize those compounds -- like ammonia -- that cannot be allowed to accumulate in a space habitat.

Here's how it works: ENose uses a collection of 16 different polymer films. These films are specially designed to conduct electricity. When a substance -- such as the stray molecules from a glass of soda -- is absorbed into these films, the films expand slightly, and that changes how much electricity they conduct.

Because each film is made of a different polymer, each one reacts to each substance, or analyte, in a slightly different way. And, while the changes in conductivity in a single polymer film wouldn't be enough to identify an analyte, the varied changes in 16 films produce a distinctive, identifiable pattern.

Electronic Noses are already being used on Earth. In the food industry, for example, they can be used to detect spoilage. There's even an Electronic Tongue, which identifies compounds in liquids. NASA's ENose needs to be able to detect lower concentrations than these devices.

Right now, Ryan is working on a stand-alone version of ENose. "Everything is in one package," she explains: polymer films, a pump to pull air (and everything in the air) through the device, computers to analyze data, the energy source. The noses could simply be posted, like smoke detectors, at various points around the habitat.

Ultimately, Ryan believes, the ENose could serve as the sensory part of an intelligent safety system. "We'd have a lot of them, connected to a central computer." Any change in the atmosphere would set off a cascade of activity.

If the signal suggests a fire, says Ryan, "then the crew would immediately be notified." But if not, then the computer would try to determine exactly what was going on. Had it detected something toxic? Had it detected something that was approaching dangerous levels? Where is it coming from?

Depending on its answers, the system could choose from a range of responses -- from notifying the crew, to turning on fans to change the direction of air flow, to turning on filters, to sealing off an area.

As a safety device, the ENose has a lot to offer here on Earth, too. With some modifications, says Ryan, an ENose could be used to check for gas buildups in offshore oil rigs. "The workers have to go down into the legs of the rigs, and they want to make sure it's not going to blow up while they're in there." Sanitation workers would benefit by knowing if any poisonous gases have collected down in the sewers. There are many other examples.

Ryan's team is working on an advanced version of the ENose that could expand its usefulness even more.

"When we first started choosing polymers for the ENose," recalls Ryan, "we used what you might call an 'Edisonian' approach." (That's a scientist's way of saying trial and error. Edison tried thousands of filaments before he perfected the first light bulb.) "We tested between eighty and one hundred polymers against each analyte." That's a lot of testing.

But, points out Ryan, Edison's approach means that you can only use the ENose to identify substances whose patterns are already known. Ryan and her team are starting to go beyond that. They're trying to develop a computer model that can predict the responses of any polymer to any analyte--"without having to test a hundred polymers," she says. This would greatly accelerate the pace of ENose design. Ryan's team has already made enough progress to select some polymers using the model.

This is exciting, she says, because a successful computer model could also be used to help ENose identify unknown compounds.

"We want to be able to look at an unknown response, and then figure out what caused it," says Ryan. Such an ENose could identify unexpected vapors on Earth or in space habitats. It could even analyze strange gases encountered on interplanetary explorations.

Picture this: An astronaut lands on an alien world. Strange landforms beckon in all directions. Where to begin? Simple. "Hey, that crater smells interesting!" Follow your ENose.

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