The ENose Sensor Unit (the darker-looking metal object), housed in its Interface Unit (white). The ruler, shown for size comparison, is 12 inches (about 30.5 cm) long.

Currently, the only early-warning system for air contaminants in the International Space Station is the astronauts' noses. As perfumers and wine connoisseurs know, the human nose can be wonderfully sensitive to the subtleties of fragrance. But it has major drawbacks as a safety device.

The human olfactory system can't detect many substances until their concentrations reach several times their hazardous level, and it's insensitive to some substances altogether. Astronauts need a better nose, and AEMC has built one.

AEMC's Electronic Nose (ENose) has a much wider dynamic range than the human nose -- from less than one part per million (ppm) to 10,000 ppm. Thus, it can detect chemicals in concentrations so small that people could not smell them, or so large that they would overwhelm human noses (or a mass spectrometer). It can detect chemicals that people find odorless. It doesn't suffer from "odor fatigue," the human tendency to grow accustomed and insensitive to smells that start small and increase gradually. And of course, it doesn't catch colds.

The ENose is scheduled to begin an experiment on the International Space Station (ISS) in the second half of 2008. It will run continuously for six months, monitoring the air for at least 10 chemicals.

This will be the ENose's second trip into space. An earlier version flew with John Glenn on the STS-95 Space Shuttle in 1998. Since that time, the ENose has increased in sensitivity and reduced in size. The current, third-generation ENose, including its Interface Unit, is about the width and length of a shoe box, and about half as high. The Interface Unit contains 2 computers -- one to manage the instrument's operation and data acquisition, and one to analyze the data -- and connects the ENose to the ISS EXPRESS Rack, through which it will transfer data to the support team on Earth.

The ENose will be able to alert the ISS astronauts to any air-contaminating spills or leaks of substances on its "watch" list. If a toxic leak does occur, forcing the astronauts to wear breathing apparatuses, the ENose will tell them when the air-filtration system has returned the air to a condition that is safe to breathe.

How it works

A person's olfactory system employs millions of receptor cells of an estimated 350 different types. Each type responds differently to the molecules in each inhaled breath. The brain compares the pattern of responses with patterns stored in its memory to determine whether an aroma represents, for example, a rose or a chocolate chip cookie.

In place of the human nose's receptor cells, the ENose uses an array of 28 films of 16 different types. The films are insulators, but are impregnated with carbon particles that enable them to conduct electricity.

Each film absorbs, to a greater or lesser degree, certain classes of chemical compounds -- alcohols or ketones, for example -- when a waft of air brings them into contact. Depending on the kind and amount of the compound it absorbs, the film swells or shrinks by a characteristic amount. Swelling drives the carbon particles apart, reducing the ability of the film to conduct electricity (or put another way, increasing its resistance). Shrinking draws the carbon particles closer together, making it easier for electric current to flow across the film (decreasing its resistance).

For an illustration of how the ENose works, click here.

A computer program reads the pattern of resistance changes across the array, compares it to the patterns stored in its memory from laboratory testing, and identifies the chemical that has been "smelled." If the chemical is in a concentration deemed to be dangerous, the ENose can sound an alarm to notify the crew or possibly, some time in the future, activate an automatic system to remove the pollutant from the air via fans and filters.

To sense mercury vapor, the latest version of ENose has 4 additional sensors that operate a little differently. Instead of causing the films to swell, mercury atoms change the material's conductivity by binding with it.

JPL's ENose team makes its sensing films with materials (mostly polymers) selected from among thousands available on the market. During the early stages of the project, the instrument's developers used the "Edisonian Method," namely trial-and-error, to find the best-suited materials. More recently, they created computer models to calculate, based on chemical and physical principles, what the response will be between an analyte (the chemical to be analyzed) and a particular polymer. They can analyze the suitability of many more sensing films much more quickly this way than by actually trying them out.

Sixteen types of film allow for more than 60-thousand possible combinations, each of which could potentially signal a different chemical. For the technology demonstration aboard the ISS, however, the current version of the ENose focuses on 10 substances, with the possible addition of formaldehyde. The chemicals and their target concentrations have been determined by the Health and Environmental Factors Office at Johnson Space Center in Houston, Texas. The ultimate goal is to detect between 20 and 30 substances, in mixtures of up to 3 at a time.

In order to develop the ability to recognize each chemical, the ENose must be trained. It is exposed repeatedly to varying concentrations of known compounds in varied order, and the resulting patterns of resistance changes across the array of films are turned into a computer algorithm.

The ENose can also be trained to recognize partial patterns caused by substances for which it has not been specifically trained -- enough to identify the chemical's functional group. Currently it can identify alcohols and aromatics, the two groups that include most of what is likely to present a problem aboard the ISS, but it could be educated enough to identify any group. Knowing to what group a contaminant belongs could be enough to enable astronauts to determine whether a breathing apparatus is necessary (they would know, for example, that aromatics tend to be more hazardous than alcohols) until the Vehicle Cabin Atmosphere Monitor identifies the contaminant. And if an automatic air-cleaning system is triggered, identifying the functional group could be enough to tell the system which filter to employ.

Since ENose's sensing array is so versatile, an ENose in space (on the Moon or en route to Mars, for example) could be upgraded to detect new substances without changing its hardware, simply by transmitting new patterns that have been calibrated on a duplicate ENose in an Earth-based laboratory. When the sensing films eventually wear out, or in cases where detecting a new substance requires a new sensing film (as was the case with mercury), the films can easily be replaced.

Innovations

Various electronic noses have been used by industry for a number of years, but the one developed for AEMC is smaller, lighter, and less power-hungry than any of its predecessors. It's also the only one reported in the scientific literature as able to determine the concentrations of the substances it smells and to identify the components of mixtures of compounds, up to three at a time. This is thanks to some very sophisticated data-analysis software developed at JPL, which can analyze the signals coming out of the polymer arrays both quantitatively and qualitatively.

As mentioned, the third-generation ENose features a new set of sensors dedicated to detecting vaporized mercury, one of the highest-priority substances on NASA's list of potential hazards. ENose can detect mercury at parts-per-billion concentrations, a feat that no other handheld device available today is able to perform.

As development progresses, the ENose will be trained to identify new substances, possibly including fire precursors and chemicals likely to be found in the Orion crew exploration vehicle that is planned to carry astronauts to the Moon. Merging the sensor with the computers and interface components into one discrete unit will further reduce mass and volume. And ultimately, a system will likely be developed consisting of several sensing units connected to a central computer to monitor the air throughout the ISS or a future spacecraft.

Links

To learn more about JPL's ENose, go to:
    http://enose.jpl.nasa.gov/
There are web sites for other electronic noses listed at:
    http://www.nose-network.org/component/option,com_weblinks/Itemid,23/