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Materials of the Future

The advanced space ships of tomorrow will be crafted from far-out materials with extraordinary resistance to the harsh environment of space.

by Patrick L.Barry

Fast forward 50 years into the future. 

Humans, seeking signs of ancient Martian life, have flown to the Red Planet aboard extraordinary ships capable of shielding the crew from the intense radiation of interplanetary space.

The successors to the Hubble Space Telescope have been in space for decades -- longer than today's satellites can usually survive -- taking breathtaking photos of the cosmos and helping unravel the history and workings of our Universe. Dozens of Earth-observing satellites with long-lived mirrors and lenses provide scientists with daily global maps of deforestation, urban sprawl, stratospheric ozone, and other environmental factors -- bringing unprecedented scientific knowledge to the political arena. 

Huge orbiting solar collectors as big as a dozen football fields generate gigawatts of power that are beamed down to the energy-hungry civilisation below. Space-based radio antennas hundreds of metres long listen for the faintest of signals from civilisation's elsewhere in our galaxy. And spacecraft propelled by vast billowing solar sails stand poised to carry their first passengers across interstellar space.

Such marvels are probably many decades away, but scientists are preparing for them now with an experiment on board the International Space Station (ISS).

The Materials International Space Station Experiment, or MISSE for short (pronounced "Missy") consists of two suitcase-like containers, each holding hundreds of advanced materials that might someday be used to build solar sails, large antennas, super-spaceships, or thousands of other far-out devices.

This tray of material samples may look a bit like the ultimate children's water-colour set, but the contents of these trays certainly aren't child's play. Scientists and engineers will use this palette to paint the future of space utilisation.

During a spacewalk recently (August 2001), shuttle crew members attached the "suitcases" to the outside of the space station and opened them up to expose the material samples to the harsh environment of space. MISSE will be the first externally-mounted science experiment on the ISS. Once opened, the suitcases will require no attention from the ISS crew until a year later, when they will be returned to Earth, where scientists will examine the materials and see how well they fared.

Communications satellites such as this one need to be made of materials that can withstand the punishing conditions of Earth orbit for years on end. Degradation of many currently orbiting satellites has been faster than expected -- particularly the power-generating solar panels. This has led to shorter-than-expected lifetimes for these expensive satellites, motivating the aerospace industry to develop more durable materials to use in their next generation of satellites.

"Materials are really the enablers for all future technology," says Bill Kinard, senior research scientist at NASA's Langley Research Centre (LaRC). Researchers at LaRC manage the experiment, which is a co-operative venture involving NASA, the U.S. Air Force, and members of the aerospace industry.

"[With] MISSE, we're testing the candidate materials and components that people are planning to use in the future," Kinard says. "The first question is, 'Are they stable? Will they survive for a long period of time in space?'"

Many people think of space as completely empty. If that were true, choosing materials for spacecraft would be easy! But space is far from empty, and many types of materials don't survive for long in the harsh space environment.

"In space, there are high-energy protons and electrons, ultraviolet radiation, atomic oxygen, high and low temperature extremes, hard vacuum, galactic cosmic radiation, micrometeors, man-made debris and a lot of other things as well," says Sheila Thibeault, LaRC's principal investigator for MISSE.

in Low-Earth Orbit (LEO), where the ISS and most satellites are, the "public enemy number one" is atomic oxygen.

Here on Earth, the oxygen we breathe is molecular. It consists of two oxygen atoms chemically bonded together (O2). Molecular oxygen is fairly stable and safe, but in the upper atmosphere, radiation often splits molecular oxygen into atomic oxygen (O). Many of these oxygen atoms recombine to form ozone (O3). However, some of the oxygen atoms drift up to LEO, where they pose a serious threat to spacecraft.

Atomic oxygen is highly chemically reactive. When it encounters other materials, it can steal atoms of carbon, hydrogen, nitrogen, and other elements from the surface. Over time, this will erode the material away.

A dramatic example of this erosion was provided by the Long Duration Exposure Facility. LDEF was a bus-sized materials exposure experiment similar to MISSE launched in 1984. LDEF orbited for more than 5 1/2 years before it was finally recovered.

Some of the materials on MISSE may someday be used to build vast, ultra-lightweight solar sails that would use the pressure of sunlight to propel humans across interstellar space. With areas exceeding that of a small farm here on Earth, these sails must be able to withstand the inevitable impacts of micrometeors -- dust- and sand-sized grains zipping through space at high speed. MISSE will help determine if candidate materials are indeed durable enough.

LaRC invited the other NASA centres as well as the larger aerospace community to submit materials to fly on MISSE. 

"This time we're primarily using the space station to perform experiments to benefit other people in other programs, as opposed to just supporting the station," Kinard says. About 1500 total samples were approved for the mission, roughly half of which were installed recently. The rest will go up in two more suitcases scheduled to fly in about 18 months.

The samples represent a wide range of materials. New solar cell materials promise to improve the lifetime of satellites, including commercial communications satellites. Lightweight radiation-shielding films may someday help protect astronauts on their long trip to Mars. Optical materials that can better withstand atomic oxygen will improve the reliability of Earth-observing satellites. Thin, micrometeor-resistant polymers could allow the construction of huge, fold-up antennas, as well as inflatable mirrors and lenses for solar power collection. And ultra-lightweight polymers flying on MISSE could someday be used to build the solar sails that finally take humans sailing to another star.

All of this from two modest, suitcase-like containers affixed to the outside of the ISS -- and, of course, the dedicated work of thousands of researchers with a vision of what space technology 50 years from now could be.

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First Science 2014