Materials: Out of This World
Space isn't empty. Space is considered an environment — an extreme environment, filled with entities that can be harmful to spacecraft.
In space, there are several environmental threats that can harm materials used to create spacecraft. These threats include ultraviolet rays and x-rays from the sun; solar wind particle radiation; thermal cycling (hot and cold cycles); space particles (micrometeoroids and debris); and atomic oxygen.
It is essential for NASA to research and understand how materials are affected by the environmental threats that exist in space. Since 2001, NASA and its partners have operated a series of flight experiments called Materials International Space Station Experiment, or MISSE. The objective of MISSE is to test the stability and durability of materials and devices in the space environment.
Testing on the International Space Station
MISSE experiments have been flown in space on five different occasions, and will be flown once more. During each mission, either one or two Passive Experiment Containers (PECs) are flown. PECs, which are attached to the exterior of the International Space Station, are about 2-feet by 2-feet and hold a variety of materials samples and devices whose reactions in space are of interest.
"Each PEC has two trays that are hinged like a suitcase. Samples are loaded onto the top surface of the trays, and then the PEC is closed to protect the samples during launch on the space shuttle," says Kim de Groh, the principal investigator for the MISSE Science program at Glenn. "The PECs are taken up to the space station, where an astronaut does a spacewalk and installs each PEC at its designated location on the outside of the space station. The PEC is opened by the astronaut and the trays are placed back-to-back and secured, exposing the samples to the space environment."
The PECs are positioned in either a ram/wake orientation or in a zenith/nadir orientation. The ram orientation is the direction in which the space station is traveling, and the wake orientation faces the direction traveled. The zenith orientation faces away from Earth into space, while the nadir orientation faces straight down to Earth. Each orientation exposes the samples to different space environmental factors.
Once installed, the samples remain outside of the space station, exposed to the environmental threats of space for a pre-determined period of time. Some of the experiments are active, meaning they produce and record data while still in space. Others are passive, meaning their data will be measured once they return to Earth.
Once the PECs are removed from the exterior of the space station, they are returned to Earth via another space shuttle mission or on a partner agency's spacecraft. Back on Earth, the effects of the space environment on each material sample are catalogued and studied.
Each MISSE PEC includes numerous different experiments with hundreds of test samples. Each individual experiment within MISSE is coordinated by a NASA center or partner agency. NASA's Glenn Research Center in Cleveland has flown experiments on every MISSE mission since 2001.
"Glenn has been very honored to be able to fly many environmental exposure experiments with more than 600 test samples on the space station as part of the MISSE program," Kim de Groh says.
Glenn's MISSE Experiments
The MISSE PECs that have already been flown are MISSE 1 & 2, 3 & 4, 5, 6A and 6B. MISSE 7A and 7B are currently flying, and will be returned to Earth during the STS-134 shuttle mission. MISSE 8A and 8B will be delivered to the space station during STS-134, scheduled for launch in April 2011.
The MISSE 7A and 7B PECs hold fifty-two experiments which explore materials, electronics and solar cells.
"On MISSE 7, Glenn has ten individual experiments with 155 samples," Kim de Groh says.
Glenn's ten MISSE 7 experiments include: Zenith Polymers Experiment; Nadir Tensile Sample Experiment; Atomic Oxygen Fluence Monitor; LIDS Docking Seals; Polymer Experiment; Atomic Oxygen Scattering Chamber; Atomic Oxygen Pinhole Camera; Flexural Stress Effects Experiment; Thermal Control Paints Experiment; and Spacesuit Fabrics Exposure Experiment.
Each experiment investigates how materials that are critical to space exploration are impacted by the exposure to the rigors of the space environment.
Space Suiting Up
When astronauts travel outside of a space vehicle—for a space walk outside of the space station or to explore the surface of a celestial body, such as the moon—they must wear spacesuits to protect themselves. The fabric that is used to create these spacesuits must be able to withstand the harsh environment of space for long durations while protecting the astronauts who wear the suits.
The Spacesuit Fabrics Exposure Experiment is designed to identify and evaluate the effect of long-term ultraviolet (UV) radiation on spacesuit fabric. Pristine fabric, or fabric with no contaminants on it, is tested, along with dust-damaged fabrics.
The experiment includes six samples of fabrics: new, state-of-the-art orthofabrics that are pristine; new orthofabrics that are dusted in a lab on the ground with a lunar dust simulant; Apollo era fabrics that are pristine; Apollo era fabrics that are dusted with lunar dust simulant in a lab on the ground; and Apollo 12 mission fabric which is dusted with real lunar dust.
The fabric from the Apollo 12 mission is a piece of the actual spacesuit worn by Alan Bean. The fabric, taken from the knee of the suit, is coated with lunar dust that accumulated on the suit when Bean wore the suit on the moon in 1969.
Comparing the effects of space on the historical fabrics with the effects of space on the modern fabrics will lead to a greater understanding of the efficacy of the new fabrics. Investigating the differences between the dusted fabrics and the clean fabrics will increase the knowledge of how spacesuit fabrics perform in a realistic setting.
"We want to document the appearance of the samples before and after flight to understand changes that occur as a result of long term exposure to the space environment," says Don Jaworske, the program manager for the MISSE Science program at Glenn and the principal investigator for the Space Suit Fabrics Exposure Experiment.
Put a Lid On It
When a spacecraft flies with humans as passengers, the spacecraft must be tightly sealed so that the air inside of the craft is breathable. The air inside the cabin must sustain life, so tight seals that protect the air are imperative.
The Low Impact Docking System (LIDS) Seal Experiment investigates how potential seal materials perform when they are subjected to the environmental threats in space.
"We are hoping these tests of different rubbers will help us develop a seal that will work well on NASA's next crew module. The seal will be used where the crew module docks to another spacecraft, such as the International Space Station, and is designed to seal in the cabin air and keep it from leaking out," says Henry de Groh III, the principal investigator for the LIDS Seal Experiment.
Thirty-four samples are included in the experiment. These include elastomer O-ring samples; aluminum samples with a microinch surface finish; and RTV (Room-Temperature-Vulcanizing) adhesive seals.
This experiment continues work begun on earlier MISSE missions, increasing and fine-tuning the knowledge of how to create the best seals that protect humans traveling in space. Each of these samples plays an important role in creating spacecraft seals that protect life by withstanding the damage caused by exposure to space.
From the very first MISSE mission to the very last, Glenn has flown (and is scheduled to fly) samples as part of their Polymers Experiment series (flown on MISSE 2, 5, 6, 7 and 8). Currently, a variety of polymer samples are flying on MISSE 7 and additional samples will be flown on MISSE 8.
"We have a wide variety of polymers that are being flown so that we can determine each polymer's atomic oxygen erosion yield, which is how quickly the polymers erode in the space environment. Erosion yields are values that spacecraft designers need to know to in order to design durable spacecraft materials and components," says Kim de Groh, who is also the principal investigator for the Polymers Experiments.
The Polymers Experiments include numerous types of polymers that are used on spacecraft, as well as polymers that are not traditionally used on spacecraft, so the effect of space on different materials can be better understood for predictive model development. Teflon, Mylar, Kapton and Tedlar are included in the Polymers Experiments.
New polymers that have never been flown, as well as new samples of polymers that have been flown on previous missions, are included in the Polymers Experiments. Certain polymers may experience a different rate of erosion based on how long they are exposed to the space environment, making the comparative data sets from different MISSE missions insightful for long space flights.
Understanding how polymers respond to the harsh environment of space is critical to creating durable, safe spacecraft.
"If a spacecraft is being built with materials that are going to be exposed to atomic oxygen and will erode away over time (like polymers), you need to know how quickly they will erode. Therefore, we are collecting erosion flight data and documenting the data for spacecraft designers," Kim de Groh says.
The data that will be generated from the MISSE 7 and MISSE 8 Polymer Experiments will contribute to the ongoing evolution of how spacecraft are designed.
"Glenn's MISSE polymer erosion data has been widely requested," Kim de Groh says. "The flight data has directly impacted spacecraft materials design."
The knowledge acquired from the previous MISSE missions, which will be increased and enhanced by the information from MISSE 7 and 8, has changed the way spacecraft are designed and maintained. From determining the best way to restore the Hubble Space Telescope to selecting materials for the next generation of landing modules, the contributions from MISSE are invaluable.
"The MISSE program has provided the space community with a unique opportunity to expose materials to the space environment for long durations," Kim de Groh says. "We are very grateful to have participated in each of the MISSE missions."
-Tori Woods, SGT Inc.
NASA's Glenn Research Center