Scientists have known for years that chemical reactions are to blame for the wear and tear that spacecraft undergo in orbit, but, according to Massa, knowledge of the reactions' elementary mechanisms has always been vague. His team's first research report on those mechanisms, which will be published in an upcoming issue of the Journal of Physical Chemistry, brings the field into sharp relief. Their simulations use the Schroedinger equation, which determines the energy of molecules and atoms as well as the transition to their optimal state, to model the position of the oxygen atoms as they seek out and break a bond within a polymer. The simulations provide a precise view of the molecules' geometry, or shape, before and after the polymers interact with the oxygen. They also reveal the activation energy for the reaction.
"This method gives us a first-principles understanding of the quantum mechanics. It shows us where the weak links are and is much more detailed than other methods," says Bruce Banks, who works with Massa's team on the project. Banks is chief of the electro-physics branch at NASA's Glenn Research Center in Cleveland.
The average simulation takes 100 to 150 CPU hours on eight processors of Maui's IBM SP supercomputer. However, the precision of a given simulation and the amount of time required vary depending upon the polymer's size. The team employs more exact methods and compares a larger number of theoretical methods when looking at smaller molecules. They then use that information to finetune and calibrate the results of the less precise techniques used on larger molecules.
The team has completed calculations on about a third of the polymers they plan to look at, and their work continues today. Already the data let researchers compare the erosion rates of materials in orbit and also identify the key degradation mechanisms present in a given reaction. In addition the results will help researchers reconcile the differences between current theoretical and experimental results, which are often contradictory and difficult to interpret due to the complicated nature of the problem.
"Initially we just wanted to understand the common materials so we could predict how they would react. We've already been very successful at that," says Banks. "Ultimately we want a shopping list of materials so we'll have a quantified erosion yield for any arbitrary polymer before we use it and can design spacecraft with that information in mind."
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