by J. William Bell, NCSA Senior Science Writer
New York, N.Y. — Lucite is so unyielding to weather and shock that it is used in aircraft canopies. Nomex shows such heat resistance that firefighter suits are made from it. And Kevlar-the stuff of bullet-proof vests-has the tensile strength of mild steel. Not one of them, however, is impervious when used as a building material in spacecraft and satellites. Atomic oxygen constantly bombards any spacecraft in low earth orbit (LEO), and these highly reactive atoms break down the materials at the chemical level.
Chemists at the City University of New York, Hunter College, are using the Alliance’s IBM SP supercomputer at the Maui High Performance Computing Center to study the interaction between atomic oxygen and spacecraft materials. By creating very precise quantum mechanical simulations of more than 30 of the most common polymers used in spacecraft and the polymers’ interactions with atomic oxygen, they are getting an explicit understanding of how the structures are damaged. They are also getting some strong hints about how to slow that damage. Understanding these interactions and the long-term durability of these materials is crucial to any number of space missions and experiments.
“In just a few days in orbit, there’s a noticeable amount of damage due to the atomic oxygen in LEO,” says Lou Massa, the chemistry professor at Hunter College who heads the project. “It’s not as if the space shuttle is going to fall apart. The structure isn’t threatened, but certain properties of that structure are. And those properties-the surface’s absorptive and reflective properties, or its ability to radiate heat and control temperature-are very important.”
Most of us think of space as a void. If Sigourney Weaver has taught us anything, it’s that in space no one can hear you scream. But the truth-and this is truth of the scientific sort, not the horror movie alien brand-is that there is oxygen out there. “The atmosphere gets less and less dense. In LEO, it’s tenuous. But it’s not so tenuous that there aren’t still 100 million oxygen atoms per cubic centimeter,” Massa says.
The density of oxygen in LEO is, of course, worthy of mention because of the damage that it can do. But oxygen’s state at that altitude, about 180 to 650 kilometers above the earth’s surface, is even more significant. Oxygen is at its most stable and is least reactive as a molecule, its common state in the earth’s atmosphere. The two atoms are bound together and share two electrons between them. In LEO, however, oxygen molecules are split by ultraviolet light from the sun and endure as atomic oxygen.
Atomic oxygen is ravenous, having two unpaired electrons and wanting nothing more than to make itself stable again by forming chemical bonds. Thus, it is highly reactive. As hungry as it is, though, the effects of atomic oxygen on a given material depend upon the reaction’s activation energy. The activation energy is the minimum amount of energy required to place molecules or atoms in a state in which they can undergo a chemical transition. In other words, it’s the ooomph oxygen atoms must have in order to break the molecular chains that make up the spacecraft polymers. When these chains are broken, new molecules form and float off into space at the expense of the polymers.
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.”
There are alternative theories to explain the damage done to materials in LEO. For example, polyethylene damage is often chalked up to what is known as hydrogen abstraction in which hydrogen atoms are knocked from the polymer’s structure and leave broken portions of the polymer to react and wreak havoc. Using their quantum mechanical approach, however, Massa’s team has shown that chain breaking, in which oxygen atoms directly attack and break the polymer’s carbon-carbon bonds, may also be a significant cause of damage.
“We’re not saying that chain breaking by atomic oxygen is the only culprit,” says Massa. “Any number of other reactions could exist, and all of these mechanisms remain to be studied.” The team has confirmed, however, that atomic oxygen in LEO has sufficient energy to break down polyethylene polymers by way of chain breaking and that the mechanism inflicts damage more efficiently than many other chemical processes.
More importantly, the team has provided a fundamental method for approaching the overall problem of polymers’ interactions with atomic oxygen. “We’ve really suggested a new way for this thing to be looked at. Quantum mechanical research has been around for years. This problem has been around for years. But the marriage of the two is unique.” says Massa.
Banks agrees. “Massa’s team is really the only group that is giving us a structural understanding of the reactions. It’s a solid contribution that will continue to grow in significance as the volume of their work increases.”
This research is supported by NASA.