In choosing materials to analyze, Pasqual picked six that are commonly used in satellites: white and black paint, aluminum, titanium, and Kapton and Teflon—two filmlike materials used to shield satellites.
“We thought they were very representative of what you might find in space debris,” Pasqual says.
He placed each sample in the experimental apparatus, which was motorized so measurements could be made at different angles and geometries, and measured its polarization effects. In addition to reflecting light with same polarization as the incident light, materials can also display other, stranger polarization behaviors, such as rotating the light’s oscillation slightly—a phenomenon called retardance. Pasqual identified 16 main polarization states, then took note of which efffects a given material exhibited as it reflected laser light.
“Teflon had a very unique property where when you shine laser light with a vertical oscillation, it spits back some in-between angle of light,” Pasqual says. “And some of the paints had depolarization, where the material will spit out equal combinations of vertical and horizontal states.”
Each material had a suffiiciently unique polarization signature to distinguish it from the other five samples. Pasqual believes other aerospace materials, such as various shielding films, or composite materials for antennas, solar cells, and circuit boards, may also exhibit unique polarization effects. His hope is that scientists can use laser polarimetry to establish a library of materials with unique polarization signatures. By adding certain filters to lasers on existing groundbased ladars, scientists can measure the polarization states of passing debris and match them to a library of signatures to determine the object’s composition.
“There are already a lot of facilities on the ground for tracking debris as it is,” Pasqual says. “The point of this technique is, while you’re doing that, you might as well put some filters on your detectors that detect polarization changes, and it’s those polarization features that can help you infer what the material is made of. And you can get more information, basically for free.”
Hundreds of millions of pieces of space junk orbit the Earth daily, from chips of old rocket paint, to shards of solar panels, and entire dead satellites. This cloud of high-tech detritus whirls around the planet at about 17,500 miles per hour. At these speeds, even trash as small as a pebble can torpedo a passing spacecraft.