Mirror, Mirror Out in Space, How To Dust Off Your Face?
One of the challenges with space telescopes and other satellite instruments is keeping their mirrors, lenses, and other surfaces free of dust and other contaminants. A Goddard researcher now has developed a way to dust these devices while the instruments are in space.
While working in his lab, Principal Investigator Fred Minetto found he could create an electric field by combining an electron gun with an item called a “fractal Penning discharge pin.” The result was a proof of concept that could be used to clean space mirrors. Ultimately, he’s hoping to create a device — which he calls a fractal wand — that could pass over a mirror or other sensor and simply wave off the dust.
Principal Investigator Fred Minetto holds the fractal Penning discharge pins he fabricated for his space-based dust buster.
“It’s really quite dramatic,” Minetto said, referring to his laboratory tests. “Our combination instrument actually blew the contamination off, in a vacuum.”
Funded by Goddard’s Internal Research and Development (IRAD) program and the NASA Office of the Chief Technologist’s Center Innovation Fund, Minetto’s work builds on the lessons NASA scientists learned when they cleaned the sensors on the Solar Terrestrial Relations Observatory (STEREO) spacecraft simply by flipping STEREO to directly face the sun. The solar wind took care of the cleaning, blowing dust and other contaminants off the spacecraft’s sensors.
This method, of course, requires the instrument to stare directly into the sun, but it did give Minetto
Putting Dust in the Cross Hairs
Solar wind is actually a stream of charged particles, mostly electrons and protons, which the sun ejects from its upper atmosphere. Minetto recreated this wind by shooting dusty laboratory samples with an electron gun, a device that produces a precise beam of electrons similar to a solar wind.
Minetto combined the gun with a Penning discharge pin, developed by American scientist Hans Georg, who won the 1989 Nobel Prize in Physics for this work. The device is a small, straight metal pin that stores charged particles by creating both a homogeneous static magnetic field and a spatially inhomogeneous static electric field. Minetto’s hybrid instrument worked well enough for him to file for and receive a patent.
“But I could only ever get movement of the dust when both the pin and the electron gun were, at most, about half-an-inch away from the contaminated surface.” Beyond that distance, “I was stumped,” Minetto said. For years he mulled over different ways of increasing the system’s strength, and then it came to him in a dream.
“It wasn’t even a mathematical thing, per se; it was visual. I saw an outline of the golden mean that I remembered from elementary-school math class,” Minetto said. The golden mean, also called the golden ratio, is when the ratio of the sum of two quantities to the larger quantity is equal to the ratio of the larger to the smaller. It’s a concept used in both math and art. In math, the golden mean has the unique property of saying ‘a plus b’ is the same to ‘a’ as ‘a is to b.’ The value of the golden ratio, expressed mathematically, is one plus the square root of five, all divided by two. The resulting number is 1.6180339887 — and on into infinity. When this is expressed in geometry, the result is an ever-curling spiral that looks like a snail’s shell.
“It’s interesting because it’s not exactly what you’d think of as a mean, or an average,” Minetto said, “and I just kept seeing it duplicated again and again and again.”
Making the Call
When he woke up, Minetto looked into the math of the golden mean and found that the self-repeating pattern that he dreamt of was the same pattern used in mathematical sets known as fractals. “I dug a little more and found that this was used in antennas for our cell phones.”
Cell phone antennas use fractal antennas to create a very strong signal in an exceptionally small space. These antennas look like a series of metal diamond shapes arranged first, point-to-point, and then into a second diamond shape. These shapes are arranged again into another diamond shape, over and over again.
This geometry results in the amplification of a signal. As the electricity flows from each metal diamond to another, it increases in strength. Because the pattern repeats over and over again, the signal keeps getting stronger and stronger. Minetto used IRAD funding to create hundreds of these copper metal crosses, all based on this initial fractal shape. Due to the machining process, Minetto’s fractal Penning pins have a more rounded appearance. “They look like little papal crosses to me,” he said.
When he combined these fractal Penning pins with the electron gun, the results were startling.
“The electron gun floods the area with negatively charged electrons. They attach themselves to the contamination, the dust, and that builds up a negative charge,” Minetto said. “Then, when the fractal Penning pin comes into play, it creates this force field that blows it all off, and at the end, you’ve got a clean mirror.”
Overall, it only took three fractal iterations to improve the strength of the electric field more than 400 percent, and, as a bonus, that meant less power was necessary to operate the electron gun. That’s a big plus when it comes to adding this technology to a satellite because power supplies on satellites are very limited.
Minetto is now perfecting this process in a small glove-box chamber in his laboratory, with an eye toward finding a way to combine the electron gun and the fractal Penning pin into the same device — his so-called fractal wand. “I’m still not sure how I’m going to combine the two,” he said. “I probably need to sleep on it.”
The Office of the Chief Technologist is involved in a variety of projects, missions, and technologies.