By 
“What kind of solar panels does NASA actually use?” was the question we had after watching Matt Damon haul clunky panels with tragically inefficient design around Mars in the space thriller “The Martian.”
For an answer, we turned to researchers at NASA Glenn Research Center in Cleveland. The campus houses a center for photovoltaic research used in space energy production, among a multitude of other research projects. Altogether, more than 100 buildings occupy 350 acres beyond the NASA-emblazoned hanger visible from Cleveland Hopkins International Airport. Facilities include wind tunnels, drop towers and vacuum chambers.
“Glenn is the spacecraft power lead for the agency,” said Jeremiah McNatt, an electrical engineer who works with solar cell and array technology in the Power Division at Glenn. “We’re also more of a research facility, versus Kennedy, which is a flight center. We’re not launching rockets, we’re building experiments.”
On a tour, McNatt showed me a pair of giant vacuum chambers, each big enough to fit a car, where researchers test all matter of technology destined for space. From top-secret projects to solar panels, the two-story-tall chambers simulate the empty environment of space.
In another facility, McNatt demonstrated a machine used to make solar cell semiconductor materials from a variety of chemical combinations, just by turning a few dials in a sophisticated computer program. Different chemical combinations—say gallium arsenide or indium phosphide—can react to sunlight differently, making cells more or less efficient.
McNatt uses the semiconductor cell-making machine, more formally called a metal-organic vapor phase epitaxy reactor, by placing a substrate with a given crystal structure onto a platform. That platform would turn up to 1,000 times a minute, and reactant gases would flow into the system through a series of tiny tubes. Soon, the machine would produce the active components of a solar cell.
If Glenn researchers wanted to find out how a particular cell might work in space, they could attach the cell to one of the high-altitude planes housed in Glenn’s hanger by Cleveland’s Hopkins airport, fly it high above the clouds where the atmosphere is thin, measure production and extrapolate for its effectiveness in space. Follow-up experiments would then be completed using a solar simulator. The lab’s solar simulators are used to recreate the light seen in space and consist of a dark box attached to a set of powerful light bulbs.
“Getting experiment space in orbit can be a long process,” McNatt said, so NASA often uses high-altitude planes.
So what kind of solar panels does NASA use?
Turns out, you won’t find a standard 72-cell silicon solar panel on any NASA spacecraft. The missions are too long and the environment is too harsh—alternating between extreme heat and extreme cold, flush with radioactivity—for terrestrial solar. As a result, NASA Glenn, in conjunction with the larger tech and university communities, has developed solar cells that can survive long-term use in space.
Commercial companies like SpaceX use terrestrial technology, but that’s because they only need it to work for a couple weeks, said Michael Piszczor Jr., chief of the Photovoltaic and Electrochemical Systems Branch at NASA.
Two types of solar cells are common outside our hospitable atmosphere. Silicon cells covered by thin glass to avoid degradation from radiation make up the 16 arrays flanking the International Space Station. Taken together, they are the largest representation of solar in space, occupying enough area to cover most of a football field.
As a runner up, multi-junction cells made of gallium arsenide and similar materials resist degradation better than silicon and are the most efficient cells currently made, with energy conversion efficiencies up to 34%. “Junction” refers to the number of light-absorbing layers in the cell. Common in space today are three-junction cells, but four and six are on the way, McNatt said.
Solar technology for space can be very expensive. A state-of-the-art solar cell can cost NASA hundreds of dollars, compared to a couple bucks for a terrestrial cell.
“If you look at a terrestrial market, you find technology has an impact on the cost, but, from my limited experience, the biggest driver of reduced cost is volume,” said Piszczor. “NASA uses low volumes of high-efficiency cells, very periodically and very specifically, so costs are higher.”
NASA has taken an interest in solar for a long time. While the very first satellites were battery powered, solar arrays became common in orbit by the ’60s. Regular silicon cells were used first, until gallium arsenide made it out of R&D in the ’90s. Now, almost everything arriving in the ionosphere is multi-junction.
Why does NASA use solar technology in space?
While solar technology can be a political football on the ground—tossed around and tackled often—in space, it encounters little opposition. For starters, power alternatives in space include batteries and politically difficult radioisotope power systems, or RPS. RPS converts heat generated by the natural decay of the radioactive isotope plutonium-238 into electricity.
Batteries have the unfortunate habit of running low on power, eventually turning a multi-million dollar spacecraft into space junk. And when it comes to chemical power, few countries on Earth want a spacecraft powered with plutonium-238 orbiting above. While this is not the same material used in bombs, most governments are not keen on the return of radioactive elements to earth. RPS systems have been used in many deep-space missions, however. A radioisotope system helps power the Curiosity rover on Mars.
“Solar paired with batteries, then, is the preferred way to power satellites,” Piszczor said. The space station uses nickel-hydrogen batteries to support its solar panels. Spirit, another Mars rover, also uses batteries paired with solar. Researchers get excited when Martian wind blows away dust that sometimes accumulates on the panels, providing an energy boost to the rover.
But NASA hopes to do more than just power satellites with the sun.
What is NASA’s goal for solar in space?
The ultimate goal is to use solar energy to propel spacecraft. NASA has its eyes on solar electric propulsion as a way to transport materials to Mars in support of a manned mission on the red planet. A key driver of this plan is cost.
The amount of chemical fuel needed to propel a spacecraft to Mars along with all the necessities of a human visit—vehicles, power-making supplies, a human habitat module—would be expensive and bulky. Solar power eliminates the need for liquid fuel, once all of that equipment is in space. A rocket would still be needed to get it above Earth.
Solar researchers are also considering how to power unmanned flights to the Sun and the outer reaches of the solar system.
“There are a lot of people interested in the Sun—predicting sun spots, solar flares and more about it in general. If we understand our Sun, maybe we can understand other stars,” McNatt said.
But high temperatures, high radiation and high solar intensity make solar-powered travel to the Sun difficult. Mercury, for example, receives 10 times the amount of sunlight that satellites in Earth-orbit get, and the first planet from the Sun is still nearly 36 million miles away from it.
The problems don’t abate traveling the other direction. Scientists wants to investigate Europa, Jupiter’s icy moon—which is probably the only other major water supply in the solar system. An ocean could reside under the moon’s icy surface, and it could be hiding clues to the original formation of life—a potential gold mine of information for scientists.
But Europa, about 485 million miles from the Sun, receives only a twentieth of the amount of light as Earth orbiting spacecraft. That means, according to McNatt, that the solar array would need to be 20 times as big or 20 times as powerful, or a combination thereof.
“The other problem with Jupiter, too, is since it has such a big body, it’s a high-radiation environment,” Piszczor said. “It’s almost like a very weak sun. In some ways, going to the Sun is easier.”
NASA, however, remains undaunted by the challenges. The space agency has accomplished many other once-unthinkable feats, after all.
Comments