Hardened solar panels ready to power Juno to Jupiter
BY STEPHEN CLARK
Posted: May 29, 2011
Inside a pristine clean room just outside the gate to the Kennedy Space Center, engineers casting brilliant beams of light on NASA's Jupiter-bound Juno spacecraft finished checking the power efficiency of its 18,600 solar cells last week.
Everything checked out, according to Tim Gasparrini, Juno program manager at Lockheed Martin Corp.
"Completing the testing and stow of solar panels is always a big pre-launch milestone, and with Juno, you could say really big because our panels are really big," said Jan Chodas, Juno project manager at NASA's Jet Propulsion Laboratory.
The mission is aiming to uncover clues about the origin of Jupiter, potentially yielding insight into the formation of the whole solar system.
Juno has three solar panels to generate electricity. The arrays will be folded up for launch, then unfurled like an accordian moments after the spacecraft leaves its Atlas rocket in space. Fully deployed, each wing measures about 9 feet wide and 29 feet long.
One array has a magnetometer boom on the end for one of Juno's research investigations.
Using a string of lights in place of the sun, Lockheed Martin employees illuminated each of the probe's three extended solar panels to verify they will produce enough electricity at the mind-boggling distance of Jupiter.
Because of their size, only one of Juno's solar wings can be extended at a time on the ground. Lockheed Martin's factory in Denver and the clean room in Florida are not large enough accommodate the simultaneous extension of all three wings.
The arrays are positioned on three plates of the six-sided core of the spacecraft, forming a triangular shape when they are deployed together in space. All together, the panels have an area of about 635 square feet, larger than most studio apartments.
"I like to tell my kids the spacecraft would almost stretch rim to rim on a basketball court," said Rick Nybakken, Juno's deputy project manager at JPL.
The solar array "walk-out" and illumination tests were a repeat from a similar check at the spacecraft's factory in Denver. But the solar panels were removed from Juno for the shipment to Florida, so officials wanted to ensure everything still worked when the components were put together again.
Engineers checked the current from the arrays when the light stand cast an eerie glow on the violet-colored panels. Officials were happy with the results, giving the all-clear to fold up the solar panels into their launch configuration and passing another milestone on the road to blastoff.
Next up will be fueling of Juno with rocket propellant, a spin test and mating of the spacecraft with launch vehicle.
Up to now, robotic probes daring enough to venture to travel to Jupiter or beyond have always been powered by a nuclear generator.
The $1 billion Juno mission will blast off Aug. 5 on an Atlas 5 rocket. Its five-year journey from Earth to Jupiter will rely solely on sunlight, setting a new distance record for a solar-powered spacecraft when it is tugged into orbit in July 2016.
Its solar panels could generate up to 18 kilowatts of electricity with the bright sunlight at Earth, but that value falls to about 450 watts at Jupiter's distance. That's equivalent to about five standard light bulbs.
"If you had a 100-watt lightbulb at Earth, it turns into a nightlight at Jupiter," Gasparrini said.
Half of Juno's electricity budget goes to its thermal system to keep the spacecraft at a comfortable temperature. The balance goes toward communications, computers, propulsion and operating the probe's seven scientific instruments and color camera.
Juno will reach nearly twice as far from the sun as NASA's Dawn asteroid chaser, another ambitious mission exploring new frontiers with two huge solar panels. The European Space Agency's Rosetta comet probe is also pushing the solar power envelope, but it won't quite reach Jupiter's distance.
That all makes for a tall challenge for Juno's electrical engineers.
"The one thing enabling this mission is the efficiency of the solar cells," said Jeff Coyne, Lockheed Martin's chief engineer for Juno's power subsystem.
Without finding a way to power Juno on sunlight, the mission likely would not have made it beyond a PowerPoint slide. That's the consensus view among the team of contractor and government engineers helping prepare the helicopter-shaped probe for liftoff.
For one thing, U.S. reserves of plutonium fuel are dwindling for nuclear-powered spacecraft, and nearly all of what's left is already spoken for by future missions. Even if plutonium was available, the nuclear power generator would introduce regulatory hurdles that might have complicated development.
Instead, Lockheed Martin came up with a solar-powered concept that could still accomplish all of Juno's objectives at Jupiter.
"I started doing a lot of calculations using manufacturers' publicized information, and we came up with an array that would work," Kindt said.
Engineers came up with a novel design using 18,600 solar cells handpicked from Spectrolab, a subsidiary of Boeing Co. based in Los Angeles. Made of gallium arsenide, each cell measures approximately 3.7 inches by 2.25 inches.
"We're the first mission to do solar power at the distances of Jupiter, but the solar cell technology has evolved over the last few decades," Chodas said in an interview. "We have every confidence that our arrays will provide enough power."
Lockheed Martin and Spectrolab put each of Juno's solar cells through multiple tests to check their expected performance at simulated solar distances from the launch from Earth to the science phase at Jupiter.
Designers also devised additional radiation testing to make sure the cells, computers and wiring will survive Jupiter's radiation belts, which give off enough electrons, protons and other high-energy charged particles to zap Juno's unprotected brains on its first orbit.
But it will take 30 trips around Jupiter for the probe to collect the data scientists need to map the giant planet's magnetic field and peer deep into its swirling inhospitable atmosphere.
Juno will fly through Jupiter's doughnut-shaped radiation belts on each orbit, exposing itself to an environment that will eventually get the best of the spacecraft, ending its mission after just 15 months at the gas giant.
NASA's Galileo probe lasted for nearly eight years in the vicinity of Jupiter, but Juno will fly through the planet's worst radiation with more regularity, approaching within 2,500 miles of Jupiter's turbulent cloud tops on each pass. Officials liken the experience to receiving 100 million dental X-rays in a little over a year.
Juno's flight computers, avionics and commanding system are wrapped inside a solid titanium box on top of the spacecraft. Known as the vault, the container is about the size of a microwave oven and weighs nearly 500 pounds fully loaded, Gasparrini.
Engineers chose to build a radiation-shielding vault early on in Juno's development. They knew it was heading for an environment never before explored, encountering the most intense space radiation of any mission in history.
Some energetic particles will still penetrate the vault, but it is sturdy enough to keep the spacecraft alive through the planned mission. After about 15 months, engineers expect Juno will start to succumb to the radiation. They plan to dive the probe into Jupiter's atmosphere in a fiery sendoff to avoid the off-chance it could collide with one of the planet's inner moons, which are potential targets for subsequent missions.
But some of the challenges aren't so obvious to an armchair observer.
Juno's circuitry and power system is unable to work with the 18 kilowatts of electricty the probe's solar panels could generate right after launch. So engineers crafted a clever way to tie together solar cells into strings, giving ground controllers the ability to route just some of the copious power into the heart of Juno.
"At Jupiter, we only need to be able to handle a certain amount of current, and when you're sitting there with 18 kilowatts, you've got a lot of current," Kindt said. "It's a thermal issue for the electronics, so we back off to manage the amount of current we have."
As Juno ventures further away from the sun, controllers will add more strings to the power system, calling on a larger portion of the solar panels' collecting area for electricity as needed.
Radiation will also threaten the efficiency of Juno's solar arrays. Unable to protect the panels in the craft's titanium vault, engineers doubled the thickness of the cover glass over each cell.
"Radiation is a big degradation factor for our solar arrays," Coyne said. "We think we've avoided radiation at first, but the mission's later orbits take Juno deeper into the radiation belt. That's a big hit on the solar cells."
Juno's off-the-shelf solar cells were not designed to withstand the radiation at Jupiter, but testing shows they should do the trick long enough to collect the mission's science data. Solar cells on satellites near Earth typically last decades.
Even taking away the low-light and harsh radiation environment at Jupiter, the solar cells still have to handle the violent vibrations and fluctuating hot and cold temperatures of every space mission.
The craft will see temperatures ranging from minus 220 degrees to 266 degrees Fahrenheit. The solar arrays should be able to take even more extreme temperatures, Kindt said.
"They will all function," Kindt said. "We have gone through all the thermal and acoustic enviroments. We're highly confident we're not going to have any problems with the arrays."
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