Test: Space-bound US-Europe water mission passes finals

Before a NASA mission is launched, the spacecraft is treated harshly for weeks. It’s strapped to a large table that shakes like the pounding of a rocket launch. It’s bombarded with louder noise than a stadium rock concert. It is frozen, baked and irradiated in a vacuum chamber that simulates the extremes of outer space. The Surface Water and Ocean Topography Mission (SWOT), a US-French mission to monitor all water on the Earth’s surface, has passed these important tests. Aside from some final reviews, SWOT is now ready to launch in December.

Some SWOT engineers at NASA’s Jet Propulsion Laboratory in Southern California have invested almost a decade in designing, building and assembling this complex mission. Watching the instruments they were working on go through the final round of testing was stressful, but the team handled the process with ease. That’s because every part from SWOT down to the nuts and bolts was tested multiple times before the satellite entered the thermal vacuum chamber for the final time. The engineers say the earlier tests generated far more fear.

Phoebe Rhodes-Wickett, Mechatronics Engineer at JPL, has spent a quarter of her life working on SWOT. She initially focused on a small component that deploys the antennas on the spacecraft’s main instrument. “When I first tested my mechanism, I was scared,” she said. The component is about the size of a box of tissues and has been tested on a full-size shaker table. “It was just this little mechanism that sat by itself. The test is noisy and you can see the mechanism moving,” Rhodes-Wickett added. “We had an error in our first round of testing. We had to redesign and retest the mechanism in a few months to certify it as spaceworthy.”

After passing this re-test, the mechanism was connected to larger and larger systems that were eventually integrated into the complete SWOT spacecraft. Each phase of spacecraft assembly creates new connections and provides another opportunity for human error to sneak in, ending with another round of testing. The Rhodes-Wickett mechanism has passed three more vibration tests since that first experience. “Every test you pass is a relief,” she said, “but by the time you get to the third or fourth test, your stress level is a lot lower.”

The mechanism is part of SWOT’s new radar instrument, which is the first of its kind in space. The Ka-band radar interferometer, or KaRIn, has two radar antennas mounted on mechanical arms. Once SWOT is in orbit, the arms will extend and extend from opposite sides of the spacecraft until the antennas are almost 10 meters apart. Just as the distance between your eyes helps you better judge distance and depth, the distance between KaRIn’s two antennas helps the instrument reveal more detail about Earth’s waters. But if the process isn’t nearly perfect — if the mechanical arms aren’t fully extended, or the antennas are misaligned by even a few thousandths of a degree — KaRIn won’t be able to make the measurements hoped for.

A risky venture

“It’s a unique part of a career at NASA that we’re always trying to build things that have never been built before,” said JPL’s Eric Slimko, chief mechanic at SWOT. That means every NASA payload starts with an unknown risk factor. Most missions gain some sense of risk level by using prototype instruments on planes and labs, but there’s still the (literally) sky-high added challenge of adapting the technology to survive launch and work in space. “We’re not able to eliminate all of these risks with one piece of paper analysis,” he said. “We have to test it.” Even standard parts are certified.

Developing tests that could prove the folding arm and antenna assembly would work just as well in orbit as they do on Earth has been “very, very challenging,” Slimko said. “For one thing, we can’t turn off gravity. But we have developed a verification program that, while we cannot replicate the exact flight environment on the ground, has full confidence that it will work in space.”

Running the program required dozens of JPL engineers to spend weeks or months at the Thales Alenia Space facility in Toulouse, France, working with colleagues at France’s Center Nationale d’Études Spatiales (CNES) space agency to complete the suite of tests as such Spaceship was assembled. The spacecraft includes hardware not only from CNES and NASA, but also from British and Canadian space agencies, with each team monitoring the performance of its own parts during testing.

Now almost all that remains is the ultimate test: the launch itself. The engineers are more than ready. “It’s fun to have a baby that you literally dreamed up, helped grow, and now you’re taking it to the finish line,” said Rhodes-Wickett. “It’s really exciting to see that something you’ve put so much time and effort into is moving forward and making a difference in the world.”