NASA’s Dirt-Scanning Satellite Is Down to a Single Peeper
Every mission to space is a rocket full of risk. And when things go wrong, inertia tends to carry every mishap towards catastrophe. But not every extraterrestrial tribulation is a mission-ender.
One such mission is the nearly billion-dollar Soil Moisture Active Passive satellite, which lost one of its two radar imagers just months after coming online. SMAP’s pair of sensors were supposed to generate high resolution, incredibly accurate maps of soil moisture—the crux of the Earth’s water, energy, and carbon cycles. The blackened electric eye doesn’t blind the mission, but it does handicap the satellite’s high-def mapping abilities. And it’s probably not coming back. Last week, NASA announced that the extra eye was done for good.
NASA launched SMAP in January to watch over the water that Earth stores in its soil. More than 97 percent of all the water on Earth is stored in the oceans; most of the rest is locked up in ice caps and glaciers (for now). Less than a single percent of all water on Earth is soil moisture. But gallon for gallon, this fraction of water is more important than any other source on the globe.
That’s mostly because soil is where plants grow. The amount of moisture in the soil determines how many plants can grow in an area, which determines how much carbon gets sucked out of the atmosphere. Even more, when those plants suck in carbon, they transpire water vapor, which keeps Earth’s surface temperature cool as it evaporates.
After its winter launch, SMAP began sending soil moisture maps home in April. But by July, something was amiss with one of its paired sensors.
Both sensors collect data from the same slice of the RF spectrum, though they use different methods. The active radar (the “A” in SMAP) bounces energy off the Earth’s surface and takes moisture readings based on the return signature. “The advantage is that it actually illuminates the surface and has a high spatial resolution,” says Dara Entekhabi, the SMAP mission’s principal investigator and climate scientist at MIT. “But it is more susceptible to interference.”
The P in SMAP is the huge, cake pan-shaped antenna hanging off its side. As its name suggests, the passive microwave radiometer just sits back and collects the planet’s emanations. It scoops up a ton of soil data—it can even see through clouds and tree cover—but only sees about 25 miles per pixel. The active radar would have brought that resolution down to just under two miles, but since the active sensor punched the big clock in the sky, the passive radiometer is doing everything on its own.
Entekhabi says the the culprit is a faulty power supply. “In order to amplify a signal, you need power,” he says. The failure could have been due to faulty parts, a freak ion static event, or one of many other low-probability events.
What does this mean for the $915 million mission? “The radar was unique in that it was a global mapping radar, and produced a map every two to three days,” says Entekhabi. On a mission level, this cripples SMAP’s ability to make high-resolution maps and collect data on how changes in soil moisture affect long term climate. The radar’s brief months of operation let NASA watch climate change in real time: seasonal vegetation changes, sea ice extent, new bodies of water created by melting permafrost. They expect to release these maps later this month. But that’s the end.
“It’s certainly a tough loss for the team and the community,” says Jay Famiglietti, NASA’s top water scientist. “Entire careers are built around flagship satellite missions like SMAP, so I cannot imagine what my colleagues are going through.” But Famiglietti points out that we should consider the loss relative to what came before.
Which is nothing. Prior to SMAP scientists built soil moisture models based on limited, ground-based observations. And these models are crucial to just about every experiment involving climate or weather. “Soil moisture is very important because it controls temperature and moisture above the ground,” says Entekhabi. In a weather or climate model, this determines how solar and atmospheric conditions play out on the ground. Instead of proxies, now those models can use direct data downloaded from SMAP.
Soil moisture is also the key component for figuring out if a region is in drought, and also a predictor of flash flooding. “In my opinion, going from virtually nothing to something like 50 km soil moisture has great potential to transform our current knowledge,” says Famiglietti.
Still, it’s not trivial to lose a millions-dollar component to a nearly billion dollar mission. Currently, two investigation boards are working to figure out what went wrong and how to prevent it in the future. “You send something into a really harsh environment through a really tough launch process into a very, very harsh environment, and once it’s up there you don’t have a chance to recover and fix anything,” says Entekhabi. Hey, nobody said space was easy.