Watering space plants is hard, but NASA has a plan

Fluid dynamics on the International Space Station makes growing tomatoes difficult.

Astronauts love plants. In the sterile, mechanical International Space Station, spacefarers are so desperate for a hint of green that they will eagerly volunteer their free time to tend to the plant experiments onboard. But a new terrarium means they will have to spend very little mandatory time gardening—once it starts working properly, anyway.

NASA scientists have been collaborating with Techshot and Tupperware since 2017 to create the perfect container for cultivating greenery in space. The result is a low-maintenance, mini-nursery called the passive orbital nutrient delivery system, or PONDS.

Each PONDS is a plastic box. Inside there is a small pot that contains orange clay sediment like you’d find on the paths at a baseball field, fertilizer, and two seeds. The outer container stores water. The idea is that the roots eventually grow out of the main container and into the water, without any human intervention.

NASA isn’t trying to take #plantshelfies away from the ISS crew. But an astronaut’s time is precious, says David Reed, launch operations director at Techshot, a research and development company that’s made a lot of the station’s hardware. Since their packed schedules are a major constraint on space experiments, the less time astronauts have to spend on plant maintenance, the better.

Two sets of PONDS have made their way to space, the first in April 2018 and another earlier this month. But they haven’t quite worked as planned. In the most recent experiment, the seed germinated, but came up short—probably due to a lack of water. “They dried out,” says Howard Levine, chief scientist of the International Space Station research office. “That was a little frustrating, but there was a lot learned.”

Nothing works quite the same way in space as it does here on Earth. Imagine a houseplant. Usually, it’s in some sort of pot with a drainage hole. The plant sits in soil within the container. When the caretaker of the plant adds water, it seeps down to the roots and then the excess water flows out of the hole on the bottom of the pot. In space, this concept completely breaks down. Aspirational botanists run into the same problems that space scientists have faced when confronting how to design the best wastewater disposal systems, fuel tanks, and even coffee cups: Nothing stays put inside its container. Even when scientists found a way to hold all the right parts together, the water in a planter would not flow downward, but continuously saturate—a no-no for most plants.

PONDS NASA ISS space vegetables
A glimpse inside PONDS. Courtesy of Howard Levine

For many years, NASA scientists solved these problems with various strategies. Lately, they’ve used “pillows”—packets of clay, fertilizer, and seeds that the ISS crew members water every three days or so. Leafy greens like romaine grow well in the pillows, which is good news for astronauts who might be short of some lettuce-y nutrients. However, the pillows don’t hold enough water to support fruiting plants, like tomatoes or bell peppers—the impetus for PONDS.

Howard Levine and his team at NASA developed the prototype using whatever materials they could find at home goods supply stores. Techshot got involved to further develop the model and present the idea to Tupperware. Engineers at the food packaging have expertise in safe plastic technology that won’t leach toxic plastics, and they understand how to mold those materials for various uses, says David Kusuma, Tupperware’s vice president of research and product innovation. Even with the collective knowledge, the team turned to outside consultants like Mark Weislogel, a zero-gravity-obsessed materials engineer from Portland State University, and other plant biologists at the Kennedy Space Center.

PONDS’ future hinges on finding the exact right amount of water to keep the plants alive and thriving. So far, the plants in the experiments have drowned and dried up, so future success depends on hitting the Goldilocks just-right equilibrium. If all goes well, Reed estimates the team will have a PONDS 3.0 ready for tomato experiments on the ISS in the next year. Eventually, he envisions a large tank that could operate autonomously and water many different edible plants at the same time. Perfecting the technology could allow for longer space missions, a crucial step in traveling to Mars.