MIAMI • It's not easy to have a green thumb in space.
Without gravity, seeds can float away. Water doesn't pour, but globs up and may drown the roots. And artificial lights and fans must be rigged just right to replicate the sun and wind.
But the National Aeronautics and Space Administration (Nasa) has decided that gardening in space will be crucial for the next generation of explorers, who need to feed themselves on missions to the Moon or Mars that may last months or years.
Nutrients like vitamins C and K also break down over time in freeze-dried foods. Without them, astronauts are vulnerable to infections, poor blood clotting, cancer and heart disease.
So the American space agency has turned to professional botanists and novice gardeners for help.
"There are tens of thousands of edible plants... It is a big problem to choose which are the best for producing food for astronauts," explained Dr Carl Lewis, director of the Fairchild Tropical Botanic Garden, which is leading the effort. "That is where we come in."
SOMETHING DIFFERENT ON THE MENU
When we are able to harvest our own lettuce here, just having a different texture to enjoy is a really nice diversion from the standard menu.
NASA ASTRONAUT RICKY ARNOLD
The Miami-based garden has identified 106 plant varieties that might do well in space, including hardy cabbages and leafy lettuces.
It has enlisted 15,000 student botanists from 150 schools to grow plants in space-like conditions in their classrooms.
The four-year project is about midway through, and is paid for by a US$1.24 million (S$1.65 million) grant from Nasa.
Using trays rigged with lights that mimic the "grow boxes" used in space, students must tend to the plants and record data on their progress, which eventually gets shared with Nasa.
"We're not using typical gardening equipment," said 17-year-old high school student Rhys Campo, who tried her hand at growing red romaine lettuce. "We have set-ups that are a lot more high-tech."
Still, some plants get overwatered, some classrooms are hotter or colder than others, and holiday breaks may leave the grow boxes unattended. In Miss Campo's class, the lettuce dried up.
Such foibles have turned out to be an unexpected but useful part of the project, said Nasa plant scientist Gioia Massa.
"If you have a plant that does well in all that variability, chances are that plant will do well in space," she told Agence France-Presse.
Astronauts living in the International Space Station (ISS) have encountered their share of failures while gardening in orbit too.
The first portable grow box for space, equipped with LED lights, called Veggie, was tested at the orbiting outpost in 2014.
Some of the lettuce did not germinate, and some died of drought.
But the astronauts kept trying, and finally took their first bite of space-grown lettuce in 2015.
Now, there are two Veggie grow boxes at the ISS, along with a third, called the Advanced Plant Habitat.
The food being grown is only occasionally harvested, and amounts to just a leaf or two per astronaut, but it is worth it, said astronaut Ricky Arnold, during a live video downlink with students last month.
"The textures of food are all very similar," he said of the freeze-dried fare available on the ISS. "When we are able to harvest our own lettuce here, just having a different texture to enjoy is a really nice diversion from the standard menu."
According to Dr Massa, a good space plant has to be compact and produce a lot of edible food.
Plants also have to do well in a spacecraft like the ISS, which has a temperature of about 22 deg C, 40 per cent relative humidity, and high carbon dioxide - some 3,000 parts per million (ppm).
"That is something plants aren't adjusted to," said Dr Massa. "On Earth, it is about 400 ppm."
Under a system Dr Massa described as akin to hydroponics, space plants also have to germinate from a plant pillow with only a small amount of dirt, do well under LED lights and be microbially fairly clean because it is hard to wash vegetables in space.
Some of the student-tested crops are expected to launch in the coming months, including dragoon lettuce and extra dwarf pak choi. By next year, tomatoes could be on the menu.
Nasa is also looking into the possibility of robotic space gardening, to automate the process.
But many astronauts say they like tending to plants, as it helps them maintain a connection to Earth.
Besides, as gardeners know, having a plot dry up or be devoured by mould isn't the end of world.
"The students learn that making mistakes is OK," said Ms JoLynne Woodmansee, a teacher at BioTech High School in Miami. "Science is all about building. You can't learn something new without making a mistake."