Optimal Lighting for Plants in Space

Plants are photosynthetic organisms that require light to harvest energy. Lighting has a big impact on plant growth: plants grow towards light (phototropism) and also take lighting cues to regulate certain processes (photoperiodism). In fact, the color of the light that a plant gets matters as well. Let’s take a look.

Plants and Different Light Colors

We’ll start by asking this question: why are plants green? Objects have a certain color because they are reflecting that color away. Plants reflect a lot of green light away, and consequently, they absorb that light poorly. Green light is the most inefficient light source for plants. High-energy, short-wavelength colors such as blue and red are the most efficient sources of light for plants. Blue light promotes leaf growth and red light combined with blue light promotes flowering.

Efficiency is the Name of the Game

Take a look at the VEGGIE plant growth system aboard the International Space Station. What color light do you see?

Astronaut Steve Swanson next to the VEGGIE Plant Growth System on the ISS

If you said purple, you’re right. And purple is … you guessed it: a combination of red and blue light, the two most efficient colors for plant growth. Making sure all spaceflight equipment is running at optimal efficiency is important. To give plants the best treatment we can, our current spaceflight growth systems run under a combination of red and blue light: purple light.

Can Sunlight be Used?

Plants need light to live and grow, and it takes a lot of energy to grow plants using man-made light fixtures. So why don’t we use sunlight instead? Let’s explore.

Why don’t we use sunlight to grow plants on the International Space Station?

Engineering challenges	Windows take a lot of time, effort, and money to design and build in space. They are not worth the cost, given the availability and low cost of other lighting options.
No long periods of darkness	The ISS orbits the Earth, so it receives daylight for 45 minutes and darkness for 45 minutes. Plants have a circadian rhythm and need long periods of darkness for their metabolism to function properly.
Flexibility	In 2017, NASA decided to send a second Veggie unit to the ISS. Adding a second Veggie would have been much more difficult if we had to modify the ISS to let in more sunlight. Using lighting technology allows us much more flexibility in what we design and where we place it in the vehicle.

Why don’t we use sunlight to grow plants on Mars or the Moon?

Engineering challenges	Windows take a lot of time, effort, and money to design and build in space. They are not worth the cost, given the availability and low cost of other lighting options.
Inadequate light intensity	Mars is much farther from the sun than Earth, so it receives less than half as much sunlight as Earth. This means plants will grow more slowly and won’t make as much food.
Extended day length	One moon day is about 29.5 Earth days long, so a lunar base would get about 2 weeks of nonstop sunlight followed by 2 weeks of darkness. This would inhibit plant growth. In comparison, Mars days are about 25 hours long, very close to our 24-hour days on Earth.
Flexibility	Using lighting technology allows us much more flexibility in the environments we design and where we place the plant growth chambers.

History of Grow Lights

People have been using light bulbs to grow plants since 1861. For the past 50 years, researchers primarily used high-pressure sodium (HPS) lamps, but these are not well-suited for growing plants in space, since they’re bulky and emit a lot of excess heat. In the search for space-age grow lights, scientists considered incandescent lamps, fluorescent lights, and other types of bulbs, but these options were too short-lived, inefficient, or potentially dangerous for continuous use in spaceflight. Astrobotanists needed a better lighting option, and eventually they found LEDs.

LED Grow Lights for Astrobotany

Light-emitting diode (LED) technology isn’t new—in fact, LEDs have been in development since 1939—but LEDs weren’t used to grow plants until NASA researchers studied them for astrobotany. In the early 1990’s, the NASA Research Partnership Center at the University of Wisconsin-Madison tested LEDs for plant lighting, which led NASA scientists to publish some of the first papers on LED-based grow lights. LEDs are well-suited for spaceflight—they are energy efficient, long-lasting, lightweight, and emit very little heat. NASA’s development of LED grow lights in the name of astrobotany has benefited many plant-related industries around the world.

Why use LEDs instead of sunlight?

We currently use LEDs to grow plants on the International Space Station in the Veggie and Advanced Plant Habitat. When we look to the future, we will probably continue to use LEDs to grow plants in space, since LEDs are much more reliable than sunlight in the context of space travel. Developments in LED technology are also making them more efficient, so growing plants under LEDs is much cheaper than it was 10 years ago. Still, growing plants under LEDs takes a lot of energy, so scientists are still considering ways to add supplemental sunlight to future space growth chambers. NASA researchers and scientists at the University of Arizona are considering fiber optic technology to see if they can concentrate sunlight and deliver it to the plants without the complicated engineering of building windows in spacecraft.

There’s no promise that fiber optics will work in space, but perhaps this technology could make it easier for us to grow plants indoors on Earth. Developments like fiber optics are part of the reason that many people find astrobotany so exciting: astrobotanists collaborate with diverse teams to design cutting-edge technology that will change the way humans live and work in space and on Earth.

Astrobotany.com