Astro-Z The Prequel: Growing Plants in Zero-G
Editor's note: This was originally posted 5 May 2003 during Don's first six month mission to the International Space Station, where he served as Science Officer.
Growing plants in the weightless environment of an orbiting spacecraft is much harder than meets the eye. Having a supply of tomato and basil seeds I tried my hand at growing them with results proving that it is not as easy as the first time I tried this for science class in second grade. I do not have the necessary ingredients to grow healthy plants to fruition. We have no dirt on space station. I have no soil or soil substitute and no fertilizer to supply the necessary macro and micronutrients throughhydroponics. However, it is possible to investigate sprouting where the new plant structures live off the nutrients from their seed starch. The best I could expect is to have healthy seedlings for a few days and then have them turn yellow, wither, and die as they starve for the lack of potassium, fixed nitrogen compounds, and all the other micronutrients. With this end in mind, I set out to construct a 0g sprouter.
Lacking soil, you need some sort of substrate that will retain both seeds and water. I considered using an old shirt or sock but decided the Russian supplied toilet paper was best. This toilet paper is not like what you normally think of as toilet paper. It consisted of two layers of coarsely woven gauze, 4 by 6 inches in dimension sewn together at the edges with a layer of brown tissue sandwiched in-between. It works very well for its intended purpose. It also makes a wonderful sprouter. It retains water well and the seeds can be anchored under the gauze weft.
My first thoughts were to arrange the seeds in little rows on the rectilinear squares of toilet paper. Only then did it occur to me that being on space station could offer new possibilities. Why not a spherical planter? Every direction is the same as the rest and a sphere would offer some interesting possibilities. The limited lighting, both from window and artificial sources, can be shared by the spherical surface if the sphere is attached to a string and allowed to freely rotate in the air currents. It will then be constantly moving and bobbing around like a helium balloon on a string and thus share the lighting with all its surfaces. The plants can enjoy the 1/R^2 potential given by spherical geometry. A spherical geometry gives increasing surface area as the plants radially expand, surface area needed for leaves and photosynthesis. They will not have to partake in foliage fights like plants do when grown on a rectilinear flat plane where surface area for foliage becomes limited as the plants grow upwards. It also allows one small spherical volume in which to circulate hydroponics via capillary action. A spherical geometry for growing plants in limited volume makes good sense if you have weightlessness.
To construct my planter, a spherical core is needed. An old pair of underwear worked well. We have supplies on station sufficient to change our underwear perhaps once every three to four days so I figured there might be a few nutrients in there as well. An old pair of underwear was folded into a sphere and held in place with a few well-placed stitches using needle and thread from our sewing kit. The toilet paper was sewn to the outer surface. A drinking straw was sewn so that its opening was in contact with the fabric and could thus be connected to a drink bag to provide a continuous water drip via capillary action. Seeds were planted with a pair of tweezers by carefully working each seed between the weft of the gauze. The sphere was initially watered and then attached to the water bag by a long thin plastic tube, which also functioned as a string and hung by a light in the node. It bobbed around in the air currents.
I became concerned, when after a few days, no sign of sprouting had occurred. The water bag was working well in keeping the sphere moist and was consuming about 100 ml a day. I touched the sphere. It was cold. The cabin air temperature was warm at 28 degrees centigrade. I measured the sphere at 18 C. Evaporative cooling was acting like a refrigerator and slowing germination. A plastic enclosure was constructed out of plastic bags that made the planter sphere into a miniature greenhouse. A few holes were poked through the side for ventilation just like you did in the jar lid when you were in second grade. The plastic enclosure slowed the rate of evaporation and resulted in temperatures around 30 C. Within two days there were sprouts. However, not all was well on the planter sphere.
Gravity plays an important role in sending roots down and stems up. Without gravity, every direction is the same. Roots and stems were exiting the seed and growing in any direction within the plane of the spherical surface. The lighting provided an outward growing cue however its effects were small compared to the effects of capillary forces. Capillary forces, subtle in nature and derived from the water interface on the damp layers of gauze, convinced the sprouts to ignore the outward direction of the light and to grow in the surface plane of the sphere. Each sprout had deployed its cotyledons, two miniature leaves that jump-start photosynthesis and provide for the growth of its real leaves. Cotyledons are the drogue chute equivalent before the main canopy is deployed in a parachute system. The capillary forces were overpowering the effect from the outward direction of light. Without gravity's direction, it was as if the sprouts were lazy and decided to give into the subtle capillary forces instead of standing upright to light's beckoning.
The cotyledons, constantly covered with a warm layer of water, rapidly molded and withered. Without a drogue chute, the main canopy will not deploy and you will have a hard landing. Without cotyledons, the sprouts died. It is imperative to make the sprouting leaves grow through the water film as quickly as possible, thus surrounding them with fresh air.
It was too late to save this batch of sprouts so a new batch was started. This time, as each sprout deployed its cotyledons they were carefully pulled by hand out from the grips of surface tension and allowed to freely spread their leaves in the surrounding air. This batch of sprouts did as well as you could expect having only their starchy seedpod as a source of nutrients. After an inch or so of growth with budding main leaves forming, they turned yellow and died. When you live inside of a metal can filled with machines and electronics, a small splash of growing green is a pleasant reminder from where we came; we all have our roots. I could not help but feel a small sadness for these living creatures that were giving me so much delight while I was powerless to help them grow strong.