Don
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ISS Science Officer Don Pettit performs in-flight maintenance
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Expedition
Six Space Chronicles #13
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By:
ISS Science Officer Don Pettit
Growing
Plants in Zero-G
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. |