April 21, 2003
The Physics of Space Gardens
Credit: ISS Expedition 6 Flight Engineer
Nikolai Budarin
Explanation: It could only happen in space: A tiny
bubble of air hangs suspended inside a droplet of water. The
droplet rests in the cup of a delicate green leaf, yet the stalk
doesn’t bend at all.
Cosmonaut Nikolai
Budarin photographed this scene on April 9, 2003. He was
peering into the Russian Rasteniya greenhouse onboard the International
Space Station (ISS), and his
snapshot illustrates some of the strange physics of gardening
in space.
First, consider what would happen
on Earth: The air bubble, lighter than water, would race upward
to burst through the surface of the droplet. Meanwhile, the leaf
would be busy tipping the heavy water onto the floor below. Everything
would be in motion, the picture a blur.
In Earth-orbit, though, the scene
is truly motionless. The air bubble doesn’t rise because it is
no lighter than the water around it–there’s no buoyancy. The
droplet doesn’t fall from the leaf because there’s no
force to pull it off. It’s stuck there by molecular
adhesion.
Sticky water. No buoyancy. These
are some of the factors spacefarers must take into account when
they plan their gardens. Suppose Budarin sprays water onto the
base of the plant. Will it trickle down to the roots? More likely
it will stick to the stem or adhere the substrate in which the
plant grows. And what happens to oxygen
emitted by plants? Without buoyancy to stir things up, air
circulation can be poor. Oxygen lingers. It doesn’t waft away
to make room for the carbon dioxide plants need. A plant can
actually suffocate in its own emissions!
Nevertheless, plants can
flourish in spaceships. Recent experiments onboard the ISS prove
it. Stay tuned this week and we’ll show you some of the eye-catching
results. | 
