Probing Question: How does microgravity affect astronauts?

Anyone over forty knows firsthand the effects of gravity's constant downward pull on our faces and bodies. It is an immutable force that Einstein called a “curvature of space-time”— but the curvature caused by gravity is a little closer to home, in our very bones.

Every day, the weight of gravity compresses the sponge-like discs between our spinal vertebrae, making us up to three-quarters of an inch shorter by evening. Most of that height loss is regained while we sleep, as the discs are rehydrated, but not all, which is why our stature slowly shrinks over a lifetime.

In the weightless environment of space, are astronauts spared the bone-compressing impact of gravity?

It's a common fallacy, says Raj Acharya, head of Penn State's department of computer science and engineering, but the reality is bone loss does occur in space. “One of the most significant concerns for NASA,” he explains, “is the deterioration of bone conditions of astronauts exposed to microgravity.” In fact, bone loss is one of the two biggest health risks (radiation exposure is the other) that astronauts face.

A past research fellow at NASA and the Department of Defense, Acharya developed algorithms used to monitor bone conditions of astronauts under microgravity conditions. Microgravity—also called zero gravity—doesn't mean there's no pressure on our bodies, Acharya notes. Most human spaceflights take place in an orbital altitude between 120-360 miles above Earth's surface, only 1.8 percent of the distance to the moon. Within that range, astronauts still are exposed to 88.8 percent of the full strength of Earth's gravitational field. (If not for this constant pull of Earth's gravity, the space station and space shuttle would drift out of orbit.)

Astronauts may be nearly weightless (the simultaneous “free fall” of the spacecraft and everything within it gives the illusion of zero gravity) but they are not massless—the mass of their bodies remains the same and it's this mass that gravity works upon. In fact, explains Acharya, astronauts lose bone mass and strength much faster in space than on Earth, since they miss the weight-bearing exercise we get from simply moving our bodies around (pushing back against gravity's resistance) on our planet's surface.

The tissue at greatest risk for astronauts is trabecular bone, the softer stuff found near joints at the end of long bones, says Acharya. “Microgravity may result in thinning of the trabecular network and result in fractures,” he says, noting that the lattice-like rods and struts in trabecular bone may become permanently thinned and weakened, making astronauts on long-duration missions very susceptible to hip and spinal fractures.

In his research, Acharya turned to fractals—fascinating geometric patterns with repeating, self-similar patterns—to better understand bone loss. “The trabecular bone has a honey-comb like network structure, which is why fractals are particularly good mathematical objects to model the trabecular structure,” he explains. “My research provides a mechanism for modeling the trabecular bone as a fractal. The deterioration of bone condition actually manifests itself as a change in fractal dimension.” Adds Acharya, “Traditionally, only bone-mass effects were used by NASA,” whereas fractal analysis allows a more in-depth look at bone-tissue architecture in its entirety.

Now that our space program is gaining a better understanding of trabecular bone loss risks, what is being done about it? In addition to exploring the use of bone-strengthening drugs and superhero-style spacesuits that replicate gravity, “NASA also is using counter measures such as exercise to combat the effects of microgravity on the bones of astronauts,” Acharya says.

One thing is for certain, he notes: Before we humans attempt the trip to Mars, we'll need some reliable measures to reduce the physical toll of life in space. That round-trip may take up to three years to complete and research suggests that astronauts could lose close to half their bone mass before they return.

So, Baby Boomers, take heed. The next time you're tempted to complain about sagging bodies and faces, remember that your proximity to Earth's gravitational field is (so to speak) actually your lucky break.

Last Updated July 22, 2011