Astronaut Scott Kelly, recently returned from nearly a year in the International Space Station, has deservedly grown in stature, with accolades coming from everyone, including President Obama. But that's not the only way he grew — by the time he returned to Earth he was a full 1 1/2 inches taller than his identical twin brother Mark. How could this be?
It's simple really — credit gravity, or a lack thereof. The human spine has bony vertebrae, and between any two are relatively soft inter-vertebral discs (yes, these are the ones that can "slip"). Normally over time — and here we're talking about years — these discs are compressed by the weight of the body, resulting in the shorter spine and loss of height that older people are familiar with. So, when the force of gravity is removed, as it is in space, these discs can expand again, and voilà — the person gets taller.
Lest one think that space travel is a solution to age-associated diminution, it's unfortunately the case that this effect doesn't last. By the time Kelly traveled from Russia, where he landed, to his home in Houston, gravity had done its job and he was back to his normal height.
While it's amusing to think about astronauts "growing" in space, there are a number of physiological changes that occur in a condition of microgravity. And some of these are not amusing at all.
For example, without gravity signalling to the inner ear (which is key to maintaining balance), a person can suffer from "motion sickness." In some cases this can result in nausea and vomiting, just as it does on earth. This condition usually lasts only the first three days or so after leaving the Earth's gravitational field.
More concerning are long lasting changes due to weightlessness, such as changes in muscle mass and bone. Since astronauts in space don't really use the back and leg muscles to resist gravity and stand, these muscles actually lose mass and start to shrink. This is why astronauts in the space station must exercise a couple of hours per day on equipment designed to simulate the earth's gravitational field — otherwise they could lose up to 20 percent of their muscle mass within a couple of weeks of beginning space travel.
And the lack of a gravitational pull can also wreak havoc on the bones. In space, the cells that break down bone (osteoclasts) are more active than those that build it (osteoblasts), which can result in as much as a 1 percent loss of bone per month. So an astronaut in space for long periods could come home with a condition similar to osteoporosis. Obviously this is another good reason to be faithful about exercising. Further, when bone is broken down, calcium is released into the blood stream, and can deposit in soft tissues where it isn't supposed to be.
Other possible changes due to weightlessness include redistribution of body fluids to the upper part of the body, which could increase the pressure in the cranium and on the optic nerve, affecting eyesight. In addition, pressure in the sinuses could increase, resulting in clogging, headaches and discomfort.
Although many of these changes can be avoided or ameliorated by proper exercise during and after space flights, it will be important to understand their effects on astronauts' health and ability to work — especially if plans to send humans to Mars are to be realistic. Since it would probably take a couple of years to get there, and even though Mars is smaller than Earth with a smaller gravitational "pull," these effects we have seen with shorter flights could well be substantial. If we don't understand how to effectively prevent or counteract them, we could end up with Martian astronauts who are too weak to work.