Fighting zero gravity
If astronauts never establish a base on Mars, it may not be because of problems related to funding or hauling enough food and fuel through space. Instead, it could be because the human body simply can’t do it.
As spaceflight has progressed over the last four decades, scientists have learned that humans aren’t designed to thrive in a zero-gravity environment. After about 30 days, the effects of weightlessness cause the body to begin to lose muscle mass, aerobic function and bone density.
The hope for a potential Mars mission is that astronauts, after a six- to nine-month flight from Earth, would be able to build a station or shelter on the Martian surface, a feat that could take additional months. But given the current rate of physical deterioration in space, astronauts will be hard-pressed just to adjust to the red planet’s gravity -- about half of Earth’s -- much less be physically prepared to erect a base camp, scientists say.
Deteriorating muscles and bones are “real showstoppers,” said William J. Evans, a physiologist at the University of Arkansas for Medical Sciences and a former advisor to NASA on physical fitness. “Unless they can prevent these [physical] losses, there won’t be any multiyear missions.”
Further refining their current exercise program to ameliorate the effects of weightlessness on the human body is NASA’s No. 1 health issue for astronauts, say scientists at NASA’s Johnson Space Center in Houston. Crew members aboard the international space station now experiment with different approaches -- referred to as “exercise countermeasures” -- during their stays, which last at least six months.
Astronauts in the space station currently are required to work out six days a week for about 90 minutes at a time. At least 30 minutes are spent revving up their cardiovascular systems on either a bicycle or a treadmill. Their goal is to keep their heart rates between 50% and 80% of their maximum rate.
Of course, exercising in zero gravity presents special challenges. Astronauts must be tethered to the devices. And in the case of the treadmill, loads of between two-thirds and three-fourths of the astronauts’ body weight are strapped onto them to more closely mimic weight-bearing exercises on Earth. The extra weight helps keep legs strong.
In space, astronauts also must spend one hour six times a week on the Interim Resistive Exercise Device, essentially a resistance-training machine that uses rubber cords to strengthen muscles and help prevent bone loss. The astronauts, whose average age is 41 and whose earthbound occupations include pilot, physician, engineer and scientist, exercise on both the space shuttle and the space station.
“It’s been a lot of trial and error,” said Don Hagan, who supervises the astronauts’ exercise program at Johnson Space Center. “We’re always tweaking it.”
Of the physical problems associated with space travel, bone loss is viewed as the most critical to overcome. Normally, bones are kept strong in large part by the stress and pressure of walking, say scientists. The small vibrations created by the foot meeting the ground and its gravity fortify the entire musculoskeletal system. Without that, bones become substantially weakened. Even now, astronauts returning from an extended stay in space are at a heightened risk of fracturing a bone if the landing is too jarring.
“In space, your body says it doesn’t need these bones,” said Evans, coauthor of “AstroFit,” a book about space travel and fitness (2002, The Free Press). “It’s really the same thing that happens when someone remains in bed for extended periods of time.”
The Interim Resistive Exercise Device, however, will be replaced soon by the Advanced Resistive Exercise Device. The newer model, which won’t be in the space station for at least a year, offers more efficient strength training and its rubber bands won’t wear out as quickly, Hagan said.
Unlike muscle mass and aerobic function, which can be restored once back on Earth, it’s less clear whether bone density can be returned to its original levels after prolonged spaceflight, say scientists. Ongoing experiments aboard the space station are investigating the matter.
“The space station has been put up there for 15 years, and we’re learning as much as we can from it,” said Hagan. “But there’s still a lot we don’t know or understand about [humans and] space travel.”
