国家地理-2008-04-24 Spacewalks 太空行走，看上去很美（在线收听）

Floating free in the hostile vacuum of space, astronauts push the boundaries of exploration. Join them on an incredible journey as spacecraft Voyager explores the solar system.

From planetary orbit, the earth and heavens look serene. But the vacuum of space is a hostile environment, lacking oxygen and permeated by radiation. Since the advent of manned space flight in the 1960s, humankind has been exploring this inhospitable realm from the relative safety of spacecraft. But occasional extravehicular activities have led astronauts to leave these confines and explore the unknown. In order for astronauts to survive in space, spacesuits provide a self-contained environment. Their tough material, along with heating and cooling elements, protect astronauts from extreme temperatures, ranging from around 250 degrees Fahrenheit to minus 250 degrees. The suits are pressurized with pure oxygen, allowing astronauts to breathe and keep their blood from boiling in the vacuum of space.

The first venture outside the spacecraft was made by Soviet cosmonaut Alexey Leonov in 1965, lasting only 12 minutes. This feat was duplicated a few months later by American astronaut Edward White during the Gemini 4 mission. Although these excursions were meant to test the feasibility of humans to exist outside the spacecrafts, spacewalks have become a relatively common part of missions in the space shuttle era. On spacewalks, astronauts have performed vital tasks that could not be accomplished from inside the shuttle. They've been able to recover lost satellites, restoring them to their proper orbit. They have serviced orbiting bodies, like replacing the solar panels on the Hubble Space Telescope. And they have become the construction crew for the International Space Station.

Conducting spacewalks is dangerous, and astronauts work in pairs, tethered to the spacecraft for safety. Walking in these spacesuits is not as easy as it seems. The thick pressurized gloves make grasping tools difficult, while the weightlessness of space creates its own issues. To ensure spacewalk missions are successful, astronauts practise their planned excursions at NASA’s Neutral Buoyancy Laboratory. Immersed in a tank of water, the astronauts neither sink nor float, creating a sensation close to weightlessness. For every hour a task is scheduled on a mission, up to 10 hours is spent practising underwater.

As NASA moves forward on the maintenance and construction of the International Space Station, spacewalks will continue to be vital to the process.

permeate: v.
if liquid, gas etc permeates something, it enters it and spreads through every part of it

the advent of sth:
the time when something first begins to be widely used

extravehicular activity: n.Abbr. EVA
Activity or maneuvers performed by an astronaut outside a spacecraft in space.

duplicate: repeat

tether: v.
fasten (an animal) within a limited area