阅读短文，回答问题Like visible light, UV rays are energy waves released by the sun, only they cannot been seen. Some exposure (暴露) to those waves can do a body good. For example, ultraviolet B (UV-B) rays tell human skin to make vitamin D. Too much exposure can be harmful, though. Overexposure to UV rays can cause DNA problems, opening the door for skin cancer.
Too much sun can also cause painful sunburns.
There are three different types of UV energy waves. They are UV-A, UV-B and UV-C rays. Each type moves at a different wavelength, with UV-A's being the longest. Rays of UV-C are considered especially harmful to human beings.
Fortunately, we have the ozone (臭氧) layer of the Earth's atmosphere. It blocks out nearly all of the UV-C rays and most of the UV-B ones.
The moon doesn't have much of an atmosphere, though. Back on the moon, things are more dangerous. Over the years, astronauts have planted six American flags on the moon. Some scientists think that powerful UV rays could have bleached all the colorful flags white by now. For this reason, the moon is a dangerous place to visit without protection. You might think that astronauts who visit the moon would come home with terrible sunburns. That didn'thappen to
Armstrong and his company. Neil Armstrong was a U. S. astronaut and the first person to walk on the moon. These astronauts all wore special spacesuits on their missions. Spacesuits were made with heavy fabrics that block out UV rays.
When Armstrong set foot on the lunar surface, he was wearing a see-through helmet (头盔) with sun shades that were made of ultraviolet-stabilized polycarbonate. It is a very tough plastic that protects astronauts' faces from UV rays. A
weaker material could put the space travelers at risk for "snow blindness" and other problems. Conditions like these arise when UV rays damage parts of the human eyes.
Sunscreen (防晒霜) defends us from the UV rays that do pass through the ozone layer. However, there's really no need for an astronaut to put on sunscreen. The risk of sunburn goes way down when astronauts are inside a spaceship. Inside
the International Space Station (ISS), for example, there is no need for special suits. Astronauts regularly do their work in nothing but common cotton shirts and pants. To protect them from sunburns, the ISS uses UV-blocking windows. So do most space exploration ships.
