Four hundred years ago, an Italian scientist named Galileo Galilei became the first person to see the craters  on the moon. Galileo was one of the first people to use a telescope to study the sky. Since then, telescopes have become the most important tool used by astronomers. Scientists never stop finding new ways to make these instruments more powerful. In the next several years, two new telescopes with different purposes are to be used.     One of the telescopes, called Pan-STARRS, could save humans from dying out. Nick Kaiser, a scientist who works on the project, says the Pan-STARRS telescope has been designed to find "90 percent of all killer asteroids  near Earth".    Pan-STARRS, like most telescopes, uses mirrors and lenses  to provide pictures of outer space. Giant mirrors are used to "gather" light. They reflect the light onto the lens of a camera, which can then record the image.     When completed, Pan-STARRS will include four telescopes which will be put on top of a mountain on the Hawaiian island of Maui. Only one telescope is in place and working now. Each telescope will take pictures of one patch of sky for about 30 seconds, and then move on to another patch. Every night, each telescope will take pictures of about 1,000 patches. Every week, each telescope will have photographed the whole sky.     Each of the four telescopes will take pictures of the same patches of sky. One telescope, working alone, may sometimes incorrectly show an asteroid. If there are three other telescopes working, astronomers can use them to see if there really is an asteroid coming our way. By using four telescopes instead of one, scientists hope to get a better picture of space. If a giant asteroid was identified, astronomers would try to break it up long before it reached Earth. How many patches is the whole sky divided into when Pan-STARRS is working?
1,000 7,000 21,000 28,000