2007年VOA慢速英语special200708150045

VOICE ONE:

I'm Barbara Klein.

VOICE TWO:

And I'm Bob Doughty 1 with EXPLORATIONS in VOA Special English. This week, we tell about a project to identify images of one million galaxies 2 that have been captured by the Sloan Digital Sky Survey. We also tell about the Perseid Meteor Shower.

We begin with NASA's new spacecraft that will visit two of the biggest asteroids 4.

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VOICE ONE:

For many years, the United States space agency has studied the major planets in
An artist's picture of the Dawn spacecraft

detail. For example, NASA is observing Mars from orbit and also on its surface with the Mars Rovers. Jupiter has also been visited several times by Pioneer, Voyager and Galileo spacecraft. The Deep Impact mission even visited a comet.

But there are objects in our solar system that remain mysterious. These are the asteroids and dwarf 5 planets.

Now NASA has designed a mission that will visit two of the important members of this group of objects. A new spacecraft called Dawn will visit Ceres and Vesta, two large bodies in what is known as the asteroid 3 belt.

The asteroid belt took shape early in the formation of the solar system, about four and one-half billion years ago. Astronomers 6 believe that the force of gravity from Jupiter prevented the rocky material between it and Mars from forming a planet. The result is that there are thousands of rocky bodies circling the sun between the orbits of Mars and Jupiter.

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Ceres is the largest object in the asteroid belt. It is over nine hundred kilometers across. It was the first object of its kind ever discovered. In August of last year, however, astronomers changed the way they define Ceres. It is now a dwarf planet. Ceres shares this classification with two other objects: Pluto 7 and Eris. But Ceres is a rocky world much closer to the sun.

Astronomers are interested in Ceres because it may hold water. Measurements of light reflected from the dwarf planet suggest this. Astronomers believe that the outer covering of Ceres may contain up to twenty-five percent water ice. This could mean that there is more water on Ceres than there is fresh water on Earth.

VOICE ONE:

Vesta was the fourth object of its kind discovered. Astronomers believe it is covered with rock that melted and then became solid again. There is also a giant hole, or crater 8, four hundred sixty kilometers across on the asteroid's south pole. Astronomers believe a massive crash took place between Vesta and another object. The force was so great that Vesta lost one percent of its total mass.

What happened to all this material? It spread throughout parts of the solar system. Five percent of all meteorites 9 we find on Earth might be from this one event on Vesta. Meteorites are small pieces of space rock that fall to Earth.

VOICE TWO:

Now Dawn will explore these bodies in greater detail than ever before. Dawn is a new kind of spacecraft. It improves on technology used in NASA's Deep Space One mission. That spacecraft visited the comet Temple One and an asteroid. However, Dawn's mission will be much longer.

Dawn's engines use what is called an ion propulsion system. Ion engines use electrical current and fuel made of the element xenon to power the spacecraft through space. Electricity comes from two large solar arrays over eight meters long. The wide flat surfaces of the solar arrays gather sunlight and turn it into electricity.

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Dawn's eyes are two cameras that are exactly alike. They can measure light in seven different colors. There are two other instruments on Dawn. One is a Gamma Ray and Neutron 10 Detector 11. This device measures a form of light, gamma rays, and neutrons 12, which are particles in the centers of atoms. The detector measures the light and particles, showing scientists what kind of substances make up the surface of the objects. This device will also show if there is water ice on Ceres.

NASA had planned to launch Dawn in July. But now the spacecraft is set to launch in September.

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VOICE TWO:

Researchers spend millions of dollars on equipment and tools they need to observe the universe. They are able to see and study objects that are millions of light-years away. However, people can observe some very interesting things with their own eyes. Any person who wants to know about the night sky can go outside, look up, and see some extraordinary sights.


A Perseid meteor

This month, people everywhere have the chance to see the Perseid meteor shower. The meteors seem to come out of the group of stars called Perseus.

The Perseids come every year, near the middle of August. The meteor shower comes from dust left behind by the comet Swift-Tuttle. The comet takes about one hundred thirty years to go around the sun, and leaves dust in its path. When the Earth passes through this trail of material, the dust hits the Earth's atmosphere.

This dust is moving very fast, about sixty kilometers a second. Because the meteors are moving so fast, they make bright streaks 13 as they burn up in the atmosphere.

VOICE ONE:

The best way to see these meteors is to get away from cities with bright lights. People in big cities will be able to see the brightest meteors. But many, many more will be visible in the darkness of the countryside. The view this year will be especially good, because there will be very little moonlight.

There will be different kinds of meteors during different parts of the meteor shower. For example, there will be a few meteors called earthgrazers near the beginning of the shower. These appear early in the evening when the constellation 14 of Perseus first rises into the sky. Earthgrazers are meteors that show up on the horizon and then trail overhead. A NASA official described them as long, slow and colorful.

There will be more and more meteors overhead as the night goes on. This year, the most meteors were visible extremely early on August thirteenth. At that time, scientists say as many as one meteor per minute crossed the sky. However, the Perseids can be seen for about one week after this time as well. This is a great time to get outside and see astronomy in action.

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VOICE TWO:

Professional astronomers have collected pictures of one million galaxies in our
A picture of the galaxy 15 NGC 6070 from the Sloan Digital Sky Survey

universe. These pictures came from a project called the Sloan Digital Sky Survey. Now, the astronomers want to sort the galaxies they have found by shape.

However, there are too many galaxies for professional astronomers to look at and sort on their own. This would take a very long time. So they have asked the public for help. Astronomers want people around the world to help look at the pictures of the galaxies on a Web site and classify them into different kinds. All you need is a computer with connection to the Internet. This project is called Galaxy Zoo.

Astronomers want people instead of computers to help because some of the pictures are very hard to recognize. They say that people are much better at identifying these patterns and shapes. Some of the images of galaxies are unclear and the human eye is the best tool to help decide what shape a galaxy really is.

Scientists say that knowing how many of each kind of galaxy there are will help them understand more about our universe. The information will help answer questions about how galaxies are made and how they change over time. It will also help answer questions about how the universe works and moves.

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Project Galaxy Zoo was first announced on July eleventh. In the first sixty hours after the announcement, the Web site had been visited almost seven million times. So far, more than eighty-five thousand people are exploring the universe on the Internet. People have sent more than twelve million galaxy classifications. However, astronomers say that they need even more people to help.

Scientists want between ten and twenty different people to sort each of the one million galaxies. This will take a long time, even with so many people working on it. However, more and more people are signing up to look at these galaxies every day. When the results help scientists answer important questions about the universe thousands of people will be able to say, I helped. If you would like to join in, you can visit the Web site at www.galaxyzoo.org.

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VOICE TWO:

This program was written by Erin Braswell and Mario Ritter who was also the producer. I'm Bob Doughty.

VOICE ONE:

And I'm Barbara Klein. Join us again next week for Explorations in VOA Special English.