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STEVE EMBER: I'm Steve Ember.

SHIRLEY GRIFFITH: And I'm Shirley Griffith with EXPLORATIONS in VOA Special English. Today we tell about an American spacecraft that has a date with a comet. We hear the latest about a space shuttle commander whose wife continues to recover from a deadly gun attack. And we remember a sad anniversary. But first, the discovery of possible planets like our own.

STEVE EMBER: Last week, American space agency scientists announced the discovery of possible Earth-like planets. The announcement came from newly released information from the Kepler space-based telescope.

NASA scientists say the Earth-like planets are among 1235 possible planets orbiting other stars that have been discovered so far.

The researchers say six new planets have been confirmed. But the Kepler mission's chief scientist, William Borucki, says 80 percent of the possible planets will probably be confirmed in the coming months and years.

Before last week's announcement, the total number of so-called exoplanets outside our solar system was just over 500. Mr. Borucki says that number increased, based on new information from the small part of the sky examined by the Kepler telescope.

WILLIAM BORUCKI: "Kepler looks at one-400th of the sky. If we had 400 of these fields of view, we would see 400 times that number of candidates."

SHIRLEY GRIFFITH: Mr. Borucki notes that they have found many possible planets in a small part of the sky. This suggests that there are countless planets orbiting stars like our sun in our galaxy. He says there must be millions of planets orbiting the stars that surround our sun.

Mr. Borucki says he was surprised to find 68 suspected planets about the size of Earth or smaller. Fifty-four of the possible exoplanets are in so-called habitable areas with moderate temperatures where liquid water could exist on the planet's surface. He says some of the possible planets could even have moons with liquid water. And he says five of the planetary candidates are both close to the size of Earth and orbit in the habitable area of their parent stars.

STEVE EMBER: NASA scientists using the Kepler space telescope also announced the discovery of six planets orbiting a star called Kepler-11. Investigator Jack Lissauer described the discovery.

JACK LISSAUER: "Kepler 11 is a surprising flat and compact system of six transiting planets. The five inner planets are especially close together, something that we didn't think would happen for worlds of this size."

NASA says Kepler-11 has the fullest, most compact planetary system yet discovered beyond our own. Mr. Lissauer says the five inner planets are among the smallest confirmed planets beyond our solar system. He says they are mixtures of rock and gases, possibly including water ice.

The Kepler space telescope looks for planets by measuring tiny decreases in the brightness of stars when planets cross in front of them. Scientists say extensive observations from earth-based telescopes are needed to confirm the existence of the planets.

Yale University astronomer Debra Fischer says non-professional astronomers around the world are helping study the new information about the possible planets. They are doing this on an Internet website called PlanetHunters.org.

(MUSIC)

SHIRLEY GRIFFITH: A spacecraft from the United States space agency has a date with comet Temple 1 on February 14th. The Stardust-NExT spacecraft is to meet the comet on Valentine's Day. This is the day when many people will be going out on a date with someone special in their lives.

This meeting is all the more meaningful because comet Temple 1 has been visited before. And scientists are very interested in seeing how this solar system body has changed over the years.

The goal of the Stardust-NExT spacecraft is to gather scientific evidence about one of the solar system's most changeable objects -- comets. Comets are balls of ice and rock that leave behind a trail of gas and dust in space as they approach the sun's warming light.

Sometimes, observers see this as a comet's tail, which can be visible to the unaided eye stretching across the night sky.

On February 14th, Stardust will approach to within 200 kilometers of comet Temple 1. The spacecraft will take 72 high quality pictures of the comet's nucleus, which is about six kilometers across. But this is not the first time that Temple 1 has been observed closely.

STEVE EMBER: In 2005, another NASA spacecraft called Deep Impact paid a visit to Temple 1. It released a small device, or probe, that crashed into the comet. This released huge amounts of material into space.

The main Deep Impact spacecraft was able to send back information to scientists on Earth. They found evidence of carbon-based chemicals, sand and, most importantly, water ice. Scientists will have a second look at comet Temple 1 with Stardust. This double take is something new for comet experts.

SHIRLEY GRIFFITH: Joe Veverka is principal investigator of Stardust-NExT. He says it will give scientists their first chance to see how comets change with each close approach to the sun.

JOE VEVERKA: "We know that comets lose material, but the question is, 'How much does the surface change and where does the surface change?' So we'll be able to answer that question by comparing our images with those taken by Deep Impact in 2005."

STEVE EMBER: Astronomers have known about Comet Temple 1 for a long time. They discovered it in 1867. They have observed it on most of its returns to the inner solar system since then. It is a short-period comet. This means it orbits the sun in a relatively short time. For Temple 1, that is about five and a half years.

Where does the comet go when it is not close to the sun? Short-period comets like Temple 1 have orbits that take them only as far as the outer planets like Jupiter. In fact, Temple 1 is a member of the Jupiter family of comets. These are all influenced by the gravity of the solar system's largest planet. Long-period comets, however, may take several hundred years to orbit the sun.

SHIRLEY GRIFFITH: Scientists have described comets as leftovers from the creation of the solar system. Some scientists believe comets brought water and carbon compounds to Earth. That means they may have supplied the Earth with the basic building blocks of life nearly four billion years ago. That is a big reason why scientists are so interested in them.

(MUSIC)

STEVE EMBER: Late last week, NASA said astronaut Mark Kelly would return to train with crewmembers on the last flight of the space shuttle Endeavor in April. Mark Kelley is the husband of United States Representative Gabrielle Giffords of Arizona. A gunman shot her in the head on January 8th at a political gathering in Tuscon, Arizona. Six people were killed in the attack, including a young girl and a federal judge.

Gabrielle Giffords has since been moved to a hospital in Houston, Texas. Astronaut Mark Kelly trains at the Johnson Space Center in that city.

Recently Mr. Kelly said he plans to command the final flight of the Endeavor. He said he hopes his wife Gabrielle Giffords will be at the launch. Mr. Kelly's brother, Scott Kelly, is the current commander of the International Space Station. NASA plans to retire the space shuttle program this year.

(SOUND)

SHIRLEY GRIFFITH: On January 28th, 1986, tragedy struck NASA's Space Shuttle program.

(SOUND)

It was only 73 seconds into the 25th flight for the program. A problem with one of its booster rockets caused space shuttle Challenger to explode. Seven astronauts were killed. Many Americans clearly remember the event as if it were yesterday. Students were watching the launch from their classrooms. One of the astronauts was Christa McAuliffe, a school teacher.

NASA grounded the space shuttle program for more than two years while investigators tried to find out what happened. The space agency developed new safety rules as a result of the accident. NASA went on to launch more than 100 shuttle flights following the Challenger disaster. But in 2003, another accident on the space shuttle Columbia claimed the lives of seven more astronauts.

STEVE EMBER: Valerie Neal supervises the human spaceflight collection at the Smithsonian National Air and Space Museum in Washington, DC. She says because spaceflight is experimental, it continues to be risky. But Ms. Neal says the United States space program always seems to recover from hardship.

VALERIE NEAL: "The fact, though, that the space shuttle program didn't close down, that we didn't close up shop and say, 'This is too dangerous. We're not going to do it anymore,' I think is a tribute to the American people and the American spirit."

SHIRLEY GRIFFITH: This program was written and produced by Mario Ritter. I'm Shirley Griffith.

STEVE EMBER: And I'm Steve Ember. Join us again next week for EXPLORATIONS in VOA Special English.

space shuttle: a spacecraft designed to be used, for example, for travelling between the earth and a space station 航天飛機；太空梭

exoplanet: a planet that orbits a star outside the solar system 【天】（太陽系以外的）外部行星

Related stories:

Space shuttle endeavour heads to International Space Station

Remembering the X-15, America's first step in manned space flight

NASA's Kepler satellite to seek distant Earth-like planets

新發現：有顆類地行星適宜生物存活

（來源：VOA 編輯：崔旭燕）