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I'm Steve Ember.

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And I'm Barbara Klein with EXPLORATIONS in VOA Special English. Today we tell about the latest exploration of the red planet, Mars. We learn about the travels of the Mars Rover vehicles. And we find out about the Mars Global Surveyor and Mars Odyssey as they map extraordinary places.

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Imagine a land were the average temperature is about sixty-seven degrees below zero. Iron-rich rocks cover the landscape. There is no water to be seen anywhere. Powerful windstorms blow huge amounts of sand hundreds of kilometers, changing the landscape from year to year. Above, two small moons quickly cross the sky like distant airplanes among the stars.

This world is the planet Mars. Once Mars was a place known only in the pages of science fiction stories. But today Mars is being explored on its surface and from above. The computer and robotic technology of four space vehicles developed by the American space agency, NASA, has opened Mars to science.

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NASA's Mars Rovers are land vehicles able to move and think for themselves. The two rovers are exactly the same. And it is easy to think of them as living things.

They each weigh about one hundred seventy kilograms. And they have computer brains. NASA says they are similar in memory and ability to a well equipped personal computer.

Each rover also has eyes -- nine of them. Six cameras on the body help each rover avoid dangers like rocks and holes.

Two other cameras are placed on a structure that NASA engineers call the mast. The mast serves as the neck and head of the vehicle. It carries cameras that work together to give the rover depth perception. That means they see the world much as people do. With two cameras working together, a rover can tell how far away objects are. And it can judge distance between objects. This is called stereoscopic vision.

Even the area that can be seen by one of these navigation cameras, its field of view, is similar to human vision. Two other science cameras give each rover color vision. And each also has a microscope camera able to produce pictures of soil and rock particles.

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The rovers are valuable to planetary scientists because they can move around on the Martian surface. Each rover has six twenty-five centimeter wheels. Each wheel has its own motor.

The rovers will not set any speed records. They can reach a top speed of five centimeters a second. But they rarely reach even that. This is because the rovers are designed to travel for ten seconds, then stop. The rovers then use their cameras to look around for twenty seconds before moving forward again. The most the rovers can travel on a Martian day is about one hundred meters.

Scientists designed the rovers for this slow, careful way of traveling. Mars is so far away that it can take up to twenty minutes for a radio signal from Earth to reach it. So, NASA engineers had to design the rovers to think for themselves sometimes, without help from controllers on Earth.

The rovers' computers have to make control decisions every day. One error in a mountainous or rocky area and the three hundred twenty million dollar vehicles could be severely damaged or destroyed.

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So far, each Mars rover has developed a highly personal story. The rover Spirit landed inside a low-lying area called Gusev Crater on January fourth, two thousand four. The vehicle was expected to survive for ninety Martian days. A day on Mars is about forty minutes longer than a day on Earth.

But Spirit is a fighter. Late last month, it spent its one-thousandth Martian day on the red planet. At first, Jet Propulsion Laboratory scientists operating the rover worried that Spirit might not survive the extreme Martian winter this year.

Last March, one of Spirit's six wheels stopped working. Now, when the vehicle moves, the other five wheels drag the sixth wheel in the sand.

NASA also reported having trouble getting Spirit to move into position so its solar energy collectors could face the sun. The solar panels gather the electricity necessary to run the rover's scientific instruments and computers.

But, Spirit was able to turn to the north. In that position, Spirit has been able to collect enough energy to keep operating. And that is not easy in the Martian environment. During the coldest Martian months, nighttime temperatures can drop to one hundred degrees below zero.

Even with its troubles, Spirit has traveled almost seven kilometers over the Martian surface. It is currently in a resting position. It is taking pictures and waiting for the Martian spring.

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NASA's Mars rover Opportunity landed shortly after Spirit, on January twenty-fifth, two

thousand four. It has traveled more than nine kilometers on the Martian surface. It has not had the technical difficulties suffered by Spirit. Last month, Opportunity reached a new place on Mars that is more beautiful and interesting than any it has yet visited. Victoria Crater is a huge round hole about seven hundred fifty meters across. It is seventy meters deep. Victoria Crater is the biggest landmark that Opportunity has visited. It is several kilometers from where Opportunity landed.

Scientists are interested in Victoria Crater because its walls cut deep into the Martian surface showing many levels of rock. Opportunity's special cameras will let scientists studying Martian geology examine material they could not have seen any other way. One scientist called the arrival at Victoria Crater a dream come true.

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From October eighteenth to the twenty-ninth, all Mars spacecraft were cut off from Earth. But this was not because of a technical failure.

Every two years, the Earth reaches a point in its orbit where the sun is between it and Mars. This event is called inferior conjunction, meaning that Mars is behind the sun. It is a period when no communication with any Mars vehicles is possible for almost two weeks.

For the rovers, that meant no traveling. Instead, NASA scientists ordered the robots to gather scientific information and to take pictures of their surroundings.

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It could be said that the two robotic rovers are making their mark on the surface of Mars. That has been confirmed by one of the spacecraft in orbit around the red planet.

On October ninth, NASA release a picture taken by the Mars Reconnaissance Orbiter. It reached orbit around Mars in March of this year. The orbiter used its High Resolution Imaging Science Experiment camera to take a picture of the Mars rover Opportunity from space.

The image is extremely detailed. The picture not only shows Opportunity. It even shows marks the rover made in the sand as it approached Victoria Crater.

NASA hopes the Mars Reconnaissance Orbiter will show areas on the surface that have been shaped by running water. Scientists who study the geology of Mars are now seeing surface structures that could not be seen with other Mars spacecraft.

And the best images are still to come. The Mars Reconnaissance Orbiter starts the main part of its detailed mapping mission this month.

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The Mars Reconnaissance Orbiter is not the only spacecraft observing Mars from space. The Mars Odyssey continues to take pictures of the surface. These images are being used to make improved maps.

Next month, Mars Odyssey will also begin taking pictures meant to give a three-dimensional view of the surface. The orbiter will do this by taking pictures of the same area from different places in its orbit.

Mars Odyssey has been in orbit around the red planet since October two thousand one. Since then it has provided the best maps of Mars yet. Odyssey also shares a close link with the rover vehicles. The maps the orbiter provided helped NASA choose the landing area for both Mars rovers. And Odyssey is the main communications link between the rovers and Earth. NASA says the orbiter will support future landing missions on Mars as well.

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This program was written and produced by Mario Ritter. I'm Steve Ember.

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