Devoir de Philosophie

Mars Pathfinder - astronomy.

Publié le 11/05/2013

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Mars Pathfinder - astronomy. I INTRODUCTION Mars Pathfinder, United States spacecraft that landed on the planet Mars on July 4, 1997. Pathfinder was the first U.S. Mars lander since the Viking probes of the 1970s and the first in a series of small, inexpensive spacecraft scheduled to reach Mars every 26 months until 2005. The lander was a closed container with four triangular sides until it landed on Mars. On the planet's surface it opened up, exposing three solar panels (see Solar Energy), instruments, and a small, wheeled rover called Sojourner. Mars Pathfinder tested technologies for future robotic Mars exploration and explored its Martian surroundings. Pathfinder and Sojourner studied rocks and Martian weather at the landing site in an ancient flood channel called Ares Vallis to gather clues to Mars's geologic and climatic history. Pathfinder's instruments sent back vast amounts of data and the camera aboard the lander provided the first panoramic three-dimensional photos of a Martian landscape. The mission received a huge amount of attention from the public and press, especially during the first weeks after it landed. On July 7, 1997, the World Wide Web site that the National Aeronautics and Space Administration (NASA) dedicated to Mars Pathfinder was visited more than 47 million times. Pathfinder sent signals from Mars until October 7, 1997, returning more than 2.6 gigabits of data, including 16,000 photos. Sojourner analyzed the chemical composition of 15 different Ares Vallis rocks. II SPACECRAFT During its flight to Mars, Pathfinder's three solar panels remained folded to form a closed, four-sided container. The panels enclosed Sojourner, a small rover named for Sojourner Truth, a 19th-century African American antislavery crusader; the word sojourner means "traveler." Sojourner's wheels were retracted close to its body as the rover knelt on one of the lander's solar panels during the flight to Mars. At launch, the whole Pathfinder spacecraft weighed 890 kg (1962 lb). Pathfinder's petal-like blue solar panels opened after the spacecraft landed, cushioned by airbags, on Mars. The panels converted sunlight into electricity for Pathfinder's systems. On Mars, Pathfinder stood 1.5 m (5 ft) tall and had a mass of 360 kg (793 lb). Extendable masts held weather instruments and Pathfinder's color camera, the Imager for Mars Pathfinder (IMP). The camera, Pathfinder's main scientific instrument, could simultaneously take two photos from slightly different viewpoints to produce three-dimensional images. Scientists used it to photograph windsocks on a mast to determine the direction of Martian winds. IMP also took pictures of dust attracted by magnets aboard Pathfinder. Pictures of the sun helped scientists learn more about Mars's atmosphere. Targets on Pathfinder provided color references for calibrating IMP. The rover Sojourner had six wheels, a mass of 10 kg (22 lb), and measured 65 cm (26 in) long by 48 cm (19 in) wide and 30 cm (12 in) high. A rectangular solar panel on top provided electricity. The rover carried the Alpha Proton X-ray Spectrometer (APXS) device for determining the composition of rocks. APXS aimed radiation in the form of alpha particles (helium atoms with two neutrons and no electrons) at a rock, then measured the radiation (mostly X rays) that bounced back. Different elements react to alpha radiation in different ways, so scientists could determine which elements were present in the rock by analyzing the results. Signals from Sojourner took several minutes to travel to Earth and back, so controllers could only see where the rover had been a few minutes before. Controllers on Earth used pictures taken by the lander to plan the rover's travels at the beginning of every day. The rover could use its own camera and laser system to avoid obstacles and find its own path to a target rock. III MISSION Mars Pathfinder lifted off on a Delta II rocket on December 4, 1996. During the seven-month flight, Pathfinder was attached to a cruise stage rocket for course corrections. On July 4, 1997, Pathfinder dropped the cruise stage and entered Mars's atmosphere. About 8 km (about 5 mi) above Mars its parachute opened. The parachute detached 21 m (69 ft) above Mars, and Pathfinder, protected by airbags, hit the surface at 70 km/h (40 mph). The lander bounced for 1 km (0.6 mi) before stopping in pre-dawn darkness. The airbags deflated and Pathfinder's solar panels opened, exposing Sojourner. After sunrise at Ares Vallis, the IMP took its first pictures. Sojourner stood up and rolled onto the Martian surface on July 5. The rover then moved about 40 cm (about 16 in) to a rock that controllers nicknamed Barnacle Bill, which it analyzed using APXS. Meanwhile, IMP photographed the Twin Peaks, a pair of hills about 1.6 km (about 1 mi) away, as part of a 360-degree panorama of Ares Vallis. On July 10 the Mars Pathfinder navigation team determined that the spacecraft had landed less than 1 km (less than 0.6 mi) from its target. Sojourner studied the composition of a boulder called Yogi and used its wheels to dig into the Martian surface. Pathfinder's primary mission ended on August 3. Pathfinder and Sojourner were designed to last 30 days and 7 days, respectively, but they both continued functioning. Controllers had to take special measures, however, to prolong the life of Pathfinder's weakening battery. In late August Sojourner entered the Rock Garden, a rocky area about 10 m (about 33 ft) from the lander, where it successfully found its own way around obstacles and measured the composition of several rocks. On September 26, with the lander battery down to 30 percent capacity, Sojourner set out on a 50-m (165-ft) trek around Pathfinder. However, on September 27 the lander stopped returning data to Earth. Pathfinder sent a signal to Earth for the last time on October 7, after 92 Martian days at Ares Vallis. Engineers believe that the onset of Martian autumn caused Pathfinder's temperature to fall below the point where its transmitter could operate. Sojourner depended on Pathfinder to relay its radio signals to the earth. The rover was programmed to return to and circle the lander if it did not hear from it for five days. If it functioned as planned, Sojourner started back toward Pathfinder on October 6. On November 4, NASA announced that it would scale back its efforts to contact Pathfinder. IV SCIENTIFIC RESULTS Pathfinder data revealed that Ares Vallis was more varied than either site visited by the Viking landers in the 1970s. The Twin Peaks, for example, were the first hills seen from Mars's surface. Ares Vallis has a soil chemistry very similar to the Viking 1 and 2 sites. Sojourner's APXS instrument found that some of the rocks that the rover studied are possibly young volcanic rocks, a sign that Mars has been geologically active in the recent past. The distribution of rocks at the site indicated that they were laid down by a giant flood. Pathfinder also photographed rocks that might have formed in or been shaped by water, providing new evidence that cold, dry Mars had a warm, wet past. Pathfinder detected sand dunes, wind-carved rocks, and a dozen dust devils, confirming the important role of wind and dust in Martian geology and weather. Scientists tracked Pathfinder on Mars's surface as the planet rotated and orbited the sun. By tracking Pathfinder, scientists learned more about irregularities in Mars's orbit and rotation, and from that data, they learned about Mars's composition and structure. Using Pathfinder data, scientists estimated Mars's core as being between 1300 and 2000 km (800 and 1250 mi) across. Pathfinder and Sojourner also gathered engineering data for planning future Mars missions. Pathfinder entered Mars's atmosphere without first stopping in Mars orbit, an approach likely to be used by future landers, and its previously untried airbag system performed flawlessly. Sojourner needed about three months to traverse less than 90 m (less than 300 ft), but demonstrated that rovers can explore Mars. Engineers predict that the next generation of Mars rovers will be able to travel thousands of kilometers, and that they could assist human Mars explorers in the 21st century. Contributed By: David S. F. Portree Microsoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation. All rights reserved.

« Pathfinder and Sojourner also gathered engineering data for planning future Mars missions.

Pathfinder entered Mars's atmosphere without first stopping in Mars orbit,an approach likely to be used by future landers, and its previously untried airbag system performed flawlessly.

Sojourner needed about three months to traverse lessthan 90 m (less than 300 ft), but demonstrated that rovers can explore Mars.

Engineers predict that the next generation of Mars rovers will be able to travel thousandsof kilometers, and that they could assist human Mars explorers in the 21st century. Contributed By:David S.

F.

PortreeMicrosoft ® Encarta ® 2009. © 1993-2008 Microsoft Corporation.

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