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difficult and treacherous challenges of robotic space exploration. Upon arrival at Mars, a spacecraft is traveling at velocities of 4 to 7 kilometers per second (km/s). For a lander to deliver its payload to the surface, 100 percent of this |
kinetic energy must be safely removed. Fortunately, Mars has an atmosphere substantial enough for the combination of a high-drag heat shield and a parachute to remove 99 percent and 0.98 percent respectively of the kinetic energy. Unfortunately, the Martian atmosphere |
is not substantial enough to bring a lander to a safe touchdown. This means that an additional landing system is necessary to remove the remaining kinetic energy. On previous successful missions, the landing system consisted of two major elements, a |
propulsion subsystem to remove an additional 0.002 percent (~50 to 100 meters per second [m/s]) of the original kinetic energy and a dedicated touchdown system. The first-generation Mars landers used legs to accomplish touchdown. The second generation of touchdown systems |
used air bags to mitigate the last few meters per second of residual velocity. The National Aeronautics and Space Administration (NASA) is currently developing a third-generation landing system in an effort to reduce cost, mass, and risk while simultaneously improving |
performance as measured by payload fraction to the surface and the roughness of accessible terrain. Legged Landing Systems The legs of the 1976 Viking mission lander represent the first-generation landing system technology (Pohlen et al., 1977). Basic landing-leg technology was |
developed for the lunar Surveyor and Apollo programs in the early 1960s. In conjunction with a variable-thrust liquid propulsion system and a closed-loop guidance and control system, legs represented an elegant solution to the touchdown problem. They are simple, reliable |
mechanisms that can be added to an integrated structure that houses the scientific and engineering subsystems for a typical surface mission. The first challenge for a legged system is to enable the lander to touch down safely in regions with |
rocks. For this the legs must either be long enough to raise the belly of the lander above the rocks, or the belly of the lander must be made strong enough to withstand contact with the rocks. Neither solution is |
attractive. Either the lander becomes top heavy and incapable of landing on sloped terrain or a significant amount of structural reinforcement must be carried along for the remote chance that the lander will directly strike a rock. The decreased stability |
because of the high center of mass is exacerbated if a mission carries a large rover to the surface. Because of the rover's configurational requirements, it is typically placed on top of the lander. The Soviet Lunokhod lunar landers are |
an excellent example of this type of configuration. A second major challenge of the legged-landing architecture is ensuring safe engine cutoff. To prevent the guidance and control system from inadvertently destabilizing the lander during touchdown, contact sensors have been used |
to shut down the propulsion system at the moment of first contact. On sloped terrain, this causes the lander to free fall the remaining distance, which can significantly increase the total kinetic energy present at touchdown and, in turn, decrease |
landing stability and increase mission risk. Implementation and testing of fault protection for engine cutoff logic has been, and continues to be, a difficult problem. The first in-flight problem associated with engine shut off occurred on the lunar Surveyor lander |
mission when the propulsion system failed to shut off at touchdown, resulting in a significant amount of postimpact hopping. Fortunately, the terrain was benign, and the problem was not catastrophic. The second in-flight problem occurred on the Mars 98 lander |
mission when the engines were inadvertently shut off prematurely because of a spurious contact signal generated by the landing gear during its initial deployment. This problem resulted in a catastrophic loss of the vehicle. As a result, the Apollo missions |
all reverted to a man in the loop to perform engine shut off. A third major challenge with a legged landing system for missions with rovers is rover egress. Once the lander has come to rest on the surface, the |
rover must be brought to the surface. For legged landers, a ramped egress system is the most logical configuration. Because rovers are bidirectional, the most viable arrangement has been considered two ramps, one at the front and one at the |
rear of the lander. The Soviet Lunokhod missions landed in relatively benign terrain, and in all cases, both ramps were able to provide safe paths for the rover. In the Mars Pathfinder mission, one of the two ramps was not |
able to provide a safe egress path for the Sojourner rover, but the second ramp did provide safe egress. For vehicles designed to explore a larger fraction of the Martian surface and, therefore, land in more diverse terrain, combinations of |
slopes and rocks could conceivably obstruct or render useless the two primary egress paths. Air-Bag Landing Systems The second-generation landing system was developed for the Mars Pathfinder mission and subsequently improved upon for the Mars Exploration Rover (MER) missions. These |
second-generation systems have a combination of fixed-thrust solid rocket motors and air bags to perform the touchdown task. The solid rocket motors, which are ignited two to three seconds prior to impacting the surface, slow the lander down to a |
stop 10 meters above the surface, from an initial velocity of approximately 120 meters/second. The lander is then cut away from the over-slung rockets and free falls for the remaining distance. The air-bag system, which was developed to reduce cost |
and increase landing robustness, is designed to provide omnidirectional protection of the payload by bouncing over rocks and other surface hazards. Because the system can also right itself from any orientation, the challenge of stability during landing has been completely |
eliminated. Because the lander comes to rest prior to righting itself, the challenge of rock strikes has been reduced to strikes associated with the righting maneuver, which are significantly more benign. The challenge of thrust termination, in this case cutting |
the lander away from the rockets, remains but has been decoupled from the problem of landing stability. The problems of rover egress were addressed systematically on the MER missions; a triple ramp-like system provided egress paths in any direction, 360 |
degrees around the lander. Although the air-bag landing system has addressed some of the challenges and limitations of legged landers, it has also introduced some challenges of its own. Horizontal velocity control using solid rockets and air-bag testing were significant |
challenges for both the Mars Pathfinder and MER missions. The Sky-Crane Landing System As Mars surface explorations mature, roving is becoming more important in the proposed mission architectures. The MER missions demonstrated the value of a fully functional rover not |
reliant on the lander to complete its surface mission. In the 2009 Mars Science Laboratory (MSL) and other future missions, the rover's capabilities and longevity will be extended. Future missions are also being designed to access larger areas of the |
planet and, therefore, will require more robust landing systems that are tolerant to slope and rock combinations that were previously considered too hazardous to land or drive on. The third-generation landing system, the sky-crane landing system (SLS), currently being developed |
for the MSL mission, will directly address all of the major challenges presented by the first- and second-generation landing systems. It will also eliminate the problem of rover egress. SLS eliminates the dedicated touchdown system and lands the fully deployed |
rover directly on the surface of Mars, wheels first. This is possible because the rover is no longer placed on top of the lander. In the SLS, the propulsion module is above the rover, so the rover can be lowered |
on a bridle, similar to the way a cargo helicopter delivers underslung payloads. The landing sequence for future missions will be similar to the Viking mission, except for the last several seconds when the sky-crane maneuver is performed. After separating |
from the parachute, the SLS follows a Viking-lander-like propulsive descent profile in a one-body mode from 1,000 meters above the surface down to approximately 35 meters above the surface. During this time, a throttleable liquid-propulsion system coupled with an active |
guidance and control system controls the velocity and position of the vehicle. At 35 meters, the sky-crane landing maneuver is initiated, and the rover is separated from the propulsion module. The rover is lowered several meters as the entire system |
continues to descend. The two-body system then descends the final few meters to set the rover onto the surface and cut it away from the propulsion module. The propulsion module then performs an autonomous fly-away maneuver and lands 500 to |
1,000 meters away. The central feature of the SLS architecture is that the propulsion hardware and terrain sensors are placed high above the rover during touchdown. As a result, their operation is uninterrupted during the entire landing sequence. One important |
result of this feature is that the velocity control of the whole system is improved, and, therefore, the rover touches down at lower velocity. Thus, there is no last-meter free fall associated with engine cutoff, and, because dust kick-up is |
minimal, the radar antennas can continue to operate even while the rover is being set down on the surface. The lower impact velocity has two effects. First, the touchdown velocities can now be reliably brought down to the levels the |
rover has already been designed for so it can traverse the Martian surface. Second, the low velocity, coupled with the presence of bridles until the rover's full weight has been transferred to the surface, results in much more stability during |
landing. Because the rover does not have to be protected from the impact energy at landing and because there is no need to augment stability at landing, there is no longer a need for a dedicated touchdown system. This, in |
turn, eliminates the need for a dedicated egress system. The SLS takes advantage of the fact that the rover's mobility system is inherently designed to interact with rough, sloping natural terrain. Rovers are designed to have high ground clearance, high |
static stability, reinforced belly pans, and passive terrain adaptability/conformability. These are all features of an ideal touchdown system. Touchdown sensing can be done in several ways. The simplest and most robust way is to use a logic routine that monitors |
the commanded up-force generated by the guidance and control computer. The landing sequence is specifically designed to provide a constant descent velocity of approximately 0.75 m/s until touchdown has been declared. Prior to surface contact, the commanded up-force is equal |
to the mass of the rover plus the mass of the descent stage (which are roughly equal) times the gravity of Mars. During the touchdown event, the commanded up-force fluctuates depending on the specific geometry of the terrain. Once the |
weight of the rover has been fully transferred to the surface of Mars, the commanded up-force takes on a new steady-state value equal to the mass of the descent stage times the gravity of Mars, approximately one-half of its pretouchdown |
magnitude. The system declares touchdown after the new lower commanded up-force has lasted for at least 1.5 seconds. This approach provides an unambiguous touchdown signature without the use of dedicated sensors. The fly-away phase of the landing sequence is initiated |
when touchdown has been declared. During the fly-away phase, separation of the rover is accomplished by the pyrotechnic cutting of the bridle and umbilical lines connecting the rover and descent stage. The descent stage then uses its onboard computer to |
guide the propulsion module up and away from the rover and land it several hundred meters away. As Mars explorers have learned the hard way, it's not typically the fall that kills you, it's the landing. Landing technology has matured |
significantly in the 40 years since NASA began exploring extraterrestrial surfaces. Each generation of landing technology has attempted to resolve the challenges posed by the previous generation. The SLS represents the latest stage in that evolution. The research described in |
this paper was carried out at the Jet Propulsion Laboratory of the California Institute of Technology under a contract with the National Aeronautics and Space Administration. From The Bridge, Volume 34, Number 4 - Winter 2004 Pohlen, J., B. Maytum, |
Prairie Resurgence in the Midwest Suburban sprawl meant the introduction of lawn monoculture: perfectly cut, well-manicured lawns that became a part of pride for many American homeowners. However, in the Midwest, a new lawn resurgence is occurring: restoring yards to the native prairies that existed in pre-settlement days. In an |
effort to manage yards and fallow farmland succumbing to invasive shrubs, more and more people are spending the time and resources to turn their property into the native ecosystem that once ruled the land. This practice is not only attracting more wildlife to areas, but it is changing the way |
people maintain their yards, as prairies require less watering and fertilizer, and no mowing! Prior to colonial settlement, prairie took over central North America, making up nearly 600,000 square miles of grassland. This complex ecosystem was home to a diverse and teeming web of life, including now-tattered bison populations. However, |
farming and development have reduced much of this landscape. Tall-grass prairie habitats, a habitat dominated by grasses that can grow eight feet high, now occupies less than 1 percent of its former range, putting it among the world’s most endangered ecosystems, according to the U.S. National Park Service. Government agencies |
and conservation groups, aided by volunteers, have undertaken numerous restoration projects across U.S. and Canadian prairieland, some of them thousands of acres in scale. In recent years, we are seeing more and more private citizens joining in, restoring prairie to their own properties, from city yards up to 100 acres |
or more around rural homes and farms. In some cases they've re-created prairie where it never was before — on land that was originally forest or wetlands before settlers plowed it for crops. Federal, state, and local programs offer financial and technical assistance, particularly for larger private projects on agricultural |
land, conservation groups also offer some help, and an industry of consultants, contractors, and native-plant nurseries has arisen for landowners who can't do it all themselves. Bigger prairies obviously offer more wildlife habitat, and connected ones allow species to spread over larger territories, preventing gene-pool stagnation, say experts. But even |
small patches or yards count as pocket refuges for native wildlife that may have few alternatives. While prairie restoration may not be successful in all parts of the country, returning our lawns to native habitats that may have existed before development will inevitably be better for the local wildlife populations. |
Also, creating native habitats in our backyard will reduce the costs we spend on watering and fertilizing any nonnative species. Read more at Yale Environment 360. Prairie image via Shutterstock. |
The Industry's Myths Reports & Consumer Guides Pesticide Industry Propaganda: The Real Story: The Industry's Myths The chemical and food industries care about the bottom line, even if that means fostering myths and distorting science to convince the public and policy makers not to regulate pesticides. The truth is that |
animal tests are valid predictors of human cancer and other health risks, that we can grow affordable food with far fewer pesticides, and that pesticide residues in our diet pose an unnecessary--and preventable--risk to which children are particularly vulnerable. Myth #1: Animal tests of pesticides don't predict human cancer risks |
because the high doses fed to animals are irrelevant to the low doses consumed by humans. What's the real story behind Myth #1? Myth #2: The amount of pesticide residues in food and water is so small as to pose no health risks -- expressed as "You have to eat |
340 oranges a day to get the dose causing health problems in animal tests." What's the real story behind Myth #2? Myth #3: We're winning the war against cancer. What's the real story behind Myth #3? Myth #4: Nobody has ever been hurt by exposure to pesticides at the low |
doses found in food and water. What's the real story behind Myth #4? Myth #5: Natural carcinogens in food are more dangerous than pesticides. What's the real story behind Myth #5? Myth #6: Alar on apples was a "scare," reflecting environmentalists' use of emotion and scare tactics, not sound science. |
What's the real story behind Myth #6? Myth #7: Restricting the use of pesticides will cause food shortages and raise the price of food. What's the real story behind Myth #7? Myth #8: Pesticides cost money, so farmers currently use as few pesticides as possible. What's the real story behind |
"Reading Without Limits" is a newly published book written by Maddie Witter. Witter is a founding teacher at KIPP Infinity Charter School in New York City. As a result of |
her work in this school, she understands what it is like to teach in a high poverty area with many struggling readers. It can often be difficult to find reading |
resources written by teachers who are familiar with this setting. While many professional development books for teachers offer countless suggestions that have worked for the authors, it is refreshing to |
read a book that is written specifically for struggling readers. The beginning introduction to KIPP Infinity Charter School draws in the reader. It makes it apparent that Witter is knowledgeable |
about what she is writing about. It is both inspiring and motivating for teachers. This background helps the reader see how the strategies supplied by Witter will be meaningful in |
their classroom. Witter offers countless ideas that engage and teach children. She emphasizes the importance of developing students who have a desire to read and the ability to read on |
or above their grade level. There are suggestions for what aspects of reading instruction should be incorporated into the classroom. The suggestions are ranked as mild, medium and spicy. The |
mild ideas are ideas that you can read about today and be ready to use in your classroom tomorrow. The medium ideas require a little planning and the spicy ideas |
require more in-depth planning. This book is suitable for teachers from elementary to middle school (and maybe even teachers of struggling high school students. This is a resource that will |
The physicians at ITEC are trained in cosmetic surgery of the eyelids including blepharoplasty and ptosis surgery. Blepharoplasty is a surgical procedure that removes excess fat, skin and muscle to change the appearance of the upper and/or lower eyelids. People of all ages choose to have eyelid surgery either to |
correct effects of aging or change a dissatisfactory aspect of the eyelids’ appearance. Usually sagging upper eyelids or bags under the eyes are the main reasons to undergo eyelid surgery. Ptosis is a condition where the patient has drooping eyelids or brows. There are two types of ptosis, congenital and |
acquired. Ptosis that is present at birth is called congenital ptosis. Although its cause is often unclear, the most common reason is improper development of the levator muscle, a major muscle responsible for elevating the upper lid. Children with significant ptosis may need to tilt their head back, lift their |
eyelid with a finger, or raise their eyebrows to see from under a drooping lid. Acquired ptosis is most commonly due to stretching of the levator muscle in the eyelid and may occur as a result of aging, trauma or muscular or neurologic disease. Myasthenia Gravis, a disorder in which |
the muscles become weak and tire easily, may first be detected by the presentation of acquired ptosis. Both blepharoplasty and ptosis surgeries are usually performed on an outpatient basis under local or general anesthesia. Trust your cosmetic eye surgery to a fully qualified eye surgeon. |
Quellinus or Quellin, Artus (ärˈtəs kvĕlēˈnəs, kvĕlˈĭn) [key], 1609–68, Flemish sculptor. His allegorical figures decorating the royal palace of Amsterdam are famous. Quellinus was the outstanding follower of the Rubens tradition in sculpture. In addition to his baroque architectural decorations, he made numerous fine busts and figurines. His cousin Artus Quellin, 1625–1700, worked under his direction at Amsterdam. The younger man soon showed an |
independent talent in works that he created for the cathedrals of Antwerp and Tournai. The refined elegance of his early art developed into a dramatic baroque illusionism in the figure of God the Father (1682) for the rood screen of the cathedral of Bruges. The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved. See more Encyclopedia articles on: |
Waldeck (välˈdĕk) [key], former principality, central Germany, now an administrative district (c.420 sq mi/1,090 sq km) of Hesse. Arolsen was the capital. An agricultural region, hilly and forested, it is drained by the Eder and Diemel rivers. A county of the Holy Roman Empire from c.1200, Waldeck was united with the county of Pyrmont in the late 17th cent., and |
its rulers were later raised (1712) to princely rank. In 1867 the prince of Waldeck-Pyrmont renounced most of his sovereign prerogatives in favor of Prussia; the title, however, continued in the family. Waldeck-Pyrmont became a republic in 1918. By a plebiscite in 1922, Pyrmont passed to Prussia and was incorporated into Hanover prov.; by another plebiscite (1929) Waldeck proper became |
part of the Hesse-Nassau prov. of Prussia. After World War II it was made part of Hesse. The Columbia Electronic Encyclopedia, 6th ed. Copyright © 2012, Columbia University Press. All rights reserved. More on Waldeck from Fact Monster: See more Encyclopedia articles on: German Political Geography |
8th Grade Literature November 12, 2007 |A pathfinder is just what the name implies. It is a guide to find your "path" when researching a particular subject. All the resources |
listed help you find the answers to questions you ask during your research project.| This pathfinder is particular to the study of the Holocaust during World War II in Nazi |
Germany. Millions of Jews were imprisoned, displaced and killed. This pathfinder is designed to help you find out what happened, how it happened, and the stories of the people who |
survived and know what really happened. When you search the OPAC in the Farragut School Library, some search terms to use are: - Holocaust, 1939-1945 --Personal Narratives - Holocaust survivors |
-- Biography - World War, 1939-1945 - Atrocities - World War, 1939-1945 - Jews - World War, 1939-1945 - Germany - Search events like Kristallnacht, etc. - Search for people |
like Oscar Schindler, Elie Wiesel, Anne Frank, Rudolph Hess, Adolph Hitler Your search should produce many resources you can choose.Your search will also list some web sites of Electronic Accesses |
to visit. If you are browsing the shelves, look at the non-fiction section numbered 940.53. General OnLine Resources World Book good basic information about the Holocaust AP Photo Archive photos |
of the scenes of the Holocaust EBSCOHost Web magazine and newspaper articles Recommended Holocaust Websites: Learn about the United States Holocaust Museum in Washington, D.C. This is the official museum |
site. Follow a group of teenagers through the Museum as they discover with the soldiers the dying and starving people they found after the war in the prison camps, the |
piles of human hair, model of the gas chambers, and a railroad freight car. Read the biography of Elie Wiesel who survived the concentration camps. A detailed biography of three |
teenagers who found their own ways to fight the Holocaust. This site also offers an interactive timeline of the Holocaust. The author of many books about the Holocaust tells of |
his survival in Auschwitz even though his entire family perished. Investigates answers to questions about the Holocaust, camps, victims and Holocaust Museum. Gather information about the camps, railway cars, tattoos, |
and badges. This site also details the Holocaust remembrances but also investigates the design of the Museum. It includes artifacts, photographs and films. This site includes historical facts and stories |
about the Holocaust and includes many authentic photographs. Ten survivors detail their lives before, during and after the Holocaust. They tell about how things gradually changed when the Nazis came |
to power. Survivors discuss the direction their lives took after the Holocaust. Stories of children who survived the Holocaust and death camps. Rescuer of over 1200 Jewish people, Schindler is |
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