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Priya wants one shelf in the town museum to show what it takes for a person to leave Earth and come back alive . Visitors will not stay for a lecture . A shelf that holds every machine ever launched will teach nothing . The first object is a rocket , because nothing else matters if you cannot get up . A launch rocket burns liquid or solid fuel and throws hot exhaust downward until the push beats Earth's pull . For a long time most of those rockets flew once . Some now return and land upright , so the same vehicle can fly again . Priya chooses a model of that kind . Landing again only means the ride up is not thrown away . Beside it she puts a small satellite , the part of space people already use . Some relay phone calls , television , and internet . Some tell a ship or a car where it is . Some watch the weather . They also leave junk . Dead machines and loose parts stay in orbit , and two of them can hit with no one there to clear the pieces . The card gets one sentence about debris . The large photograph is the International Space Station , a shared laboratory in low Earth orbit . A crew has been on it , one group after another , for more than twenty years . She writes why exercise is daily : when the body has no normal weight , muscles and bones grow weaker if a person only floats . Air , water , and food have to be brought up or recycled . It is already a test of how a body lasts off the ground . Mars almost loses its place . Rovers have driven there and studied rocks and soil , looking for the planet's past and for signs that life might once have been possible . The card will not say they found life . A person would face months of travel , radiation on the way , and a landing through air too thin to slow a ship as Earth's air does . A rover can do part of that work with no crew aboard . A drawing of a future city stays in the drawer . She keeps the Moon to a few lines . Water ice has been found in cold , shadowed places . Ice could give a later crew water , and water can be split for oxygen , but that is a possibility , not a town . A lander can look before anyone stays . By closing time the shelf holds four things : a rocket that returns and lands , a satellite with a warning about debris , the station where crews already live , and a model of a wheel that has rolled where no person has walked . Priya locks the case .
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