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## Orbits

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**Orbits**• Falling objects are actually in orbit! • The only problem is that the ground gets in the way. • If an object has a sufficient velocity horizontal/parallel to the ground, the ground will curve away underneath it at the same rate as it isfalling.**Orbits**• Falling objects are actually in orbit! • The only problem is that the ground gets in the way. • If an object has a sufficient velocity horizontal/parallel to the ground, the ground will curve away underneath it at the same rate as it isfalling. Objects in orbit are all falling simultaneously creating the illusion of “weightlessness”**Orbits**• The shape/altitude of the orbit depends on the speed of the object. • Objects in orbit are “coasting” after an initial shove. • A satellite can change orbits with subsequent brief shoves.**Orbits and the Atmosphere**• “Coasting” in orbit works only if nothing is slowing down the satellite. • The Earth's atmosphere can provide significant drag. • Fortunately the atmosphere gets significantly thinner with altitude. • Above about 300 kilometers the drag is negligible (almost).**Orbits and the Atmosphere**• “Coasting” in orbit works only if nothing is slowing down the satellite. • The Earth's atmosphere can provide significant drag. • Fortunately the atmosphere gets significantly thinner with altitude. • Above about 300 kilometers the drag is negligible (almost).**Orbits and the Atmosphere**• “Coasting” in orbit works only if nothing is slowing down the satellite. • The Space Shuttle returns to the surface of the Earth by slowing down ever so slightly, but just enough to dip into the atmosphere. • The atmosphere then does the work of slowing the Shuttle from almost 17,000 m.p.h. to a stop. Artist's concept**Orbits and the Atmosphere**• What we call “space travel” typically involves a trip no further from the Earth than Charlottesville is from Philadelphia.**“Deep” Space Exploration**• Only nine space missions carrying humans have ventured beyond Earth orbit – Apollos 8, and 10 through 17. • 24 people (three went twice) have made the trip. 12 of them set foot on the Moon. • In the scale model of the Solar System where the Sun is a basketball and the next nearest star is 5000 miles away this voyage is a trip of about six inches. • A trip to Mars would involve a voyage of 50 feet on this scale. • Moon missions took two weeks. Mars missions will take 2 years.**Artificial Satellites: Low Earth Orbit**• A satellite in “low Earth orbit”, just outside the atmosphere, completes a circuit of the Earth in about 90 minutes. • The critical horizontal velocity to achieve this orbit is about 8 kilometers/second (17,000 miles per hour).**Artificial Satellites: Geostationary Satellites**• Kepler's Laws imply that satellites take more time to orbit if they are at larger distances. • A satellite in circular orbit 42,000 km above the Earth takes exactly one day to orbit. • Such a satellite hovers over a fixed pointon the ground. Link to nice summary of geostationary satellites**Artificial Satellites: Geostationary Satellites**• Kepler's Laws imply that satellites take more time to orbit if they are at larger distances. • A satellite in circular orbit 42,000 km above the Earth takes exactly one day to orbit. • Such a satellite hovers over a fixed pointon the ground. Link to nice summary of geostationary satellites**Artificial Satellites: Geostationary Satellites**Link to nice summary of geostationary satellites**Artificial Satellites: Interplanetary Orbits**• Interplanetary spacecraft are artificial satellites of the Sun. • The “cheapest” most energetically favorable orbit is an ellipse that just touches the Earth's orbit at one end and the planet's orbit at the other.**Artificial Satellites: Interplanetary Orbits**• Interplanetary spacecraft are artificial satellites of the Sun. • Encounters with the planets can add a velocity boost to the satellite's voyage.**Seeing Satellites**• You can be in darkness on the surface of the Earth while a satellite is in daylight above you. • Satellites areeasily visible after sunset and before dawn.**Seeing Satellites**• You can be in darkness on the surface of the Earth while a satellite is in daylight above you. • Satellites areeasily visible after sunset and before dawn.