Orbits

1. 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.

2. 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”

3. 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.

4. 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).

5. 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).

6. 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

7. Orbits and the Atmosphere • What we call “space travel” typically involves a trip no further from the Earth than Charlottesville is from Philadelphia.

8. “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.

9. 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).

10. 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

11. 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

12. Artificial Satellites: Geostationary Satellites Link to nice summary of geostationary satellites

13. 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.

14. 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.

15. 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.

16. 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.