1
Orbits
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Falling objects are actually in orbit!
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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 is falling.
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 is falling.
Objects in orbit are all falling simultaneously
creating the illusion of “weightlessness”
3
Orbits
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The shape/altitude of the orbit depends on the speed of the
object.
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Objects in orbit are “coasting” after an initial shove.
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A satellite can change orbits with subsequent brief shoves.
4
Orbits and the Atmosphere
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“Coasting” in orbit works only if nothing is slowing down the
satellite.
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The Earth's atmosphere can provide significant drag.
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Fortunately the atmosphere gets significantly thinner with altitude.
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Above about 300 kilometers the drag is negligible (almost).
5
Orbits and the Atmosphere
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“Coasting” in orbit works only if nothing is slowing down the
satellite.
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The Earth's atmosphere can provide significant drag.
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Fortunately the atmosphere gets significantly thinner with altitude.
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Above about 300 kilometers the drag is negligible (almost).
6
Orbits and the Atmosphere
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“Coasting” in orbit works only if nothing is slowing down the
satellite.
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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
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What we call “space travel” typically involves a trip no further
from the Earth than Charlottesville is from Philadelphia.
8
“Deep” Space Exploration
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Only nine space missions carrying humans have ventured
beyond Earth orbit – Apollos 8, and 10 through 17.
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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.
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A trip to Mars would involve a voyage of 50 feet on this scale.
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Moon missions took two weeks. Mars missions will take 2 years.
9
Artificial Satellites: Low Earth Orbit
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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
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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.
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Such a satellite hovers over a fixed point on 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 point on 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
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Interplanetary spacecraft are artificial satellites of the Sun.
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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
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Interplanetary spacecraft are artificial satellites of the Sun.
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Encounters with the planets can add a velocity boost to the
satellite's voyage.
15
Seeing Satellites
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You can be in darkness on the surface of the Earth while a
satellite is in daylight above you.
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Satellites are easily visible after sunset and before dawn.
16
Seeing Satellites
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You can be in darkness on the surface of the Earth while a
satellite is in daylight above you.
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Satellites are easily visible after sunset and before dawn.