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Dear
Katie and Elizabeth,
An
artificial satellite that is outside Earth’s atmosphere
will move in a circular orbit if its speed is large enough
and the direction of its motion (velocity vector, V) is “horizontal.”
“Horizontal” means perpendicular to the line from
the center of the Earth to the satellite (position vector,
R) as shown in the figure. The speed required for a satellite
to stay in an orbit that encircles the Earth at an altitude
of 600 kilometers is about 17,000 mph (7.6 kilometers per
second). The satellite stays in orbit because as it travels
around the Earth, the Earth’s gravity, which is directed
towards the center of the Earth, causes the direction of the
satellite’s velocity vector to change, always keeping
its direction horizontal (Figure 1).
Figure 1 Gravity Turns the Satellite’s Velocity Vector
A
familiar example of how the Earth’s gravity operates
to change the direction of the velocity vector of an object
is the motion of a baseball that is thrown horizontally. The
Earth’s gravity causes the path of the baseball to curve
downward until the baseball hits the ground. The baseball
is actually in an “orbit,” but because the baseball
is traveling at a very low speed compared to a satellite,
probably less than 100 mph, its path is a part of an elliptic
“orbit,” most of which is inside the Earth (Figure
2). Satellites are moving so fast that the curved paths that
they follow curve around the Earth instead of hitting it.
It is interesting to think about what a baseball would do
if it were traveling fast enough – if the Earth had no
atmosphere, mountains, buildings, etc. to get in the way,
and if a baseball could be thrown horizontally at a speed
of around 17,684 mph (7.91 km/sec).
Figure 2 (not to scale) Baseball Travels in a Portion of an
Elliptical Orbit
It
is difficult to get satellites moving at the very high speeds
required to put them in orbit and keep them there. They are
“launched” into space using rocket-propelled launch
vehicles (RPLV). Rocket propulsion is used because it can
produce very high speeds in short periods of time. A RPLV
takes off vertically, but pitches over as it climbs so that
when the second stage burns out the RPLV is traveling horizontally.
Its third stage then fires and gives the satellite cargo the
velocity it needs to stay in orbit after it is ejected from
the third stage. The third stage usually enters the Earth’s
atmosphere and burns up like a meteor.
Thanks
for your question,
Aubie and Dr. Cochran
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