MadSci Network: Astronomy
Query:

Re: What keeps satellites revolving around the world?

Date: Sat Dec 9 22:44:39 2000
Posted By: James Steele Foerch, Instructor, Pine Creative Arts Academy
Area of science: Astronomy
ID: 976054127.As
Message:

Dear Rosa,
     Our moon, the International Space Station, Mir and all the satellites
are falling! Our earth has very strong gravity and just as you are pulled 
right down to earth when you jump off a swing, the satellites are being 
pulled down equally hard. However, they have been pushed to speeds of 
18,000 miles per hour or more, so as they fall, they miss the earth. That's 
hard to imagine, so here's how Isaac Newton explained it in the 17th 
century.
     Imagine throwing a baseball as far as you can. It curves down and 
finally bounces on the ground because our gravity pulls it down. Now load a 
cannon ball in a really huge cannon and FIRE! The projectile flies a long 
ways, but gravity pulls it down and it eventually curves down and smashes 
into the ground. Our space program, NASA, uses powerful rockets to push 
satellites so fast that they fall past the horizon, all the way around the 
earth!
     If space was a perfect vacuum, satellites would orbit forever. 
However, even 300 miles up where the Hubble Space Telescope and shuttles 
operate, there are a few molecules of air which eventually slow down a 
satellite. If the satellites aren't sped back up with thrusters or by the 
shuttle, they eventually fall back to earth.
     Unlike all our artificial satellites, the moon is moving away from us 
and slowing down our rotation at the same time. But that's another question 
for the MADSCIENTIST!  Thank you for writing and keep an eye on the sky. 
You can often see satellites moving among the stars.
Jim Foerch
James C. Veen Observatory
Lowell, Michigan, USA, Earth


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