Expert: James Gort - 8/17/2005

Question
Man made "space junk" as it orbits earth eventually enters back into the earth's atmosphere and burns up. Was doesn't the moon do the same thing?

I presume it is because the moon is in "equilibrium" with gravitional pull from the earth and sun keeping in check. Objects closer to earth would have greater gravitional pull from the earth causing the orbit to decline.

Am I right?  

Answer
Hello Randy,

You've asked an extremely difficult question, requiring a good deal of orbital mathematics to prove.  But I'll give you the short (but still accurate) answer.

The distance actually has little to do with it.  If earth didn't have any atmosphere and there was a perfect vacuum in space, small objects like space junk could orbit 100 feet above the ground and (as long as they had enough initial velocity), they'd stay in orbit almost forever.

But as it is, space junk in orbit does encounter some friction with a few air molecules, solar wind particles, etc.  So the junk loses some energy through heating, and eventually goes into a lower energy orbit, closer to earth.  As it continues, it loses more energy and eventually burns up in the atmosphere.

It's much more complicated with the moon.  The large size of the moon means it has enough gravity to partially distort the earth (causing tides).  The moon is also distorted by the earth's gravity.  So this constant bulging (in different areas) interacts with the earth's daily rotation and the moon's monthly revolution.  Energy is also lost through heating, but since the moon's revolution is greater than one day, the lost energy is translated into the moon actually receding!  If the moon's orbital revolution happened to be less than one day (so a lunar month would be less than a day), the moon would be approaching earth!

It all can be proved mathematically, but it's not easy.

Hope that answers your question.

Prof. James Gort