Friday, June 27, 2008

Planetary gravity: There I go again!

A little more than a year ago, in my blog entry entitled "How Could I Have Been So Wrong?" I reflected on my misperception of the actual distances of those stars closest to our solar system. It turns out that star closest to us is "only" 4.12 light years away--not the 26 I had expected. Vega, at 26 light years, is not the one closest to us.

Recently, I made another discovery that really stunned me. I had always imagined that, in addition to the challenges of dealing with a cold, non-definitive surface, the "giant" planets in our solar system--Jupiter, Saturn, Uranus, and Neptune--would have a crushing level of gravity. It turns out that I greatly over-estimated the gravity levels on these planets. According to figures at, Jupiter does have a gravity force 2.5 times that of Earth--but still not the tens, if not hundreds, of times I had expected. I really don't know whether 2.5 Gs would actually "crushing," but at the risk of coming across as somewhat Eartnocentric, I would rather not find out from personal experience--call me a "fuddy duddy" if you have to. There would be certain other unpleasantries--quite aside from gravity--in attempting to stand on the surface of Jupiter. (Those interested might like to consult my blog entry "Song Facts" for a discussion of some of the nasty aspects of solar system travel). Saturn and Neptune have gravities a little above that of earth--something like 1.1 times--and Uranus' gravity level is actually a tiny bit less than that of Earth--maybe about 95%, as best I can judge from the graph.

In case you are wondering, Venus has a gravity level slightly less than that of the Earth, and Mercury and Mars seem to come in at about 80%. It looks like Pluto is only about 15% that of Earth, but that still does not seem a legitimate reason for the unconsciounable act of some to try to demote Pluto from its rightful status as a planet.

How can this be? It turns out that a body's surface level gravity is proportional to the body's actual mass and inversely proportional to the square of the distance from the surface to the center. Because the "gas giants" are much less dense than "rocky" planets like Earth, these distances are quite long.

It has been a long time since my high school physics days. My intuition had been more the other way around--that the mass would be squared and the distance left to itself. Again, I was wrong.

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