Re: Cidri -- Mission of Gravity

[Rick <rick_ww@lightspeed.ca>]

Yes!  What a great story!

I really like Hal Clement.  Have you read his story, “Close to Critical”?
That was a mind bender.

Warm regards, Rick.



> On Aug 18, 2020, at 12:19 PM, Neil Gilmore <raito@raito.com> wrote:
> 
> Read Mission of Gravity, by Hal Clement.
> 
> Neil Gilmore
> raito@raito.com


> 
> On 2020-08-18 12:58, Rick wrote:
>> When I was in High School, I was wondering if there was
>> a density of a planet where it could get really large, and
>> keep about 1 gee of surface gravity.  (I was wondering,
>> because… Cidri.)
>> No constant density.  As worlds got larger their radius went
>> up (so lower surface gravity), but the large mass
>> increased the surface gravity faster.
>> So larger mass wins out.  As worlds get larger, you have
>> to keep lowering the density.  However, low density means
>> that worlds can get quite large, and still keep a 1 gee
>> gravity.  If I recall correctly, you could get worlds with about
>> triple Earth’s radius, on a world with no iron core, just rock.
>> Beyond that surface gravity just started going up.
>> (And that does not count rock compressing into denser
>> forms, deep in the core of such a world.)
>> Note that Jack Vance wrote some stories set on “Big Planet”
>> ("Showboat World”, and others).  Big Planet formed with
>> almost no metals, and was HUGE.  Well shiny, heavy
>> metals like iron.  It had plenty of metals like calcium,
>> aluminium, lithium, and ones with low atomic masses.
>> If Cidri is hollow, then you can make it as large as you want.
>> (But the inside of the world is in zero gravity, not counting
>> centrifugal force due to rotation.
>> Warm regards, Rick.