Re: Set the controls for the heart of the Sun

Carl Feynman (
Fri, 22 Aug 1997 09:00:30 -0400

At 10:34 AM 8/17/97 +0000, Mark Grant wrote:
>On Fri, 15 Aug 1997, Carl Feynman wrote:
>> then just dropping the whole ball in. The Sun is, after all, gas all the
>> way down, and much less dense than the bomb, so according to my calculations
>> the bomb should fall to the core in about half an hour.
>I thought (based on memories of my astrophysics class a few years ago)
>that the outer layers of the sun were about as dense as water? Still much
>less dense than lead, but far more dense than air...

The center of the Sun is far denser than ordinary lead. The bomb, at zero
pressure, has about the density of silicon. It is true that on its way down
it will encounter layers denser than this, but by the time it gets there, it
will be under much more pressure, and will be compressed to be much smaller,
so it will still be denser than the surrounding gas. In fact, it will
always be at least several times denser than the medium it is faling through.

This is provided that it is not substantially hotter than its surroundings,
since being hotter makes it expand and reduces its density. While it's
dropping, it will be heated by compression, but the medium around it is also
heated by compression, so as long as the bomb is cooler at the surface it
will stay cooler all the way down. So it is important to deliver it to the
surface of the Sun in a way tha leaves it no hotter than the Sun itself.
That means that it has to be placed rather gently on the surface of the Sun,
rather than being dropped in at orbital velocity (200 km/sec). Forrest
Bishop's message suggested doing this by halting an orbiting bomb in its
orbit using a gravitational slingshot manuver. Unfortunately, this won't
work because such manuvers are limited in their delta-v to the escape
velocity for the bodies concerned, which for planet-size objects are only
tens of kilometers per second. I was imagining using mass drivers on the
bomb to bring it suddenly to a halt by catching a bunch of mass going the
other way in the same orbit. I'm not sure it's feasible; there's a lot of
waste heat.