From: kjenks@gothamcity.jsc.nasa.gov
Subject: Re: The thrust problem (was Re: Moving Venus to Earth orbit)
Originator: kjenks@gothamcity
Keywords: thrust
Organization: NASA/JSC/GM2, Space Shuttle Program Office
Date: Mon, 5 Oct 1992 16:15:56 GMT

I, kjenks@gothamcity.jsc.nasa.gov, wrote:
>> My ground-based lasers would get you a net thrust, albeit a small,
>> inefficient one.  But we've got several centuries before we can 
>> change the atmosphere, anyway.  Lasers are patient.  We can wait.

In article <1992Oct2.202211.1@fnalc.fnal.gov> higgins@fnalc.fnal.gov (Bill Higgins-- Beam Jockey) writes:
>I've been waiting for somebody to point out (because I haven't
>mustered the technical argruments) that Ken's proposal stinks because
>the lasers are high-exhaust-velocity, low-thrust devices.  You have to
>move a few trillion trillion kilograms, you're going to need *serious*
>thrust.

That all depends on how fast you want to accellerate it and how many of
the "high-exhaust-velocity, low-thrust devices" you have.  If you're
patient, or if you can put a LOT of lasers in place, you can move
anything.

>If you are moving your planet with rocket engines, get some with
>exhaust velocity just a little bigger than escape velocity (reason why
>left as exercise for student) and with as much thrust as you can
>afford.  

But how do we get the thrust OUT of the atmosphere?  My main reason for
proposing lasers (well, as terry@thisbe.Eng.Sandy.Novell.COM (Terry
Lambert) points out, ANY old light source) is that light can travel
through the atmosphere and generate net thrust for the whole planet.
That's harder to do when hurling matter away.

If YOU are moving YOUR planet with rocket engines, how do you get the
thrust out of the atmosphere?  It doesn't do any good to detonate a
bunch of nukes on one side of a planet if the detonation products don't
leave the gravity well.  Bill mentioned escape velocity: that's the
minimum velocity necessary for any body to escape the planet's gravity
well.

>Where do you put them?  At the point on the surface where the planet's
>orbit intersects it.  

Actually, you can put them anywhere.  If it's not "At the point on the
surface where the planet's orbit intersects it," you have some
off-center thrust, but it's still thrust.

>[...]  If it's a fixed installation, you fire a burst
>once a day.  If the rocket engines are on wheels, not inconceivable on
>Venus (Geoff Landis would love this), you track the sub-orbital point.

If the "rocket engines" are fixed, but can swivel through 180 spherical
degrees, you can aim them wherever you need them.  Sure, you want to
fire them along your orbital path (either prograde or retrograde), but
if you have to fire at an angle, you still get net thrust.  Beam
propogation through the thick atmosphere makes you WANT to aim straight
up (thinnest there), so you may want to fire only at sunset or sunrise,
but that's not a hard requirement.

>On Earth, you'd want to fire at sunset to make the orbit bigger, or
>sunrise to make it smaller.  

If you can only aim straight up, that's true.  If you can aim, you can
fire for the half of each day when the planet's orbital velocity vector
is in view.

>[...]  Interesting problem:  suppose the engine
>is mounted on a barge in the Pacific.  Where do you move it to each
>day to get optimal effect from your thrust?  Turns out to be an
>inverted analemma... (proof also left as an exercise for the student).

I say it doesn't matter where you put it, you still get thrust.

>On Venus... let's see, the rotation is retrograde, right?... does that
>mean you'd fire at dawn to move outward?  Nope, I think you still want
>to fire at sunset.  

No, you were right the first time.  On Venus, fire straight up at dawn
to increase your orbital energy.  On Earth, fire at sunset.

It's all academic, of course.  You need a LOT of lasers (or flashlights
or radio stations) for a LONG time to budge a planet.  And you have to
get there to put them in place and coordinate them carefully to get
the desired effect.

It might make a nifty device in a science fiction story, and 
entertaining speculation on Usenet, but that's about it.

-- Ken Jenks, NASA/JSC/GM2, Space Shuttle Program Office
      kjenks@gothamcity.jsc.nasa.gov  (713) 483-4368

     "NASA turns dreams into realities and makes science fiction
      into fact" -- Daniel S. Goldin, NASA Administrator
Newsgroups: alt.sci.planetary