🚀 go-pugleaf

In article <1992Oct5.205154.1@fnalo.fnal.gov> higgins@fnalo.fnal.gov (Bill Higgins-- Beam Jockey) writes:
>Ken, how much does the Earth's orbit change because of [solar] radiation
>pressure?  Kilowatt per square meter, albedo 0.4.  I'd do the
>calculation but I'm really getting hungry and I want to go home to
>dinner.

Since the net thrust supplied by solar radiation pressure is
perpendicular to the orbital velocity vector (that is, thrust is away
from the Sun), it doesn't add anything to the planet's orbital energy.
That net thrust simply counters a tiny fraction of the mutual Sun/Venus
gravitational attraction, making Venus' orbit a little wider.  So the
effects of solar incident radiation are already built into Venus'
current, stable orbit.

In order to change your orbit from a stable, circular one, you must
apply net thrust along the velocity vector.  Solar incident radiation
does not.  Applying continuous thrust along the radius vector does
affect the balance between centrifugal force and gravitation (which is
the definition of an orbit), but it just moves that balance a little
bit in favor of the centrifugal force side of the equation.  (Yes, I
know, there's no such thing as centrifugal force.  But it's a handy
construct when balancing forces here.)

If the Sun suddenly stopped radiating, that extra little outward force
due to radiation pressure would stop, and the orbits of the planets
would move inward just a little bit (probably becoming more
elliptical).  If the Sun's output increased a lot, the planets would
move outward just a bit.  How much?  I don't know.  Anybody wanna run
some figures?

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

     "Nothing in the world can take the place of persistence.
      Talent will not; nothing is more common than unsuccessful men
      with talent.  Genius will not; unrewarded genius is almost a
      proverb.  Education will not; the world is full of educated
      derilicts.  Persistence and determination alone are omnipotent.
      The slogan 'Press On' has solved and always will solve the
      problems of the human race."  -- Calvin Coolidge