[net.space] Time warp

kallis@pen.DEC (05/19/85)

Steve Pearse just asked about "gravity slingshots."  I was under the
impression we'd been doing this for more than a week.  However:

	The idea of a gravitational "slingshot" is actually a net
transfer of energy from a massive primary (Jupiter, Tellus, Saturn)
to a much smaller body in a [Solar, in this case] passing orbit so
that the orbit is altered.  This happens naturally with comets and
asteroids (such as Jupiter's "family" of comets).
	In space sciences, the idea is to set up a condition to exploit
this effect -- in this case by sending a probe so that the net effect
of the *moving* planet will cause a velocity change that brings about
the desired result (e.g., acceleration to outer-Solar-System orbits).
	Why Jupiter?  Why not?  It has the biggest moving gravity well
available.
	To answer Steve Pearse's precise question, the probe speeds up
(gains energy) and the planet slows down.  However, since no energy
is created or destroyed in the process, the amount the probe speeds
up and the planet slows down is proportional to their masses.  And
since the probe's mass is nearly infinitesimal in comparison to
the planet, the amount the planet slows down is nearly infinitesimal
to the amount the probe speeds up.

Steve Kallis, Jr.