QUESTION:
Why is Pluto's gravity different in certain sections of Space and does
this effect the time it takes to receive images?

ANSWER from Dick Shaw on May 15,1996:
Pluto is a strange and interesting world, so I am not surprised at
your interest in it.  There are many mysteries about Pluto that need
to be solved, including how it formed, why it's orbit is so peculiar
(compared to the other planets), and what it is made of.

There are a few things astronomers know pretty well, though.  One is
the effect of gravity on Planets and other objects in space.  It is
because of our understanding of the law of gravity, and of related
laws of motion, that we are able to predict Pluto's orbit, and also
the relative mass of Pluto and it's moon, Charon.  Another thing
astronomers know about gravity is that its effect is in direct
proportion to an object's mass, and that the effect diminishes with
the square of the distance from the object.  That is true no matter
where in space the object is.  So if we were magically able to
double Pluto's mass, it's gravitational attraction to Charon would
double too.  On the other hand, if we were instead to double the
distance between Pluto and Charon, the gravitational attraction
between them would be one fourth as great.

You may know that Pluto's orbit around the Sun is quite eccentric.
In fact, for a portion of it's orbit it's distance from the Sun
is actually less than that of Neptune.  (This curious state of
affairs will remain true until sometime in the year 2000.)  Using
what we know about the law of gravity, we can say that the Sun's
gravitational pull on Pluto is slightly greater now than it will
be in 120 years or so when Pluto is farthest from the Sun.  So you
see, it's not that Pluto's gravity changes as it moves through
space, but rather that the gravitational attraction between Pluto
and the Sun changes slightly as Pluto revolves about the Sun.

And now for the second part of your question.  When you ask about
how long it takes to receive images, I suppose you mean how long
does it take light to travel from Pluto to HST, where the on-board
cameras form an image of the planet.  Since light travels at the
same speed everywhere in the Universe, how long it takes to complete
a journey from Pluto to Earth depends upon how far apart they are.
Light takes about 8.3 minutes to travel from the Sun to the Earth.
Since Pluto is (currently) about 30 times farther away, it takes
like light a bit over 4 hours to make the trip.  But wait!  In 120
years or so Pluto's eccentric orbit will carry it to roughly 50
times the distance from the Earth to the Sun.  Can you figure out
how long it will take light to travel from Pluto to Earth then?
Will it take more or less time than it does now?