In article ,
Russell Wallace wrote:
..."Each Mercury year we have to do a burn to correct the orbit, and we
carry enough fuel to do at least three burns.""
I'm curious, why isn't the orbit around Mercury stable?

For a guess, precession from the Sun's gravity.

For an orbit around Mercury, any sort of perturbation matters more than
you might think, because choice of orbit is an important part of
spacecraft temperature control. In particular, you typically don't want
an orbit that passes low over the center of the daylight hemisphere,
because there, half the spacecraft's sky is full of furnace-hot planet.
Perigee(*) preferably should be near the day-night line, where surface
temperatures are more moderate. Putting perigee near one of the poles
achieves that over a full Mercury year, but perturbations will make the
perigee location gradually move, requiring occasional corrections.

(I don't know, offhand, exactly what orbit is planned for Messenger, but
the above is a good guess.)

(* Pedantically it should be something like perihermion, but I'm inclined
to just use "perigee" as generic for planets. )
--
spsystems.net is temporarily off the air; | Henry Spencer
mail to henry at zoo.utoronto.ca instead. |