On Wednesday, October 24, 2018 at 12:41:50 AM UTC-6, Quadibloc wrote:

My memory was not playing tricks on me. MESSENGER also encountered the Earth once,
and then Venus twice, to get to Mercury. But then it limited itself to only
*three* encounters with Mercury before having a low enough relative velocity for
its purposes.

Given that Jupiter takes 12 years to orbit the Sun, and Saturn takes 30 years to
do so, it's not surprising that such complicated methods aren't used for
missions to the outer planets, and instead only relatively simple and
straightforward gravity assists are used.

While the New Horizons mission was able to do a flyby of Pluto, putting a probe
in orbit around Pluto would be difficult.

And putting a probe in orbit around a closer, but still distant, body would seem
to be out of the question. Jupiter has intense radiation belts, and presumably a
similar problem would exist for the other gas giants.

Placing a space probe at Jupiter's Trojan points might seem hazardous - there
are asteroids there - but such hazards are overstated.

But it would be easy enough to send a probe to do a Jupiter flyby, and have the
flyby provide a gravity assist that leads to the probe orbiting between Jupiter
and Saturn, and the period of the orbit could be chosen so that the probe won't
encounter Jupiter again for a long time.

However, a probe independently orbiting the Sun would seem to have the problem
for my purpose that its position could not be determined with high accuracy.

While Hipparcos has done great work in deterimining the distances to many stars,
ideally to be sure of the accuracy of a distance, one would like photographs of
the star's position against a background of more distant stars. That's the
conventional way that stellar parallaxes were obtained.

The Earth's orbit around the Sun provides a 2 AU baseline for such observations.
A probe orbiting a planet more distant from the Sun than the Earth could provide
a longer baseline. Observations with a baseline from the site of the probe to
the Earth could be made simultaneously, avoiding the need to account for the
Sun's own proper motion.

Of course, this is an expensive way to obtain a small improvement in accuracy,
which is likely why it isn't being considered. But given the uncertainty in the
cosmic distance scale, it would be nice to do something...

John Savard