On Sat, 27 Mar 2004 07:53:08 -0500, "Allen W. McDonnell"
wrote:

Why should mariner 10 in 1973 require 1 Venus flyby to set up 3 Mercury
flyby's and the 2004 Messenger mission require one Earth flyby, 2 Venus
flyby's and 3 Mercury flyby's before it actually goes into orbit in 2011?
Getting to Mercury is less energy intensive than getting to Jupiter.

Mercury and Jupiter aren't very different, in launch performance.
Getting to either one isn't that difficult... stopping when you get
there is. Galileo needed the most powerful launcher in the world and a
heavy IUS to get to Jupiter and have enough fuel to slow into orbit.
Ulysses used the same booster, plus a PAM, and went straight to
Jupiter, but it didn't have to stop there. Cassini used the most
powerful booster available, and still needed multiple gravity assists
to be able to enter Saturn orbit.

Using
'energy saver' orbits is fine so long as you have all the time in the world,
but NASA has developed the IMO bad habit of selecting the cheapest orbits no
matter what the time delay.

It's not just a NASA thing, look at the ten-year flight of Rosetta, or
BepiColombo's planned four year, multiple-gravity-assist flight plan
to Mercury. ESA has chosen this method also.

Oh, and "Deep Impact" will launch on a direct path late this year. No
gravity-assists.

Brian