Satgen324  In Orbit Pt8  InterPlanetary  by GM4IHJ            10 June 1995

It should be clear from previous satgens that the Sun dominates all Solar
system orbits ,excepting those very close to particular planets or moons.
It follows therefore, that if we want to go from one planet to another ,
we must expect most of the journey to be controlled by Solar gravity , ie
following part at least of an elliptical orbit around the Sun.

Planning a flight from Earth to Mars, we find that the economical orbit
using least fuel , is of the type described ( long before the first
Sputnik flight ), by the German author Dr Walter Hohmann. This HOHMANN
orbit is a half ellipse . We start from near the Earth at the perihelion
of the orbit, do half an orbit round the Sun, ending out at aphelion,
where we meet Mars in its orbit.

Unfortunately nature never is this simple. We cannot ignore earth gravity
at the start, nor can we ignore Mars gravity at the end. So the practical
orbit is less than perfect. We plot a circular parking orbit for the space
craft around the Earth at say 1 million kms radius. We plot an accurate
orbit of smaller radius around Mars, using only the mass of Mars. The
trick comes in joining these two orbits.

Once in Earth parking orbit we wait until Mars is in the right start
position so that 258 days later it and our spacecraft will arrive together
at the aphelion end of a Sun centred orbit, which has started from a
rocket firing to lift the spacecraft from earth orbit into a Solar orbit.

This situation is untidy because at times our craft is subject to the
gravity of two large bodies ( a planet and the Sun), and there is no
easy mathematical solution to the "Three body problem" as it is called ,
where the spacecraft (the 3rd body) is subject to two lots of gravity, as
occurs at either end of the orbit. The practical solution is less than
perfect as its name " A Patched Conic " reveals. So interplanetary flight
is not very precise, and we correct this as the space craft approaches the
target planet , by using its onboard target and star sensors/cameras , to
accurately locate the craft with respect to the planet, and thereby
calculate a suitable thrust level and direction, for a short rocket burn,
to put the craft into a safe orbit around the target.

This engine firing near the target can be dangerous. Most recently JPL
Mars Observer was given a command sequence to open up fuel supplies to its
rocket and fire it , only after it had temporarily switched of its
electronics, so that no inadvertent spark could ignite a fuel leak. After
a successful burn the space craft should have switched its electronics
back on and regained radio contact with the Earth control station. Mars
Observer never came back on, and a lengthy enquiry has failed to establish
the reason for this. So with this sort of procedure being necessary, no
one should be surprised that as yet orbit tracking data for this type of
flight has not been made available to the general public.