Transfer orbits
Transfer orbits allow spacecraft to move from one orbit to another. Usually
they require a burn at the start, a burn at the end, and sometimes one or more
burns in the middle. The
Hohmann transfer orbit typically requires the least
delta-v, but
any orbit that
sects both the origin orbit and destination orbit may be
used.
Gravity assist and the Oberth effect
In a gravity assist, a spacecraft swings by a planet and leaves in a different
direction, at a different velocity. This is useful to speed or slow a spacecraft
instead of carrying more fuel.
This maneuver can be approximated by an
elastic collision at large distances, though the flyby does not involve any
physical contact. Due to Newton's Third Law (equal and opposite reaction), any
momentum gained by a spacecraft must be lost by the planet, or vice versa.
However, because the planet is much, much more massive than the spacecraft, the
effect on the planet's orbit is negligible.
The
Oberth effect can be employed, particularly during a gravity assist
operation. This effect is that use of a propulsion system works better at high
speeds, and hence course changes are best done when close to a gravitating body;
this can multiply the effective
delta-v.
Interplanetary Transport Network and fuzzy orbits
It is now possible to use computers to search for routes using the
nonlinearities in the gravity of the planets and moons of the solar system. For
example, it is possible to plot an orbit from high earth orbit to Mars, passing
close to one of the Earth's
Trojan points. Collectively referred to as the
Interplanetary Transport Network, these highly perturbative, even chaotic,
orbital trajectories in principle need no fuel (in practice keeping to the
trajectory requires some course corrections). The biggest problem with them is
they are usually exceedingly slow, taking many years to arrive. In addition
launch windows can be very far apart.
They have, however, been employed on projects such as
Genesis. This spacecraft visited Earth's lagrange L1 point and returned
using very little propellant.
