Prof. Fred Watson, Astronomer-In-Charge at the Australian Astronomical Observatory, Siding Springs, was guest speaker at the Macarthur Astronomy Forum on 15th July 2013. Yes, that's right, it was on my birthday and I am seven months younger than Fred - and wondering where he gets his energy!
Fred always structures his talks so that the newcomers to astronomy understand clearly what he is aiming to pass on.
Fred Watson at Macarthur Astronomy Forum, 15th July 2013.
Fred's talk began with a description of geo-navigation, demonstrating the importance of great circles and trade winds to aeronautical travel.
The title of Fred's talk.
I'm going to skip that and cut straight to what I learned from his presentation, i.e. how do spacecraft travel from one planet to another?
They begin with a nearly circular low Earth orbit and at the appropriate part of the orbit, the engines are burned in a sequence which puts the spacecraft into a highly eliptical orbit reaching much further out into space. This is known as a Hohmann Transfer Orbit. At aphelion, the engines are fired again to enter into a more stable (almost circular) orbit at a much higher altitude.
From such an orbit, the spacecraft is then blasted into a solar orbital path, which in the image above takes the craft on a six month journey to the orbit of Mars. If you have done your sums, Mars will meet you at that the point where the orbits of Mars and the spacecraft intersect.
In the above case the transfer orbit allows you to reach the Moon in three steps.
Below is the route taken by the Cassini Mission to Saturn. It used a series of orbital maneouvres to send it past Venus, Earth and Jupiter to obtain a series of flyby gravitational boosts which got it to Saturn with a minimum amount of fuel.
The final trick is to use Lagrangian Points around each planet to enhance the ride.
In conclusion, we learnt how spacecraft use Hohmann Transfer Orbits, Flyby Gravitational Boosts and Lagrangian Points to navigate the Solar System and that by mapping the gravitation of all the planets, a network of low-energy pathways through space can be identified.
This opens up the possibility of very low energy (but slow) routes between the planets for robotic cargo-carrying spacecraft. The various null points (Lagrange Points) act as interchanges between different possible routes.