'Standard Orbit, Mr Sulu'

It's a pretty good bet that you've seen the Enterprise orbiting a planet in the Star Trek standard orbit. Of course, we can only really speculate why such an orbit was standard in Star Trek. Perhaps it's a good orbit for scanning planets, or avoiding being attacked by things on the surface. What we can be sure of is that Captain Kirk's standard orbit isn't actually a very good orbit from a navigation point of view. It's circular, but it's neither close to the planet nor far away. It's actually not the best orbit for doing anything.

So what are the standard orbits for Orbiter? I'd say there were five standards. Why five? The reason is that different orbits are good for different things. Things that are easy in one orbit can be incredibly expensive in fuel in another. So doing the right thing in the right orbit can make the all the difference.

The Low Orbit

When you take off from any planet, including Earth, the first stable orbit you will (hopefully) come to is a low circular orbit - rather like the one on the right. Unlike Captain Kirk's standard orbit, this orbit is actually the best orbit for some things.

For a start, it's the lowest energy stable orbit. Gravitational energy, by convention, is negative. Even with all the kinetic energy tied up in the high speed movement of your craft, your overall energy is still very negative.

Low Earth orbit is sometimes described as being 'halfway to anywhere'. The reason is that in Low Earth orbit, your overall energy is about half as negative as it is sitting on the surface. Add the same energy again, and you reach escape velocity.

Understanding energy is a big part of good navigation in Orbiter. Here are three principles to remember

Kinetic energy is proportional to velocity squared.

At 14 kilometres per second, you have FOUR TIMES as much kinetic energy as you do at 7. If you double the energy of a low orbit, you will escape from the planet. This therefore happens at sqrt(2) x the speed in low Earth orbit.

Increase in energy per second = thrust x velocity

The faster you are going, the more energy you can gain by opening the throttle.

Overall energy in any orbit is a constant

Your gravitational energy and your kinetic energy equal a constant in any orbit. The closer you get to a planet in a given orbit, the lower your gravitational energy, and the more of your energy will show up as velocity - you will move faster.

It is these principles that lead to the standard orbits, and their advantages.

Advantages of a low orbit

So what are the advantages of a low circular orbit?

Disadvantages of a low orbit


The Highly Elliptical Orbit

This orbit has its Periapsis low - just as low as the low orbit. But the Apoapsis is high - a long way above the planet. For Earth, I often use an orbit with a period of a few days.

Advantages of the highly elliptical orbit

Disadvantages of the highly elliptical orbit

The low hyerbolic orbit

This orbit still has a low Periapsis - just as low as the low orbit. But now there IS no Apoapsis.

Advantages of the low hyperbolic orbit

To have this speed at infinity You need this speed at Periapsis
0 km/s 10.9 km/s - Escape velocity
1 km/s Escape velocity +45.6 metres per second
2 km/s Escape velocity + 181.9 metres per second
5 km/s Escape velocity + 1092 metres per second

Disadvantages of the low hyperbolic orbit

The high circular orbit

This orbit is a circle a long way above the planet. In fact, the further the better, as long as the orbit is stable. Overall, this is the least useful of the standard orbits

Advantages of the high circular orbit

Disadvantages of the high circular orbit

The Hohmann transfer orbit

This orbit is an ellipse just touching the two orbits of two planets. This is what you tend to create with the transfer MFD to go from Earth to Mars. It's the most efficient way to go between most sets of orbits. An orbit which is very close to a Hohmann transfer is shown in yellow between the two green planetary orbits.

Advantage of the Hohmann transfer orbit

Disadvantage of the Hohmann transfer