Course Corrections

Although we can launch our spacecraft fairly closely to its intended trajectory, we'll still have to make minor adjustments to its speed.  This will keep it precisely on the path we want.

In the fall of 1999, the Mars Polar Climate Orbiter was lost because of errors that were made in its mid-flight corrections.  Strange as it may seem, the cause was a human failure to use the correct units of measurement!  One design team used forces in Imperial units – pounds – and the computer guidance software expected forces in SI metric units – Newtons (the difference in the size of these units is about a factor of two).  The climate orbiter needed a very precise insertion into Martian orbit, as it was to use aerobraking in the atmosphere to slow it down to orbit the planet.  Off by many kilometres, the orbiter instead burned up in the atmosphere.  Part of the reason for the large course corrections needed for the climate orbiter was its assymetrical design.  The large single solar panel captured the solar wind, and required gyroscopic stabilization.

Why will we have to do this?

• the solar wind will affect our velocity, especially if we have large arrays of solar panels that act like sails.  The solar wind is the stream of particles that is ejected from the Sun at high velocity.  It acts on a spaceship just like the wind will act on a sailboat on Earth.  In fact, large sails to capture the solar wind are one proposed means of interplanetary propulsion.  Because the solar wind is variable, we cannot predict exactly what it will do to our spaceship's path.
• the gravity of the Sun, Earth, other planets, and even comets and asteroids continues to affect our spaceship after launch.  Even a very tiny difference in the initial velocity will make the effect of these gravitational fields slightly different from our initial predictions.

Since we must reach Mars within just a few kilometres of our intended destination, some mid-course corrections are essential.  This requires a rocket engine that can be restarted easily.  Methylhydrazine and nitrogen tetroxide have proven to be the most practical fuels for this purpose.  These fuels are easily stored, since they have relatively high boiling points, so can be kept as liquids by pressure alone.

Equally importantly, they are hypergolic, so do not need an ignition source.