Mission to Mars

The Launch

The way that one reaches Mars is sometimes surprising, but in actual fact it is quite easy to understand. The best way to understand how one undertakes a flight to Mars is to consider a familiar example, such as the process whereby one gets a football from the quarterback to the down field receiver.

In the illustration above imagine that the quarterback at a and the down field receiver at b are both running across the field (shown as yellow dashed lines). The problem is how to get an object (the football) from a to the down field receiver. Transparency Master The diagram to the left shows a slightly more graphic representation of what happens. First the quarterback must anticipate (mentally calculate) the expected position of the receiver after the ball has travelled down field. Second, the quarterback must compensate for the effect of gravity on the flight path of the ball. Finally, the quarterback must make absolutely sure that the ball has enough speed so that it will arrive at position c at exactly the right moment (i.e. when the receiver gets there). The quarterback must launch the ball at exactly the correct speed and angle to make a perfect pass, because once the free flight of the football has begun, nothing can be done ... the laws of physics are in charge...no more human intervention is possible.

A trip to Mars is very similar to throwing the football...in essence, one is throwing a spacecraft to Mars. The words "flight to Mars" are, in fact, a bit inaccurate. To some the word "flight" might imply that the spacecraft is under some sort of rocket power during the trip, when in fact the spacecraft is in "free-flight".

Once the spacecraft is lifted into Earth orbit the rocket engines are fired to "throw" the spacecraft into a free-flight path (ballistic trajectory) towards Mars as shown in the diagram above. Note that the rocket thrust is applied so that the rocket accelerates (speeds up) in a direction parallel to the Earth's orbital motion...not outwards towards Mars. Because the spacecraft has increased its speed, its path (orbit) around the Sun will gradually expand. IF the launch speed has been large enough the path of the spacecraft will converge with the orbit of Mars. Once en route, only the momentum of the spacecraft, and the force of gravity (from the Sun) affect its path. Just as in the case of the football quarterback, the trick is to choose exactly the right launch speed and direction to ensure that Mars and the spacecraft arrive at c at exactly the same time. When to launch In order for the spacecraft to arrive at Mars, its launch speed and launch time must be carefully selected. As shown below, the key idea is to ensure that the time it takes the spacecraft to get from a to c is exactly the same amount of time it takes Mars to get from b to c . Transparency Master Many launch speeds and launch times are possible. The actual launch speed and launch time are a selected compromise between 1.) minimum travel time 2.) minimum fuel requirements. The rules are The more fuel you use the more expensive (and to some extent, difficult) the trip becomes. The less fuel you use the cheaper the trip is, but the longer (and possibly more dangerous) the trip becomes. All mission plans are designed as a compromise between minimum fuel and minimum travel time.

Launch Checklist

1. Calculate how much rocket fuel you can carry into low earth orbit for your launch to Mars.
2. Calculate the speed increase that you get from (using) your rocket fuel.
3. Calculate the time it will take to reach the orbit of Mars.
4. Calculate how far (in its orbit) Mars will travel during the time the spacecraft is en route.
5. From the orbital positions of the Earth and Mars, calculate the launch date required so that during the space trip, Mars will have moved along its orbit to exactly the point where the spacecraft meets the orbit of Mars.

Blast off and good luck.

[ The outbound trip ]