Trajectories to other planets

A journey to Mars takes several months. During that time, both Earth and Mars move a considerable distance around the Sun. The spacecraft’s path needs to meet Mars at its future position.

It’s similar to passing a ball to someone who is running: you allow for how far they will move while the ball is travelling.

Following a curved path

A spacecraft leaving Earth is already moving around the Sun along with our planet. Firing its engine changes that motion, placing it on an orbit that can carry it towards its destination.

For the transfers shown here, much of the journey is spent coasting along part of an ellipse under the Sun’s gravity. Small engine burns can adjust the course along the way.

The departure date matters because the spacecraft and its destination need to reach the same place at the same time. Some dates allow a journey that uses much less fuel. These favourable launch opportunities occur roughly every 26 months for Mars.

Explore the journey

The visualisation models transfers from Earth to Mercury, Venus, Mars, Jupiter or Saturn, using planetary positions from NASA’s Jet Propulsion Laboratory.

Choose a planet and a departure date to see a calculated route and the changes in speed it requires. Start with Mars and notice how the departure direction differs from the direction towards Mars’s current position.

Then move the departure date forward. Watch how the route and the required changes in speed vary as the planets move around their orbits.