When a trip to Mars is possible
Low-energy coast · about every 26 months
You do not leave when Mars is nearest Earth. You leave while Mars is still about 44° ahead of Earth. The ship then coasts for about 260 days on an ellipse around the Sun. In that time Mars travels to the meeting point, and the ship arrives there with it.
The gold band is that coast on the best day. Cyan on Earth's orbit is when you can leave. Red on Mars's orbit is where Mars is when you arrive.
The gold arc leaves the ecliptic, by about an astronomical unit. Mars's orbit is tilted only about 2° to Earth's, but the best day is the day Mars is almost exactly opposite Earth. The only ellipse joining those two points then stands on edge, so the ship swings far above or below the disc and comes back to Mars. The ribbon is also lifted slightly toward you so it remains visible above the ecliptic disc.
A straight line, straight out from Earth's orbit toward Mars, is not a path a coasting ship can fly. The ship is already moving with Earth, about 30 km/s along the orbit. Leaving perpendicular to that orbit means cancelling that speed, then thrusting the whole way. With the engines off, the Sun bends every trajectory into an ellipse around it.
Open a date in the list, then press Play or drag the timeline. The marker leaves Earth and reaches Mars at the end of the band. Real missions sometimes leave a little earlier and fly faster than this minimum-energy coast. The dates here are the low-energy window, recomputed from the same positions as the planets on the model.
Heliocentric · Spacecraft
