Mars

Launch dates to get to Mars at Opposition: May 2018, July 2020, Sept 2022, Oct 2024, Dec 2026, Feb 2029

The launch window being roughly ±4 weeks; Our current technology make other dates impractical due to the ∆V required.

Mars Oppositions

In 2027 Mars and Earth will pass at 101,420,000km and in 2029 at 96,820,000Km. then in 2031 82,780,000Km, 2033 63,280,000Km and 2035 56,910,000km are the best flight opportunities before they start getting longer again.

So in 2035 the journey time from Earth to Mars is less than 2027

Mars Opposition history

Mars oppositions are closest if the red planet is close to its perihelion when it comes into opposition to the Sun from Earth. Closest approaches can become closer than 56 million km at these times, if the perihelion at its heliocentric elliptical length of 336 deg is hit well. Very close encounters of Mars to Earth happened in Halley's and Newton's time in 1687 and 1719, in Messier's time in 1766, in the time of John Herschel in 1845, and in 1924 when galaxies had just been recognized by Hubble as island universes. Then in 2003 when they passed just 55,760,000 Km to each other.