Mars Launch Windows (2000 - 2100)
Earth and Mars orbit the Sun at different speeds, so they only line up once every 26 months or so, a phenomenon called Opposition. Opposition doesn’t occur at the same place in their orbits each time, instead shifting forward at every cycle. Probes being sent from Earth to Mars can only launch in a short window that opens a few months before Opposition. Because the orbits of Earth and Mars are eccentric (meaning they are not perfect circles), at some points the Opposition between them is closer, and some points it is farther.
There is a 15 to 17 year cycle dictating the closest and farthest Oppositions, which I have noted below (when I say “closest” and “farthest,” I mean they are at a relative local minimum and local maximum respectively; every launch window opens in the months leading up to the absolute minimum distance between the two planets, the distance just varies on a cycle. The planets are truly farthest apart roughly one year before Opposition, and this point also fluctuates closer and farther on the same cycle)
April 2001 (Mars Odyssey, April 7)
June/July 2003, PLANETS CLOSEST (Mars Express/Beagle 2, June 2; Spirit, June 10; Opportunity, July 8)
August 2005 (Mars Reconnaissance Orbit, August 12)
August 2007 (Phoenix, August 4)
October 2009 (no missions launched this window)
November 2011, PLANETS FARTHEST (Fobos-Grunt/Yinghuo, November 8; Curiosity, November 26)
November 2013 (Mars Orbiter Mission, November 5; MAVEN, November 18)
March 2016 (ExoMars Trace Gas Orbiter/Schiaparelli EDM lander, March 14)
May 2018, PLANETS CLOSEST (InSight/MarCO, May 5)
July/August 2020 (Upcoming: Mars 2020, July 17 - August 5; HX-1, July 23 - August 5; Rosalind Franklin, July 25; Hope Mars Mission, July)
August/September 2022
October 2024
November 2026, PLANETS FARTHEST
November/December 2028
February 2031
April/May 2033
July 2035, PLANETS CLOSEST
August 2037
September/October 2039
October/November 2041, PLANETS FARTHEST
November 2043
December 2045/January 2046
March 2048
May/June 2050, PLANETS CLOSEST
July/August 2052
August 2054
October 2056
November 2058, PLANETS FARTHEST
November/December 2060
January/February 2063
April/May 2065
July 2067, PLANETS CLOSEST
August 2069
September/October 2071
November 2073, PLANETS FARTHEST
November/December 2075
December 2077/January 2078
March/April 2080
June/July 2082, PLANETS CLOSEST
August 2084
October 2086
October/November 2088
November 2090, PLANETS FARTHEST
November/December 2092
March 2095
May 2097, PLANETS CLOSEST
July/August 2099