Opposition Class
Outbound Venus Fly-By

Outbound

Outbound trajectory

 

1.  Earth Departure:  October 27, 2021
2.  Venus Fly-By:  April 9, 2022
3.  Mars Arrival:  October 16, 2022

Inbound

Inbound trajectory

1.  Mars Departure:  December 17, 2022
2.  Earth Arrival:  August 18, 2023  

In 1925 the German mathematician Dr. Walter Hohmann discovered that the most fuel-efficient route to Mars occurs when Mars and Earth are in conjunction, which is when the two planets are at their furthest distance from each other (400 million kilometers).  This is because the spacecraft can take advantage of the gravitational pull of the sun and of the velocity of the planet from which it is departing by traveling in an ellipse that minimizes changes in course.  A mission that uses this type of trajectory for both legs of the trip is referred to as a conjunction class mission.  The average stay on the surface for a conjunction class mission is 300 to 550 days. 

It just so happens that the stay can be reduced to 30 to 80 days by flying past Venus in one of the legs of the mission.  The fly-by augments the ship's velocity by using a technique know as gravity assist, which in turn reduces the amount of fuel required for the ship to reach its destination.  This type of mission is referred to as opposition class.  However, even with this maneuver the opposition class mission requires more fuel than the conjunction class, because in the conjunction class the ship waits for a launch window with a more fuel-efficient trajectory.  

There is some debate as to which type of trajectory, opposition or conjunction, should be used on the first manned mission to Mars.  The NASA team that wrote "The 90 Day Study on the Human Exploration of the Moon and Mars" recommended an opposition class mission primarily because it felt the shorter duration meant the trip would be safer.  But Robert Zubrin in his book The Case for Mars argues that an opposition class mission is more dangerous.  One of the dangers cited by Zubrin is the increased radiation exposure resulting from a flight path that takes the ship closer to the sun.

The launch window for a mission to Mars occurs approximately every 25 months.