Planning a trip to Mars is no different than planning an excursion. When planning a trip the first thing that comes to mind is when will it take place? On one hand, Holiday trips, for instance, are bound to the holiday calendar and the availability of the travellers, on the other hand, work trips are often related to one specific event or series of events (meetings, fairs, conferences…).  A trip to Mars relies on other decisive criteria such as the distance between the two planets and the current propulsion technologies. If selected, both of them will define the launch windows, the type of the mission and the duration of transit and stay on Mars. Indeed, by choosing an adequate launch window, energy consumption can be optimised and crew stay on Mars can be shortened so that there is enough time to complete the mission goals without increasing crew’s risk exposure (radiation, technical failures, low gravity).  

Chapter 1: Duration of the mission

Earth and Mars travel around the sun in elliptical orbits as announced by the first Keplerian Law. Knowing that both have different orbiting velocities, the distance between the two planets is minimized every two years. In other words, Earth-Mars distance varies within a cycle of two years. For the distance to be minimized in both transit legs, a round trip to Mars can’t last less than two years. This plan is called a conjunction class mission. However, scientists nowadays based on technological advances predict that in a few years, it will be possible to fly to Mars in less than 6months and therefore have a round trip that doesn’t exceed an Earth year, which can also be referred to as an opposition class mission.

Opposition Class Mission and Short Trips to Mars

Opposition class missions are short-duration missions. The first leg of the mission relies on the Hohmann Transfer Orbit just like conjunction class missions. However, these missions don’t require a long stay period on Mars since astronauts don’t have to wait for the planets to align in order to return. The return phase of the mission uses a high energy trajectory. More propellant is therefore needed. Opposition class missions are generally not considered as a plausible alternative because they require advanced propulsion technologies such as rockets with a delta-V superior to 7.0 [km/s]. 

To decrease the quantity of loaded propellant, some theoretical research suggests that a short trip to Mars can be reached using pre-positioned fuel supplies, although in-flight refuelling in space has not already been tested. 

SpaceX predicts that Starship will be able to get around 100 passengers to Mars in less than four months with more liberty to choose the surface stay duration on Mars. They plan to use high energy trajectories for both legs of the round trip and therefore will need pre-positioned fuel supplies and in-flight refuelling. 

Conjunction Class Missions: Long Duration Missions

A conjunction class mission is a long-duration mission to Mars that uses Hohmann transfer orbit for the go and return leg from Mars. It requires current propulsion technologies with a Trans-Earth Injection Delta-V of no more than 3.0 [km/s]. 

In addition to that, the duration of the transit to and from Mars can go from 180 days up to 270 days. 

These types of missions have long-stay durations on Mars. Indeed the crew will have to spend around 517 days on the Martian surface before going back to Earth, waiting for the two planets to have optimal positions for a Hohmann transfer. Hohmann transfers are elliptical orbits that allow the spacecraft to escape the planet’s gravitational field using the least amount of propellant. 

The main goal of a crewed mission to Mars is to send a group of astronauts to and back from Mars in the least amount of time possible with the least amount of propellant. For this reason, NASA has developed a 12 step round trip to Mars using a fast conjunction class mission. The Mars Direct plan also relies on such a plan. However, it only needs to load propellant for the first leg of the mission as the spacecraft is supposed to refuel In-situ on Mars using propellant that was produced by a nuclear generator previously sent to Mars. 

However, not all conjunction class missions have the same duration of transit and stay on Mars. These factors can drastically vary according to the launch window chosen. Therefore getting to Mars with the least amount of propellant and with the shorted delays depend on when the spacecraft is launched. 

Chapter 2: Lift-off date

How to choose a launch window? 

All conjunction class missions use Hohmann transfer for both legs of the trip, however, according to the launch window chosen for the mission, durations can vary. 

Choosing a launch window therefore has to rely on the mission’s priority. Does the mission require long stay durations on Mars? Can the technologies afford 3 years-long trips without increasing the crew’s exposition to risks and danger? 

Nowadays, many scientists advocate that 2033 is an optimal launch window choice. Indeed, it allows a transit duration of 180 days with no more than deltaV=3.5 compared to for instance the 2022 launch window with deltaV=4.  

The 2033 launch window has the shortest travel distance between the two planets during the go and the return phase. Moreover, Mars stay time is 517 days which is very similar to the rest of the conjunction mission opportunities. At last, the 2033 launch window enables astronauts to spend less time in transit, using less propellant and having a relatively short round trip of 2.2 years with relatively fewer risks and less radiation exposure. It is very likely that by 2033, in-flight refuelling would be tested and propulsion technologies would allow safer travel with more abort possibilities. 

Chapter 3: Weather on Mars

General Overview

Mars is the last planet of the inner four terrestrial planets in the solar system at an average distance of 141 million miles from our Sun.  It revolves around the Sun every 687 days and rotates every 24.6 hours (nearly the same as Earth).  Mars has two tiny satellites, named Deimos and Phobos (shown below).  They are most likely small asteroids drawn into Mars’ gravitational pull.  Deimos and Phobos have diameters of just 7 miles and 14 miles, respectively.  An interesting side note; the inner moon, Phobos, makes a revolution around Mars in slightly more than seven hours.  This means since it orbits Mars faster than the planet rotates, the satellite rises in the west and sets in the east if observed from the Martian surface.

Atmosphere and Weather

The Martian atmosphere is composed primarily of carbon dioxide.  However, unlike Venus, the Mars atmosphere is very thin, subjecting the planet to a bombardment of cosmic rays and producing the very little greenhouse effect.  Mariner 4, which flew by Mars on July 14, 1965, found that Mars has an atmospheric pressure of only 1% to 2% of the Earth’s.  Temperatures on Mars average about -63 degrees C.  However, temperature’s range from around -140 degrees C. in the wintertime at the poles, to +21 degrees C. over the lower latitudes in the summer.

Although the water in Mars’ atmosphere is only about 1/1000th of the Earth’s, enough water vapour exists that thin, wispy clouds are formed in the upper layers of the Martian NASA/Martian Surface from Pathfinderatmosphere as well as around mountain peaks.  No precipitation falls, however.  At the Viking II Lander site, frost covered the ground each winter.

Seasons do exist on Mars, as the planet tilts on its axis about 25 degrees.  Whitecaps of water ice and carbon dioxide ice shrink and grow with the progression of winter and summer at the poles.  Evidence of climatic cycles exists, as water ice is formed in layers with dust between them.  In addition, features near the south pole may have been produced by glaciers which are no longer present.

In general, Mars has highly variable weather and is often cloudy.  The planet swings from being warm and dusty to cloudy and cold.  Mars long ago was likely a warmer, wetter planet with a thicker atmosphere, able to sustain oceans or seas.