Trajectories Getting to Mars
Let’s now take a look at some of the actual science behind a Martian expedition. To get to Mars you basically have 4 options when it comes to your trajectory. You can use a Conjunction class mission, an Opposition class one, or a Cycling one. The final option is a Brachistochrone trajectory, which barring some unforeseen scientific breakthrough, will not be useful anytime soon. The first trajectory classes to look at are Conjunction and Opposition. In both classes, the transit time from Earth to Mars is around 180 days (198 for Conjunction, 177 for Opposition). It is possible to decrease that if you have a sufficient propellent margin, or more efficient engines, but that extra capability would be better used increasing your payload capacity. In an opposition-class mission, the spacecraft stays at Mars for 40 days, (you could probably stay for up to around 100 before your return Delta-V requirements became unworkable), before performing a trans-Earth injection manoeuvre. The return journey takes around 342 days, and features the spacecraft diving sunwards and swinging by Venus for a gravity assist. The total round trip would be a 560 day excursion, of which only about 40 would be spent at Mars. The amount of Delta-V required varies, but would be around 8 to 12 kilometres per second. In a conjunction-class mission however, the spacecraft would stay at Mars for around 558 days, followed by a return journey taking 197 days, making the total around 1005 days. Of that, just over half the time would be spent at Mars. The last option is a Cycling trajectory. In this mission profile, a spacecraft is placed into an orbit which leaves it performing flybys of Earth and Mars periodically. Once it has been injected into this orbit, it will continue swinging between Earth and Mars forever, (with small course adjustments every so often). There are many different cycling orbits to chose from, but the most popular one is the Aldrin Cycler, named after legendary astronaut Buzz Aldrin who proposed the orbit in 1985, as part of his longstanding goal of getting humans to Mars. The Aldrin Cycler has an Earth-Mars transfer time of 146 days, after which it takes 480 days outside Mars’s orbit, before reencountering Mars and spending 146 days returning to Earth to start the cycle again. However, it cannot slow down to enter orbit, as that would break the cycle. As such, the Cycler would essentially act as an extra habitation module which a spacecraft going to Mars could utilise without increasing their propellant requirements. Which of these options is superior is a hard question to answer. Opposition class trajectories are very well suited to flag-and-footprints missions, and perhaps short-term scientific expeditions, if you take the hit on Delta-V and stay on the surface for a full 100 days. Plus, you get the added science from the Venus flyby on the return trip. However, a Conjunction class mission also has its own advantages, with a shorter amount of time spent in interplanetary space, and a much larger amount of time being spent planetside. This reduces the crews exposure to the deep space radiation environment, and gives them a much greater capability to explore the planet, but it comes at the cost of a much longer mission, meaning there is a higher chance of equipment failure. A Cycling trajectory is very similar to a Conjunction class mission, with a transit time of 146 days and a surface stay of 480 days. In fact, that mission profile could be flown normally, without using a Cycler, although it would have a slightly higher Delta-V cost. The advantage of a Cycler is it reduces the amount of stuff you need to lug around with you as you perform your transfer burns, basically giving you extra living space for free. However, it does dramatically increase the risk factor of the mission. Also depending on how your spacecraft is designed, you may have enough propellent to do the mission without using a cycling orbit, and bring your entire spacecraft with you. One thing is certain though, once voyages between Earth and Mars become commonplace, a Cycler will be very useful.
By Clarence, @aerospace_guy © World Space Club
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