While an old idea (30 years old) Ballistic Capture is being looked at with renewed interest regarding Mars. What is Ballistic Capture?
When a spacecraft is sent to Mars, the current method is one ‘shoots’ it towards Mars to where it will be with as much speed as possible and then when the spacecraft is close enough it must slow down and wait for Mars to catch up to it and pull it into its orbit. There are a few drawbacks however; It is quite expensive AND relies heavily on timing.
Ballistic Capture? The spacecraft aim’s way ahead of where the planet is going to be and goes just slightly slower. This results in the spacecraft being slingshot away from the planet resulting a speed boost that laces the spacecraft in orbit around the planet. So what’s the down side? The six months it would normally take, well add on another two. BUT the fuel usage is about 25% less meaning either you save money or you can send more cargo for the same amount – you decide! Oh and one other thing - there is no free-return trajectory.
The downside? It takes a minimum of ~8 months to enter Mars orbit as opposed to 6 with a Hohmann orbit, and you also lose your free-return possibility. But you use 25% less fuel for a given payload, so you can either save money, or launch way more payload for the same amount of money).
Free return trajectories are not really possible from Mars unless you are cargo or maybe some sort of unmanned spacecraft. Earth& Mars are no longer in the proper positions to return to Earth using the same method. In theory, you could go on a more elliptical path that would take you closer to the Sun and return to Earth, but in that case, you'd return to Earth going much higher speeds than necessary to safely enter Earth’s orbit. Since your propulsion system will be needed in order to make an Earth encountering orbit, it would end up defeating the whole purpose of Ballistic Capture.
Actually there is one possibility, but any trajectory using ballistic capture would take an unseemly amount of extra time during transfer (at least four more months on a round trip, quite possibly more) – and while the transfer window is longer (it can run outside of transfer windows) the actual time taken to get there would make running transporting people to and fro, and give them more time in deep-space for things to go wrong and/or rads - radiation absorbeddose. Concepts such as radiation-protection and other deep-space hazards being such unknowns I would not expect any of our early missions to Mars to utilize it.