I was recently discussing with my colleagues across the pond, the potential for mounting a cheap mission to some alternative, yet interesting destination in the inner Solar System, by exploiting a 'ride-share' with a more important mission, possibly one organised by NASA or ESA.
It struck me that since there have been, and what's more will continue to be, many, many missions to Mars, that would mean therefore a piggy-back on a Mars-bound mission might be a terrific opportunity. But that's all well and good in principle, what form should this alternative destination take?
As a result of further discussions, the idea of the Martian Trojans raised its head. These are asteroids hanging around at gravitationally stable points, either the Lagrange 4 (so 60 degs in advance of Mars in its orbit around the Sun) or at the Lagrange 5 (lagging 60 degs behind Mars).
But are there such objects? It turns out the answer is yes, and here is a wiki page about them. Generally they have quite high inclination orbits.
Some doubt remained as to whether a Martian Trojan would even be accessible from a mission already bound for Mars, but I was very much of the 'yes' persuasion as the answer to that question.
Not content with this big question mark, off I decided to go and made some mods to my software development, Optimum Interplanetary Trajectory Software (OITS) and soon I had a version capable of solving precisely this aforementioned problem.
The results? Look first at Figure 1.
We find that the ΔV to divert from a mission to Mars in 2028 and intercept with an L5 Trojan is generally on the rather-high side, over 2 km/s, which is quite challenging, naturally depending on the propulsion system available, but not infeasible.
However, without any expectation of finding any lower ΔV, I decided to focus instead on one of the only two known L4 Martian Trojans - refer Figure 2.
Lo-and-behold if we apply a minor ΔV on the way to Mars to slow our spacecraft down, and then wait for the spacecraft to arrive in the vicinity of Earth's orbit again, almost a complete ellipse, then applying a ΔV at this return point of 1.4 km/s, results in intercept of 1999 UJ7 in June 2031.