Among orbital nerdery ToughSF directs to Marco Bassetto and Alessandro Quarta, Solar Sail-Based Mars-Synchronous Displaced Orbits for Remote Sensing Applications. The notion is to orbit the Sun to shadow a planet, on the unstable positions of always-over-the-pole. Which means orthogonal to the ecliptic and all five its Lagrange points: "displaced nonKeplerian".
This is accomplished by solar-sails. The authors believe that Mars' 1/10 Earth mass and 1/3 surface-gravity allow a reasonably low-altitude as can map the poles. Of course Mars also gets less than half the Solar irradiance but space is cheap out there. Just unfurl twice the sail bro. If I read figures 11b and 13b correctly, they'd float about 0.01 AU above or below the Martian ecliptic plane which is 1.5 million-kilometers or, probably better, gigameters.
This would constrain a Mars mission not by propellant but by battery life and/or solar panel degradation. Like docking Deimos.
Injection into that orbit, from a base in high Earth orbit, is also handled here. There's a boost of 1 km/s deltaV to kick off to intercept the Martian DNKO. The sail takes it from there to bring the craft to where it needs to be. Minimum 949 days if the sail is mid-performance, 498 if high-performance.
These are functions of the "lightness number" β, the higher the... betta'. The paper knows of 2021-designed sails all β shy of 0.01. Bassetto & Quarta believe these rookie numbers can be improved. Lower perf does get you closer to Mars however.
There seems no plan to park these over Venus' poles. Six years ago I mooted a network of balloons perhaps angled against the vortex, per pole. From data-relays to polar mapping to weather observation, this does everything a gigameter-off sail could do.
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