Hello! I was wondering if there are anyone here who could help me with a little physics and logic pertaining to the behavior of planets. I've been working on a science fiction story set in a fictional planetary system containing an earth-sized planet and a mars-sized planet. It's important to me that this system makes somewhat sense. I know there are way too many factors to account for to get everything perfectly right, but I don't want to mess up on the basics What's important to the story is the following: 1 - The planets are tidally locked, so that the mars-sized planet can always be seen in the sky over one side of the earth-sized planet. 2 - The planets are so close to each other that the illumination of the mars-sized planet lights up the night sky enough to affect the eco-system of the earth-sized planet (for example, the light being reflected creates a different way to camouflage). 3 - They're not close enough to potentially crash into each other obviously 4 - The rotational period is as short as possible (meaning, not so long that the temperature change between night and day makes it unlivable, but not so short that it affects the relations between the two planets either). The numbers and such that I’ve come up with so far, based on comparisons with what I’ve read about the relations of Earth, the moon, Mars, Pluto and Charon: The Larger Planet: Mass: 5.3211877 * 10^24 kg Equatorial Radius: 6,218.465 km2 Surface Area: 485,036,000 km2 Surface Gravity (eq.): 9.182 g Rotation Period: 32h 4m 4.9s (how long I’d like it to be, but I have no clue) Orbital Period: 323.337245 days Axial Tilt: 12.92934 ° Mean Surface Temperature: 17 °C Min. Surface Temperature: -95 °C Max. Surface Temperature: 63.5 °C One year is equal to: 118.3% of an Earth year The Smaller Planet: Mass: 7.02396788 * 10^23 kg Mass compared to Planet: 13.2 % Equatorial Radius: 4088,845 km2 Distance between them: 105,546 km (based on the distance between Charon & Pluto) So I guess the main thing is; how close can the two planets be to each other and how fast can they spin around each other without catastrophic repercussions? And one last thing - this one might be better suited for a different forum, so just ignore it if it's too of topic: Is it possible for a certain area of the planet to have low enough tectonic plate movement to preserve a crater proportionally equal to Mimas' Herschel crater for billions of years? Even if the planet is earth-sized with a magnetic field, atmosphere and everything to a certain approximation of what Earth has?