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Orbital Calculator is free software I've written for Windows and Linux (in both 32-bit and 64-bit modes), that calculates all things orbital. Its primary objective is to calculate orbital characteristics around any gravitational mass, but it has many functions including calculating satellite visual footprints, whether an object (tower, mountain etc) is visible over the horizon from any point on the ground and so forth.

I wrote this software so Sci-Fi writers could get the science right. For example, it is easy to discover the surface gravity of a gravitational mass, or at what distance its 'geostationary' orbit lies.

It is now available in version 2.0.4, which adds the ability to calculate Hohmann transfer orbits and the distances between stars.

This is an example result of a Hohmann transfer orbit from Earth to Mars:
ΔV @ inner orbit: 2.929 km/s
ΔV @ outer orbit: 2.637 km/s
Average Velocity: 26.536 km/s
Transfer Time : 258d 06:41:44

If you know the right-ascension and declination and the distance of a pair of stars from Earth (perhaps culled from Wikipedia or a star map), the program can calculate the distance between the stars.

The program doesn't have to be installed (just unzip it and run - meaning it can be run from a USB stick) and comes with an extensive help system with explanations of all the functions and how to use them.

Link to description page and list of functions: https://philip-p-ide.uk/doku.php/blog/articles/software/orbital_calculator

Link to download page: https://philip-p-ide.uk/doku.php/blog/aardvaark/downloads