satellites). The orbit decay model has been tested against ~1 year of actual GPS measurements of VELOX-C1, where the mean decay measured via GPS was 2.566 km across Dec 2015 to Nov 2016, and the orbit decay model predicted a decay of 2.444 km, which amounted to a 5% deviation. An open-source Python based software, ORBITM (ORBIT Maintenance the Propulsion Sizing), is available freely on GitHub for Python users using the above model. Proof of simplified model By the conservation of mechanical energy, the energy of the orbit is simply the sum of kinetic and gravitational potential energies, in an unperturbed two-body orbit. By substituting the Vis-viva equation into the kinetic energy component, the orbital energy of a circular orbit is given by: Where G is the gravitational constant, ME is the mass of the central body and m is the mass of the orbiting satellite. We take the derivative of the orbital energy with respect to the radius. The total decelerating force, which is usually atmospheric drag for low Earth orbits, exerted on a satellite of constant mass m is given by some force F. The rate of loss of orbital energy is simply the rate at the external force does negative work on the satellite as the satellite traverses an infinitesimal circular arc-length ds, spanned by some infinitesimal angle dθ and angular rate ω. The angular rate ω is also known as the Mean motion, where for a two-body circular orbit of radius R, it is expressed as: and... Substituting ω into the rate of change of orbital energy above, and expressing the external drag or decay force in terms of the deceleration αo, the orbital energy rate of change with respect to time can be expressed as: Having an equation for the rate of change of orbital energy with respect to both radial distance and time allows us to find the rate of change of the radial distance with respect to time as per below. The assumptions used in this derivation above are that the orbit stays very nearly circular throughout the decay process, so that the equations for orbital energy are more or less that of a circular orbit's case. This is often true for orbits that begin as circular, as drag forces are considered "re-circularizing", since drag magnitudes at the periapsis (lower altitude) is expectedly greater than that of the apoapsis, which has the effect