An autonomous spacecraft propellant-gauging system, the system including a propellant tank, one or more heating devices, at least one temperature sensor and a processor. The heating devices are used to heat up the propellant tank, and the temperature sensors sense the temperature of the propellant content of the propellant tank. The processor controls operations of the heating devices and the temperature sensor. The processor further executes an algorithm to automate gauging of the propellant content of the propellant tank based on a reduced order model (ROM) and a number of parameters, and reports out an estimate of the mass of the remaining propellant of the propellant tank.