  We propose a new method to estimate ion escape from unmagnetized planets that
combines observations and models. Assuming that upstream solar wind conditions
are known, a computer model of the interaction between the solar wind and the
planet is executed for different ionospheric ion production rates. This results
in different amounts of mass loading of the solar wind. Then we obtain the ion
escape rate from the model run that best fit observations of the bow shock
location. As an example of the method we estimate the heavy ion escape from
Mars on 2015-03-01 to be $2\cdot 10^{24}$ ions per second, using a hybrid
plasma model and observations by MAVEN and Mars Express. This method enables
studies of how escape depend on different parameters, and also escape rates
during extreme solar wind conditions, applicable to studies of escape in the
early solar system, and at exoplanets.
