Trajectory modeling is one of the most important aspects of any mission design. The trajectory should be able to propagate the S/C to the final destination while optimizing the flight duration, the total change in velocity and also the total launch mass. The Spacecraft Trajectory Optimizer (STO) tool described in this report first solves the Gauss Lambert problem and generates initial departure and arrival conditions which can also be expressed as porkchop plots. These initial conditions are then used as input to optimize the flight steps which are based on a patched conic approximation with the elliptical transfer with respect to the Sun and the hyperbolic transfers at the departure and arrival planet's sphere of influence. The tool is completely based on MATLAB 2007 or later and uses ODE45 for trajectory propagation and FMINCON with Active-set algorithm for optimization. The results obtained in house were compared with four Mars Sample return orbits calculated at ESOC and there is a very good correlation between the required change in velocities and transfer duration for e.g. Orbit case: O22S, ESOC values: total Delta V = 3.946 - 4.119 [km/s], TOF = 329 - 342 [days] &amp; STO values: Delta V = 3:986 [km/s] &amp; TOF = 335 [days]. The in house data was also used as an input in the System Tool Kit (a professional trajectory calculation software) for modeling an interplanetary trajectory to Mars and the S/C arrived at Mars without any optimization. Therefore, even though the STO does not have all the capabilities of a professional software it can be used for preliminary mission analysis as it offers quite accurate results for interplanetary transfers. / <p>Validerat; 20131127 (global_studentproject_submitter)</p>