A new Cartesian implementation of the direct simulation Monte Carlo (DSMC) method, named the hypersonic aerothermodynamics particle (HAP) code, is presented. This code is intended for rapid setup and simulation of rarefied flow problems, and as a framework for evaluating new physical models and numerical techniques. Unique features include the use of nonuniform Cartesian adaptive subcells, a collision probability modification to reduce errors associated with spatial averaging in collision probabilities, and automatic planar element approximation of analytically defined two or three-dimensional surface geometries. In this work, simulations are performed using both HAP and an established DSMC code for a rarefied hypersonic flow over a flat plate, and excellent overall agreement is found. Additional simulations are employed to demonstrate reduced dependence on cell size through a proposed collision probability modification. Results are also presented for a threedimensional HAP simulation of hypersonic flow over a blunted cone, and reasonably good agreement with experimental data is observed.