A trace is a particle that is used to follow the positions and velocities of certain objects during a model run. The resulting path through space followed by the trace can be plotted. Only one object may be traced per trace command. Traces may be added at any time. The contents of all traces can be erased with the trace purge keyword, and all traces can be deleted with trace delete. A summary of all traces can be obtained by using trace list. Traces are sampled at a rate controlled by the trace interval command; all traces will use this sampling rate.