We investigate the interaction of plasmas with a spacecraft in low Earth orbit (LEO) using NASCAP/LEO, a commercial, electrostatic particle simulation code, adapted especially to the design of LEO spacecrafts. Charged particles that accumulate to a spacecraft surface cause spacecraft charging, which leads to a drop in the floating potential of the spacecraft. The key parameters that affect the floating potential of a spacecraft are the current generated by solar arrays, the bias voltage applied on interconnectors, and the area and direction of conductors constituting the spacecraft surface. Of these parameters the current generated by solar arrays plays the most important role. For example, floating potential is lowered to -18V from -1.5V when the solar arrays are in operation. The charge is more or less abrupt at the point when the solar arrays generate a quarter of the maximum current. Comparing the simulation results with the ETP data from KITSAT-3, we estimate the temperature of plasmas in the low altitude ionosphere surrounding KITSAT-3. The obtained temperatures are 0.1~0.2 eV in the nightside and 0.2~0.3 eV in the dayside.
