The point of an orbit that is closest to the planet or other celestial body being orbited. Spacecraft often carry out mapping of planetary surfaces during the time of periapsis passage. The spacecraft's periapsis altitude can be increased by firing its thrusters when apoapsis is reached, and the craft's altitude can be reduced by using the thrusters to slow the vehicle, or by aerobraking. Selective firing of the thrusters at periapsis can also increase or decrease the vehicle's altitude at apoapsis.