MITSUBISHI | Power Tank Retract. 1.0 mm Blue | MI-SN220-BEColor Blue The Uni PowerTank will write on wet paper; upside down; in temperatures down to 20 degree C, and even in zero gravity. Just like a Space Pen except the whole PowerTank pen is less than half the price of a single 'space pen' refill. 0 mm Ball writes a 0. 4 mm line. Retractable with chunky rubber grip. Ideal for outdoor and professional use.