zeptowatt (zW) The sextillionth [U.S.] or quadrillionth part [U.K.], 10-21 [0.000 000 000 000 000 000 001] of a watt or the approximate power of Galileo space probe's radio signal (when at Jupiter) as received on earth by a 70 meter DSN antenna, or the Deep Space Network, which is an international network of communication facilities that suppor...