Making multifilament wires for electric solar wind sails

High-velocity protons stream copiously from the surface of the Sun. Harnessing their momentum for interplanetary travel is the goal of a novel propulsion system known as E-sail. Invented in 2004 by Pekka Janhunen of the Finnish Meteorological Institute, E-sail consists of long electrically conducting tethers that fan out from the spacecraft like the spokes of a bicycle wheel. Each tether consists of a web of interlaced wires, whose holes allow micrometeorites to pass through and whose electric field penetrates up to 100 m into the surrounding plasma. When protons encounter the positively charged tethers, they’re deflected, and that generates thrust. Although the E-sail converts momentum efficiently, propelling a modestly sized spacecraft requires the tethers to be very long. To meet that criterion without overburdening the spacecraft with inertia, the wires must be very thin. In a new paper, a team led by the University of Helsinki’s Henri Seppänen reports the culmination of a four-year project to make an entire tether of micron-scale wires automatically. The prototype tether is 1 km long, consists of nearly 100 000 loops formed by three 25-μm-thick aluminum wires bonded to a 50-μm-thick base wire, and weighs just 10 g. The figure shows a typical section, with one loop highlighted in red. Making, checking, and repairing—if necessary—so many bonds entailed customizing a manual wire bonder and incorporating a microscope, image analysis software, and computer control. A shorter, manually produced E-sail is currently being tested aboard Estonia’s first-ever orbiter, the tiny ESTCube-1. (H. Seppänen et al., Rev. Sci. Instrum. 84, 095102, 2013.)