  The scientific mission of ESTCube-1, launched in May 2013, is to measure the
Electric solar wind sail (E-sail) force in orbit. The experiment is planned to
push forward the development of E-sail, a propulsion method recently invented
at the Finnish Meteorological Institute. E-sail is based on extracting momentum
from the solar wind plasma flow by using long thin electrically charged
tethers. ESTCube-1 is equipped with one such tether, together with hardware
capable of deploying and charging it. At the orbital altitude of ESTCube-1
(660--680~km) there is no solar wind present. Instead, ESTCube-1 shall observe
the interaction between the charged tether and the ionospheric plasma. The
ESTCube-1 payload uses a 10-meter, partly two-filament E-sail tether and a
motorized reel on which it is stored. The tether shall be deployed from a
spinning satellite with the help of centrifugal force. An additional mass is
added at the tip of the tether to assist with the deployment. During E-sail
experiment the tether shall be charged to 500~V potential. Both positive and
negative voltages shall be experimented with. The voltage is provided by a
dedicated high voltage source and delivered to the tether through a slip ring
contact. When the negative voltage is applied to the tether, the satellite body
is expected to attract electron flow capable of compensating for the ion flow,
which runs to the tether from the surrounding plasma. With the positive voltage
applied, onboard cold cathode electron guns are used to remove excess electrons
to maintain the positive voltage of the tether. In this paper we present the
design and structure of the tether payload of ESTCube-1.

Instrumentation and Methods for Astrophysics;Instrumentation and Detectors