To minimize propellant carried, we have chosen the most practical,
high-efficiency spacecraft engine that is commercially available today:
the Xenon-ion
Electrostatic Propulsion ion thruster.
The original UMBRAS occulter was designed with minimizing the spacecraft mass and cost in mind, while maximizing the science and minimizing technology development. In order to do this, efficient engines must be used. The task of visiting many, many different science targets results in a great deal of propellant being carried onboard.
To minimize propellant carried, we have chosen the most practical,
high-efficiency spacecraft engine that is commercially available today:
the Xenon-ion
Electrostatic Propulsion ion thruster.
This engine has been successfully tested and used on the Deep Space 1 NASA mission which tested the technology for primary propulsion onboard a spacecraft. The tests have been successful, and we assume it as the baseline propulsion for most external occulter missions.
Because the external occulter spacecraft uses these thrusters, there are severe restrictions on where it can operate. These ion thrusters have very low thrust (each engine produces a push equivalent to only the weight (as felt on Earth) of a few sheets of 8.5"x11" paper! If it were anywhere near Earth, Earth's gravity would pull so hard on the external occulter and the space telescope that they would not be able to maintain alignment long enough to do the job of blocking a star.
To complicate matters, smaller versions of these engines, or possibly cold-gas thrusters, are used to help control the position and attitude of the occulter with respect to the telescope during observations. These thrusters also use fuel, and must not consume too much.
As a result, both telescope and occulter must operate far from Earth. They must lie at least as far from the Earth as Luna (our moon). Many NASA and ESA science missions have chosen to adopt orbits around the far Sun-Earth Lagrangian Point or 1-a.u. heliocentric orbits.
