An Ion Beam Shepherd is a concept in which the orbit and/or attitude of a spacecarft or a generic orbiting body is modified by having a beam of quasi-neutral plasma impinging against its surface to create a force and/or a torque on the target. Ion and plasma thrusters

Ion thruster

An ion thruster is a form of electric propulsion used for spacecraft propulsion that creates thrust by accelerating ions. Ion thrusters are categorized by how they accelerate the ions, using either electrostatic or electromagnetic force. Electrostatic ion thrusters use the Coulomb force and...

 commonly used to propel spacecraft can be employed to produce a collimated plasma/ion beam and point it towards the body.
The fact that the beam can be generated on a "shepherd" spacecraft placed in proximity of the target without physical attachment with the latter provides an interesting solution for space applications such as space debris removal, asteroid deflection and space transportation in general.
The Technical University of Madrid

Technical University of Madrid

The Technical University of Madrid or sometimes called Polytechnic University of Madrid is a Spanish University, located in Madrid. It was founded in 1971 as the result of merging different Technical Schools of Engineering and Architecture, originated mainly in the 18th century...

 (UPM) is exploring this concept by developing analytical and numerical control models in collaboration with the Advanced Concepts Team

Advanced Concepts Team

The Advanced Concepts Team is a group of scientists, researchers and young graduates that operate within the European Space Agency. Located at the European Space Research and Technology Centre, in the Netherlands, the team was instituted in 2003 with the objective of fostering advanced research on...

 of the European Space Agency. The concept has also been proposed independently by JAXA and CNES

CNES

The is the French government space agency . Established under President Charles de Gaulle in 1961, its headquarters are located in central Paris and it is under the supervision of the French Ministries of Defence and Research...

.

How it works

The force and torque transmitted to the target originate from the momentum carried out by the plasma ions (typically xenon) which are accelerated to a few tens of kilometer per second by an ion or plasma thruster. The ions that reach the target surface lose their energy following nuclear collision in the substrate of the target material.
In order to keep a constant distance between the target and the shepherd spacecraft the latter must carry a secondary propulsion system (e.g. another ion or plasma thruster) compensating for the reaction force created by the targeted ion beam.

Applications

The concept has been suggested as a possible solution for active space debris removal ,, as well as for accurate deflection of earth threatening asteroids .
Further in the future the concept could play an important role in areas such as space mobility, transportation, assembly of large orbital infrastructures and small asteroid capturing in Earth orbit.

Control

Beam divergence angles of ion and plasma thrusters, typically greater than 10 degrees make it necessary to have the shepherd flying not more than a few target diameters away if efficient beam overlap is to be reached. Proximity formation flying guidance and control as well as collision avoidance are among the most critical technological challenges of the concept.

External links

• http://www.aero.upm.es/ Universidad Politécnica de Madrid
• http://www.esa.int/act, ESA Advanced Concepts Team