The past decade has seen two key developments that will have a lasting impact on the character of space missions.

(i) The 'faster-better-cheaper' initiative adopted by the National Aeronautics and Space Administration (NASA), with its goal to deliver programs with higher value at lower expense, shorter turn-around times, and increased safety.

(ii) The rapid advances in microelectromechanical systems (MEMS), which promise the integration of a multitude of functions in sensing, actuation, computation, and communication into microscopic components.

This special issue on Space Applications for MEMS shows the synergy between these two areas and demonstrates, with contributions provided from an extensive list of researchers, the potential of MEMS components and devices in a new generation of smaller, lighter, and more intelligent satellites. But let us first clarify some naming conventions from each respective field, which might otherwise cause some confusion. MEMS are micro-electromechanical systems, i.e., devices at a typical scale from hundreds down to fractions of micrometers. Thus, micro stands for one millionth of a meter. On the other hand, when referring to micro-satellites, the aeronautics and astronautics community usually denotes systems less than 100 kg mass. Furthermore, satellites below 10 kg are often dubbed nano-satellites, and so-called pico-satellites comprise less than 1 kg mass. In summary, the terms micro-satellite, nano-satellite, or pico-satellite are somewhat informal denominations.

As recent research efforts have shown, MEMS technology can be the basis for spacecraft components with improved performance in vastly reduced volume, by compressing increased functionality into single microchips. Applications span from propulsion and navigation to scientific instrumentation and communications. Arguably, in no other field is reduction of size and mass as important as in space, where lower mass or power consumption can translate into huge cost savings. For example, the launch cost is a significant factor in every space mission. To haul one kilogram out of Earth's gravity well, approximately 63 MJ of energy are needed. While this number in itself is not `astronomical', the cost for this task is high because of the specialized and sophisticated launch vehicle required. To put 1 kg into low earth orbit (LEO) currently costs approximately $20~000. To reach higher orbits or even interplanetary space presents significantly higher costs. These numbers give an indication that MEMS technology, with its potential to shrink complex instruments into a single microsystem, is extremely attractive.

This special journal issue starts with a thorough and up-to-date overview of the state of the art in MEMS, written by Jack Judy. It is followed by a discussion of the recently developed MEMS-specific processing technique of deep reactive ion enhanced (DRIE) etching and its application to the fabrication of microthrusters. The four subsequent papers also address micro propulsion, during free flight as well as during docking maneuvers. Sensors and actuators for in-flight monitoring are discussed in the following contribution, followed by four papers on MEMS-based communications systems in space. The final paper proposes microsystems to support a novel inflatable satellite architecture.

I would like to thank all the authors for their hard work, their excellent contributions, and their patience with the refereeing process. I would also like to express my gratitude to the anonymous reviewers who gave many valuable comments and suggestions, ensuring and improving the quality of this special issue. Finally, my special thanks go to Mary Ann Romig who handled the administrative parts of the editorial process.