## 1 Introduction  
An important research field of the Institute of Space Systems is modeling of the space debris environment (cf. Figure 1).  
For more than two decades, the Institute has been responsible for developing the European reference model for description of the space debris environment. The model is called MASTER (Meteoroid and Space debris Terrestrial Environment Reference) and is distributed by the ESA's Space Debris Office ([[https://sdup.esoc.esa.int](https://sdup.esoc.esa.int)]([https://sdup.esoc.esa.int](https://sdup.esoc.esa.int))). The space debris group of the Institute has many years of experience in the field of scientific description of the various sources of space debris, especially in the small particle size regime. This includes the comparison of the scientifically generated particle population with actually obtained measured values. For the validation of the MASTER population, a tool is used that simulates the detection capability of different detection sensors such as radars and telescopes. This tool, called PROOF (Program for Radar and Optical Observation Forecasting), uses the MASTER population and simulates detection rates of the observational measurements [1]. It allows a comparison of simulations with actual measurement campaigns and is therefore used to validate the MASTER model. The first version of PROOF was developed in cooperation with the Astronomical Institute of the University of Bern (AIUB) and the Fraunhofer Institute for High Frequency Physics and Radar Techniques (FHR, formerly FGAN). The current version of PROOF is described in [2].  
The ongoing research at the Institute covers a broad spectrum of space debris related topics ranging from providing a sophisticated model for the space debris environment over the evaluation of upcoming mega-constellations to Space Surveillance and Tracking (SST) infrastructures, precise orbit determination and Active Debris Removal (ADR). This article is a summary publication that aims to present the current state of research at the Institute. Many of the current activities are based on the expertise in the field of space debris modeling. From this main area, many spin-offs have been derived. The contribution to the full research field of space debris is significant when it comes to the MASTER model. The provided space debris population is used in engineering tools that provide risk assessments for operational spacecraft, e.g., debris and meteoroid flux levels or collision avoidance maneuver frequencies, but also provide detailed insight into the space debris situation in terms of highly populated orbit regimes and temporal evolution of the environment. In addition, future population trends are provided that are especially interesting for risk analyses of future satellite missions and the upcoming mega-constellations. The research also spans the development and evaluation of different ADR measures, which combines the longtime experience in orbital dynamics with the development of satellite hardware that will help stabilize the constantly growing space debris environment. Furthermore, the Institute provides support to any space observation segment in terms of design, operation and maintenance. The current state of research in these fields is mentioned and selected research results are presented and discussed.