Comets are small solar-system bodies that consist of several kinds of ice, minerals and organic substances. The core material is still very much like the primordial matter from which the planets of our solar system were made, about 4.5 billion years ago. When a comet approaches the sun, it warms up and starts to emit gas and dust, which results in spectacular comet tails.

Asteroids, also known as minor planets, are rocky and metallic objects, which orbit the Sun. They range in size from a diameter of 1000 km, down to the size of pebbles. They have been found all over the planetary system. Most, however, are contained within a main belt that exists between the orbits of Mars and Jupiter, a region known as the asteroid belt, as well as beyond Neptun's orbit (Kuiper Belt).

The investigation of these objects with spacecraft has a relatively short history. A cometary nucleus (Halley's comet) was photographed up close for the first time in 1986 by the Vega 1/2 and Giotto missions. These spacecraft also measured the structure and composition of the emitted gas and dust particles. IWF has made important contributions to these missions through experiments (magnetic field measurements) and theoretical studies (gas and dust jets).

The objective of the Rosetta mission (as part of ESA's "cornerstone" program) is to observe over a long time the nucleus of comet Churyumov-Gerasimenko and to put a lander on the surface of the nucleus. Amongst others, IWF built an instrument to investigate the emitted dust particles (MIDAS) and is involved in another dust experiment (COSIMA) as well as an experiment to investigate the physical properties of the nucleus' surface (MUPUS). The magnetic properties of the comet's nucleus will be measured by two experiments: RPC-MAG and ROMAP.

A further mission to investigate comets and asteroids is the NASA-mission Deep Space 1, which started in October 1998. During the encounters with asteroid Braille and Comet Borelly many useful data were obtained on the structure of these two objects.