A breadboard 4.5K helium sorption cooler for use in vibration-sensitive space missions was developed and successfully tested. This type of cooler has no moving parts and is, therefore, essentially vibration-free. The absence of moving parts also simplifies scaling down of the cooler to small sizes, and it contributes to achieving a very long lifetime. In addition, the cooler operates with limited dc’s so that hardly any electromagnetic interference is generated. This cooler is a favorite option for future missions such as ESA’s Darwin mission, a space interferometer in which the sensitive optics and detectors can hardly accept any vibration. The system design consists of a hydrogen stage cooling from 80to14.5K and a helium stage establishing 5mW at 4.5K. Both stages use microporous activated carbon as the adsorption material. The two cooler stages need about 3.5W of total input power and are heat sunk at two passive radiators at temperatures of about 50 and 80K—radiators which are constructed at the cold side of the spacecraft. We developed, built, and tested a demonstrator of the helium cooler. This demonstrator has four sorption compressor cells in two compressor stages. Test experiments on this cooler showed that it performs within all specifications imposed by ESA. The cooler delivered 4.5mW at 4.5K with a long-term temperature stability of 1mK and an input power of 1.96W. So far, the cooler has operated continuously for a period of 2.5months and has not shown any sign of performance degradation.