Scientists report that an active optical frequency reference based on molecular iodine was successfully tested for the first time in space. Results of the JOKARUS experiment (German acronym for iodine comb resonator under weightlessness) could be a step toward laser interferometric distance measurements between satellites, and future global navigation satellite systems based on optical technologies. The system used to demonstrate the first optical frequency standard based on molecular iodine in space. Courtesy of HU Berlin/Franz Gutsch. The experiment demonstrated the fully automated frequency stabilization of a frequency-doubled 1064-nm extended cavity diode laser (ECDL) on a molecular transition in iodine. The 1064-nm diode laser module was completely encapsulated in a 125- × 75- × 22.5-mm package. It delivered an optical power of 570 mW within the linewidth of the free-running laser of 26 kHz (full width half maximum, 1 ms measurement time). Scientists used a polarization-maintaining, optical single-mode fiber to divide the laser light into two paths. The laser beams were then modulated, frequency-doubled, and processed for Doppler-free saturation spectroscopy. The centerpiece of the laser system is a microintegrated ECDL MOPA (master oscillator power amplifier), with an ECDL as a local oscillator (i.e., a master oscillator or MO) and a ridge waveguide semiconductor amplifier acting as a power amplifier, or PA. During frequency reference tests carried out in May 2018 on board a sounding rocket, the compact laser system demonstrated its suitability for space. To provide a basis for comparison, a frequency measurement with an optical frequency comb from a separate experiment was performed during the same space flight. A microintegrated diode laser module (ECDL-MOPA) from the Ferdinand-Braun-Institut emitting at a wavelength of 1064 nm was successfully used in space. Courtesy of FBH/schurian.com. The JOKARUS payload was developed and implemented under the direction of the Humboldt-Universität zu Berlin (HU Berlin) and by the Ferdinand-Braun-Institut (FBH). A quasi-monolithic spectroscopy module was provided by the University of Bremen. The operating electronics were provided by Menlo Systems. The results of the JOKARUS experiment represent a milestone toward the use of optical clocks in space. Optical clocks in space could be used for satellite-based navigation systems that would provide data for accurate positioning and for fundamental physics research, including the detection of gravitational waves and measurements of the gravitational field of the Earth. A press release on JOKARUS is available here.