Microgravity Testing of a Phase Change Reference on the International Space Station

Orbital sensors to monitor global climate change during the next decade require low drift rates for onboard thermometry, which is currently unattainable without on-orbit recalibration. Phase change materials (PCM), such as those that make up the ITS-90 standard, are seen as the most reliable references on the ground and could be good candidates for orbital recalibration. Space Dynamics Lab (SDL) has been developing miniaturized phase change references capable of deployment on an orbital blackbody for nearly a decade. To determine if and how microgravity will affect the phase transitions, SDL conducted experiments with ITS-90 standard material (gallium, Ga) on the International Space Station (ISS) and compared the phase change temperature with earth-based measurements. The Miniature On-orbit Thermal Reference (MOTR) experiment launched to the ISS in November 2013 on Soyuz TMA-11M with the Expedition 38 crew and returned to Kazakhstan in March 2014 on the Soyuz TMA-10 spacecraft. MOTR tested melts and freezes of Ga using repeated 6-hour cycles. This paper provides a brief history of the development of PCM references for space applications and describes the experiment hardware, microgravity considerations, and the pre-flight, flight and post-flight data analysis.