Interviews with the “Cold Stowage” team at NASA’s Kennedy Space Center in Florida, USA, that ensure science on the International Space Station stays cool.

Many experiments run on the Space Station require freezing samples for analysis later on Earth, or are sent into space frozen and thawed out in orbit. The European-built MELFI (Minus Eighty Lab Freezer for ISS) keeps these samples at the right temperature.

From a virus to algae or muscle biopsies, many samples have been sent and stored in space over a decade years that the freezers have been operating. The technology behind keeping the science cool in space has found its way back to Earth too, limiting losses when transporting liquid gas in tankers.

Built by ESA and transferred to NASA and JAXA, MELFI is a versatile storage freezer. The four compartments can be set at different temperatures ranging from −98 °C to +4 °C to preserve biological samples such as blood and urine that will be returned to Earth.

Building a freezer that can reach such cold temperatures while meeting other standards for operation on board the Space Station was not an easy feat. The facility must be reliable and easy to maintain, energy efficient, quiet, and have little effect on the Space Station’s microgravity environment.

A closed thermodynamic loop using nitrogen gas and a Brayton Machine rotating at over 90 000 revolutions per minute was selected to meet these requirements.

Nitrogen gas is an ideal cooling agent as it abundant, odourless, and nonreactive. In the closed system, nitrogen gas is compressed, cooled, and expanded through tubes surrounding the compartments. It exchanges heat with everything it comes into contact with, which accelerates the cooling.