MIRI was the last instrument on the James Webb Space Telescope to reach the temperature required for scientific operations, marking an incredibly important stage in Webb's progress towards full science mode.

The coldest instrument on Webb

The James Webb Space Telescope, known as Webb, is the largest, most powerful telescope ever launched into space and MIRI is one of four scientific instruments on board.

MIRI was last to reach this important milestone as it operates at 7 kelvin (-266 Celsius), a colder temperature than the other instruments.

To reach this temperature, MIRI needed to be actively cooled by a cryogenic refrigerator, or cryocooler. The cryocooler was specially developed by NASA's Jet Propulsion Laboratory while the Science and Technology Facilities Council's (STFC) RAL Space in the UK were responsible for overall instrument thermal engineering.

If too much heat is transferred to the cooler, MIRI cannot reach temperature. Therefore, a key role for RAL Space's thermal engineers was to develop a design that minimised this heat transfer.

This is because the instrument sees in mid-infrared light and every object emits infrared radiation, or heat, all the time. Therefore, MIRI must be cold to make sure its detectors do not pick up any background heat from the instrument itself, obscuring what the astronomers actually want to see.

Dr Bryan Shaughnessy, Thermal Engineering Group Leader at RAL Space said: “I have been involved in the thermal design, analysis and testing for MIRI for almost 20 years so to finally see the instrument reaching 7 kelvin in space and well on its way to science operations is incredibly exciting. Our team has worked hard to make sure everything works at this challenging temperature and the last few months monitoring the cool down has been the culmination of all of our efforts. I'm looking forward to seeing the first results at the end of the commissioning process."

The mission so far

After successfully launching on Christmas Day 2021, Webb spent a month travelling through space to reach its new home in orbit 1 million miles from Earth. Over the last two months the commissioning team have been working on aligning the 18 mirror segments and cooling down the scientific instruments.

Scientists, engineers and astronomers from STFC's UK ATC and RAL Space are actively involved in this complex and painstaking commissioning process. Working alongside international colleagues at the Space Telescope Science Institute in Baltimore monitoring and commanding MIRI and Webb in shifts, 24 hours a day.

This work follows years of testing, preparation and rehearsal ​by the UK team both before and after the instrument was delivered to NASA in 2012. While still at RAL Space in Oxfordshire, MIRI underwent 100 days in a thermal vacuum chamber where it spent 85 days below room temperature to make sure all the components would work correctly at the low temperatures needed. Further cryogenic testing was then carried out with the other Webb instruments at NASA's Goddard Space Flight Center and the integrated telescope and instruments were thoroughly tested at Johnson Space Center.

Professor Gillian Wright, European Principal Investigator for MIRI and Director of UK ATC, said: “It is fantastic to know that the instrument has successfully reached operating temperature. The MIRI cryocooler is a real technical first, and it is a great testament to the talents of our JPL partners, who led the cryocooler development, that the cool down process has worked as anticipated. As the last instrument to reach operational temperature this a significant milestone in MIRI and Webb's lifetime."

Now MIRI has successfully cooled down the commissioning process will continue and the next few months will be spent testing MIRI's systems and making final alignment adjustments to Webb before we can expect the first spectacular images of the cosmos from Webb this summer.

RAL Space MIRI team at the Space Telescope Science Institute in Baltimore​. Credit: STFC RAL Space.

Team MIRI

Webb's mission to explore the universe is led by NASA, the European Space Agency (ESA) and the Canadian Space Agency. The UK played a major role by leading the European Consortium which, partnered with JPL and other US institutes, designed, built and tested MIRI. This work included STFC's UK Astronomy Technology Centre (UK ATC), STFC RAL Space, University of Leicester, and Airbus UK.

The MIRI development has been funded by the UK Space Agency and STFC, part of UK Research and Innovation, and ESA.

All the MIRI partners have key roles in preparing the commissioning and calibration of the instrument and support post-launch verification.​

Find out more:

​From the Science and Technology Facilities Council​

From NASA's Jet Propulsion Laboratory​