A French team took part in the full-scale tests recently completed on the James Webb Space Telescope (Webb), the successor to the Hubble Space Telescope. Webb is a programme being led by NASA in partnership with Europe (through the European Space Agency, or ESA) and Canada.

For these cryogenic tests, which began on August 22^nd and ended on September 26^th, the telescope and its instruments were placed inside the giant vacuum chamber (Chamber A) at NASA's Johnson Space Center in Houston, Texas. This is the only cryogenic chamber in the world large enough to house the telescope and its 6.5-metre mirror (by comparison, Hubble's mirror spans just 2.4 metres). Then, Webb was slowly heated up and pressurized again to allow the reopening of the chamber.

Inside the chamber, the temperature is cooled to as low as –253°C. The chamber also has a star simulator, a device consisting of several mirrors and infrared light sources used to verify the optical quality and alignment of Webb's mirror by comparing actual images obtained with simulated ones. This is the first time the telescope and instruments have been tested together. The instruments had previously undergone a series of three cryogenic tests on their own inside a smaller chamber (the Space Environment Simulator) at NASA's Goddard Space Flight Center in Greenbelt, Maryland, from 2013 to 2016. "A number of French research laboratories are closely involved in one of the four instruments on Webb, the Mid-InfraRed Instrument (MIRI), and more especially its Mid-InfraRed IMager (MIRIM)," says Anne Peyroche, President of CNRS, the French scientific research centre. They are the LESIA space and astrophysics instrumentation research laboratory (Paris Observatory-CNRS-UPMC-Paris Diderot University), the LAM astrophysics laboratory in Marseille (CNRS-AMU), the IAS space astrophysics institute (CNRS-Paris Sud University) and the AIM astrophysics joint research unit (CEA-CNRS-Paris Diderot University), with CEA's Institute of research into the fundamental laws of the Universe (Irfu) as prime contractor. Oversight is being assured by CNES.

Webb's travels are not over yet, as it will next be taken to California for integration with the spacecraft and sunshield. The full Webb observatory will then be sent by sea to Kourou, French Guiana, where Jean-Yves Le Gall, President of CNES, which is overseeing MIRI, is "eagerly awaiting the telescope, which has shown once again with the completion of cryogenic testing what international cooperation in space can achieve, for its launch atop an Ariane 5 late in the Spring of 2019." Webb will then be positioned in its final operating orbit at the L2 Lagrange point 1.5 million kilometres from Earth, that is, about four times further than the Moon. "If anything goes wrong out there, we obviously won't be able to send up a team to repair it like we did for Hubble¹. That's why these final tests conducted by teams from France on the JWST and its instruments are so important," underlined Daniel Verwaerde, Chairman of CEA, the prime contractor for MIRI.