When the James Webb Space Telescope launches at the end of the year, it will be the largest, most important and most complex space telescope ever built. With more than 20 years of research and development for such an anticipated mission, when will we have the first data and the first images?
Contrary to what one might think at first glance, it will not be immediately after launch.
The telescope will take off, folded up in the Ariane 5 fairing. It will take 20 days to unfold and 30 days to reach its destination 1,5 million kilometers from Earth (for comparison, Hubble is only 550 kilometers from Earth), from where it will be able to carry out scientific observations in a wavelength domain that is more difficult to access otherwise: the infrared.
The launch of the JWST, or “Webb” for short, marks the beginning of a crucial phase for scientific observations, known as “commissioning” or “in-flight recipe”. For six months, all the subsystems that make up the telescope will be started and tested; including of course the four scientific instruments, including "MIRI" (for Mid-Infrared Instrument) to which France has contributed.
Deployment and test phases: a millimeter space ballet
The first phase starts 31 minutes after launch. It is the “deployment”: first that of the communication antenna; then 3 days after launch, as the telescope crosses the moon, the rest of the deployment begins. For 12 days, the observatory will slowly go from its folded configuration to fit into the Ariane 5 fairing to its unfolded form.
Solar panels and the heat shield get the ball rolling - for the shield, the process will be phased in alongside other deployments. Then, the observatory will slide along the tower which connects it to the heat shield and to the rest of the telescope. The stabilizer and the instrument radiators, placed behind the heat shield, will in turn be put in place. These are used to evacuate the heat emitted by the instruments.
Cool the telescope to allow it to observe in the infrared
The four JWST instruments observe in the infrared. On Earth, it is difficult to observe at these wavelengths because any object emits radiation as a function of its temperature : at terrestrial temperatures, the maximum emission is in the infrared. The telescope and the primary mirror must therefore be cooled to increase their sensitivity and avoid spurious signals from the entire observatory, which includes the telescope, instruments and subsystems.
In the case of the space telescope, the instruments start from the terrestrial 300K (25 ℃) to arrive at a temperature of 50K (-225 ℃) in the shadow of the heat shield. In the vacuum of space, the only way to "passively" cool is by “radiative dissipation”: we lose energy by emitting photons but we cannot count on convection by air… since there is no air.
Paradoxically, although the space is very cold, the vacuum implies that it is difficult to cool oneself. This stage therefore takes time: almost four months to fully stabilize.
For the three instruments observing the near infrared (between 0,6 and 5 micrometers), passive cooling to 50K is sufficient to attenuate the emissions from the telescope to allow observations.
For the MIRI instrument on the other hand, the only instrument to observe the average infrared (between 5 and 25 micrometers), it is necessary to reach an even lower temperature of 7K (-266 ℃): the thermal radiation at 50K is too great in the infrared and disturbs the measurements. So we had to add active cooling with a "Cryocooler".
Commissioning: a crucial test phase
The tests will take place in space, from the first moments after launch until it arrives in its stable orbit, at Lagrange point L2. Here, the gravitational pull of the Earth and that of the Sun is such that the telescope has its back to the Sun and the Earth at all times, making it easier to cool the telescope and its instruments.
The tests are controlled remotely from the control center at the Space Telescope Science Institute in Baltimore in the United States. There, night and day for six months, teams will take turns, two people per subsystem for around fifty subsystems, for example tracking the orbit or communicating with the Earth.
Of course, many tests have already been performed on Earth, in the world's largest test chamber that mimics vacuum conditions in space, in Houston in the United States. However, these tests only concerned the mirrors and instruments but not the largest parts: even the largest test chamber in the world is unable to accommodate the unfolded JWST ... the heat shield alone being about the size of a tennis court.
In addition, ground tests were optimized to test optics, but not sufficient to prepare for scientific observations. Additional specific tests must be performed in space to calibrate the instruments by observing already known sources (observed with other instruments before).
The in-flight acceptance phase is therefore the culmination of three years of preparation, planning and training in order to select the best observations and prepare the analysis software that will be used to detect and characterize any problems.
Time is also an important factor, as six months of preparation before acquiring data, for a mission with a nominal duration of five years, is a sizeable fraction. Everything has been done to keep this time as short as possible.
In details: 15 days after the launch, and for a period of 25 days, the various systems will be started and tested to ensure that everything works normally. At the same time, the observatory will reach its final orbit around the Lagrange L2 point 30 days after launch.
40 days after launch and for 80 days, the mirror will be tested and aligned. In parallel, the first part of the instrument calibration, called internal calibration (i.e. using internal lamps and not looking at the sky) will be carried out. These tests have already been carried out on the ground but must be redone in space, in particular to see if the space environment and the thermal profile of the telescope is as expected.
120 days after the launch and for 60 days until the end of the commissioning (six months after the launch) take place the external calibrations of the instruments. These will be the first images of the sky taken with scientific instruments. All instruments in their different modes of observation will be tested, to ensure that they are ready for science.
The first observations
From 155 days after the launch until the end of the commissioning, some observations will be made (Early Release Comments): once processed, these will be the very first images available to researchers and the general public that will illustrate the possibilities of the James Webb Space Telescope. For now, the instrument teams are not yet aware of what will be observed.
Then, around June 2022, when the tests are finished and it is time to move on to the operational phase, all the data accumulated during the testing phases will be made public and accessible: researchers around the world will be able to examine them. up close, and prepare for the exploitation of their future scientific data.
Christophe Cossou, CEA engineer, developer for the JWST / MIRI instrument at the Astrophysical Laboratory, instrumentation, modeling of the CEA / CNRS, University of Paris; dan dicken, Project Scientist, Paris-Saclay University et Pierre-Olivier Lagage, CEA researcher at the Astrophysical Laboratory, instrumentation, modeling of the CEA, CNRS, University of Paris
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