The James Webb Space Telescope (JWST) is the next of NASA’s Great Observatories; following in the line of the Hubble Space Telescope, the Compton Gamma-ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope. JWST combines the qualities of two of its predecessors, observing in infrared light, like Spitzer, with fine resolution, like Hubble. Credit: NASA, SkyWorks Digital, Northrop Grumman, STScI
1. What’s happened since the telescope launched?
After the successful launch of the James Webb Space Telescope on December 25, 2021, the team began the long process of moving the telescope into its final orbital position, unfolding the telescope and – as everything cooled – calibrating the cameras and sensors onboard.
The launch went as smoothly as a rocket launch can go. One of the first things my colleagues at NASA noticed was that the telescope had more remaining fuel onboard than predicted to make future adjustments to its orbit. This will allow Webb to operate for much longer than the mission’s initial 10-year goal.
The first task during Webb’s monthlong journey to its final location in orbit was to unfold the telescope. This went along without any hitches, starting with the white-knuckle deployment of the sun shield that helps cool the telescope, followed by the alignment of the mirrors and the turning on of sensors.
Once the sun shield was open, our team began monitoring the temperatures of the four cameras and spectrometers onboard, waiting for them to reach temperatures low enough so that we could start testing each of the 17 different modes in which the instruments can operate.
The NIRCam, seen here, will measure infrared light from extremely distant and old galaxies. It was the first instrument to go online and helped align the 18 mirror segments. Credit: NASA/Chris Gunn
2. What did you test first?
The cameras on Webb cooled just as the engineers predicted, and the first instrument the team turned on was the Near Infrared Camera – or NIRCam. NIRCam is designed to study the faint infrared light produced by the oldest stars or galaxies in the universe. But before it could do that, NIRCam had to help align the 18 individual segments of Webb’s mirror.
Once NIRCam cooled to minus 280 F, it was cold enough to start detecting light reflecting off of Webb’s mirror segments and produce the telescope’s first images. The NIRCam team was ecstatic when the first light image arrived. We were in business!
These images showed that the mirror segments were all pointing at a relatively small area of the sky, and the alignment was much better than the worst-case scenarios we had planned for.
Webb’s Fine Guidance Sensor also went into operation at this time. This sensor helps keep the telescope pointing steadily at a target – much like image stabilization in consumer digital cameras. Using the star HD84800 as a reference point, my colleagues on the NIRCam team helped dial in the alignment of the mirror segments until it was virtually perfect, far better than the minimum required for a successful mission.
3. What sensors came alive next?
As the mirror alignment wrapped up on March 11, the Near Infrared Spectrograph – NIRSpec – and the Near Infrared Imager and Slitless Spectrograph – NIRISS – finished cooling and joined the party.
NIRSpec is designed to measure the strength of different wavelengths of light coming from a target. This information can reveal the composition and temperature of distant stars and galaxies. NIRSpec does this by looking at its target object through a slit that keeps other light out.
NIRSpec has multiple slits that allow it to look at 100 objects at once. Team members began by testing the multiple targets mode, commanding the slits to open and close, and they confirmed that the slits were responding correctly to commands. Future steps will measure exactly where the slits are pointing and check that multiple targets can be observed simultaneously.
NIRISS is a slitless spectrograph that will also break light into its different wavelengths, but it is better at observing all the objects in a field, not just ones on slits. It has several modes, including two that are designed specifically for studying exoplanets particularly close to their parent stars.
So far, the instrument checks and calibrations have been proceeding smoothly, and the results show that both NIRSpec and NIRISS will deliver even better data than engineers predicted before launch.
The MIRI camera, image on the right, allows astronomers to see through dust clouds with incredible sharpness compared with previous telescopes like the the Spitzer Space Telescope, which produced the image on the left. Credit: NASA/JPL-Caltech (left), NASA/ESA/CSA/STScI (right)
4. What was the last instrument to turn on?
The final instrument to boot up on Webb was the Mid-Infrared Instrument, or MIRI. MIRI is designed to take photos of distant or newly formed galaxies as well as faint, small objects like asteroids. This sensor detects the longest wavelengths of Webb’s instruments and must be kept at minus 449 F – just 11 degrees F above 