The four cutting-edge instruments on Space Telescope have begun to power up, and the observatory is ready to take its first images of its target star.
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This star, called HD 84406, is located 241 light-years from Earth and is part of the constellation Ursa Major. The images will not be used for scientific purposes, but will help ground teams align the 18 segments of Webb's 6.5-meter-wide primary mirror.
The images will be taken by Webb's Near Infrared Camera (NIRCam) , which must first cool to its operating temperature of minus 153 degrees Celsius.
“At the beginning, we’ll have 18 individual blurry images,” said Mark McCaughrean, a scientist on the JWST Science working group and a senior advisor at the European Space Agency (ESA), who is familiar with the process. “At the end, we’ll have a nice sharp image.”
NIRCam will continue to stare at the star HD 84406 while Webb's specialists move the mirror sections in nanometer-scale steps to create a perfectly smooth surface. This work is expected to last until the end of April. Only after that will the individual science instruments begin to fully use their camera on objects in the near and distant universe. The first proper images are expected to be revealed to the public in late June or early July.
McCaughrean said none of the other three instruments could take on NIRCam's job of helping align the mirror. The telescope's success depends on NIRCam, and it must not fail.
"If NIRCam failed, we wouldn't be able to align the mirror," McCaughrean said. "So it's essentially two cameras in one. If one fails, we still have the other one."
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Of the remaining three instruments, the Mid-Infrared Imager (MIRI) has already been partially activated during the telescope's journey to its destination. In the case of the other two – the Near Infrared Spectrograph (NIRSPec) and the Fine Guidance Sensor/Near Infrared Imager and Slitless Spectrograph (FGS/NIRiss) – control teams have now turned off the heaters that kept them warm during the journey phase.
These heaters allowed the instruments to gradually release the air trapped inside them and prevent water condensation and ice buildup.
It will take weeks for the instruments to reach their operating temperature. For MIRI, that temperature is just 5.5 degrees Celsius above absolute zero (minus 273 degrees C), the coldest possible temperature at which the motion of atoms (which is the source of heat in the universe) stops. The spectrographs can operate at slightly higher temperatures, at minus 236 degrees C.
These extremely low temperatures are key to Webb's ability to perform its scientific tasks. The telescope was designed to image the oldest stars and galaxies that formed in the universe in the first few hundred million years after the Big Bang. But because of the expansion of the universe, the light emitted by these galaxies is visible only at infrared wavelengths. Since infrared light is essentially heat, the faint signal would not be detectable if the telescope itself were radiating any heat.
While cameras, such as NIRCam and MIRI, will produce stunning images of stars and galaxies, spectrographs will provide detailed information about the chemical composition of these distant objects, McCaughrean explained.
The James Webb Space Telescope arrived at its destination, Lagrangian Point 2 (L2), on January 24. L2 is a point on the Sun-Earth axis that is 1.5 million kilometers from Earth. The gravitational interaction between the two bodies creates stable conditions at L2, making it a popular spot for astronomy missions. A spacecraft at this point orbits the sun in sync with Earth, making circles as it accompanies Earth around the sun.
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The James Webb Space Telescope launched on December 25 after a decade of delays. The $10 billion mission, dreamed up by astronomers in the early 1990s, has surpassed the limits of what is technically possible. Once its mirrors are aligned and its instruments calibrated, Webb is expected to revolutionize many areas of astronomy. In addition to the first stars and galaxies, Webb will contribute to the study of exoplanets, star formation, dark matter, and even the solar system and its asteroids.
