Hubble’s Successor: James Webb Space Telescope

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This year, 2021, will witness a historical event for humankind that will revolutionize our understanding of the universe. James Webb Space Telescope (Webb or JSWT), the largest and most complex telescope, is finally going to launch this December. Webb is a collaborative effort between NASA, the European Space Agency (ESA), and the Canadian Space Agency (CSA). It is an infrared observatory built to succeed Hubble and expand its discoveries. To understand how Webb’s design fulfills its purpose, we shall discuss two of its main parts; the mirrors and the sunshields.

This telescope was initially proposed in 1996. In the beginning, it was called the Next Generation Space Telescope (NGST). The current name was given in 2002, after NASA’s former administrator, James Webb. The original plan was for Webb to be launched in 2007. Nonetheless, unfortunate events led to one delay after another; first to 2011, then to 2014, then to 2018, and then finally to the current date. Throughout these years, the budget has also ballooned from 500 million dollars to 9 billion.

Webb is supposed detect light from 100 million to 250 million years after the Big Bang*. For that, it needs to detect the light that traveled from the very distant first stars and galaxies 13.6 billion years ago. When it does, we shall be able to see how they exactly were at that time.

However, if these stars and galaxies emit visible light, why did scientists choose infrared light for Webb? Because as the space between these objects stretches—according to Einstein's General Relativity—the light they emit shifts to longer wavelengths (near- and mid-infrared range). The more distant objects become, the higher these shifts become.

Unlike Hubble’s 2.4-meter mirrors, Webb’s mirrors are 6.5 meters—the difference is illustrated in Figure 1. The size of the mirror determines the sensitivity of a telescope; larger mirror permit seeing more details. However, using large mirrors is difficult even on ground, and it is a first that such a size is launched into space. Faced with that challenge, Webb’s team had to find a way to build mirrors that are both light and strong; that way was using Beryllium. Beryllium has both light weight and durability, in addition to the capacity to maintain shape across a large range of temperatures. To know more about the manufacturing of the mirror, watch this video.

Figure 1. Hubble's primary mirror vs Webb's primary mirror. Source.

Another challenge is fitting such large mirrors into the rocket; the solution was folding mirrors. The primary mirror is made of hexagonal folding blanks that would unfold after the launch. The hexagonal shape was chosen because it makes the segmented mirror nearly circular, and gives a high filling factor, which means that the segments fit together without gaps. This shape also focuses the light on one compact point for the detectors. Finally, after the mirror blank is fine-tuned, gold-coating is applied to improve the mirror’s reflection of infrared light. More about the gold coating process can be found here. Video.

Figure 2. Webb's folding gold-coated mirrors. Source.

In detecting infrared light, heat-emitting objects are a source of interference; thus, the mirror must be kept extremely cold. Possible heat sources include the observatory itself and other surrounding celestial bodies such as the Sun, the Earth, and the Moon. To eliminate such interference, five-layered sunshields were put between the telescope and the main spacecraft as seen in Figure 3. For further protection, the mirrors are positioned to remain constantly between these bodies and the telescope. Sunshields also maintain a thermally stable environment for the mirrors, allowing them to preserve proper alignment when their orientation changes. You can watch this video to see the sunshields deployment tests.

Figure 3. Sunshields separating the mirrors from the Spacecraft Bus.

*Further reading: The Big Bang: The Beginning of all Beginnings, SCIplanet Magazine, Winter 2014 Issue.

References

forbes.com
spacenews.com
jwst.nasa.gov/content/about/index.html
jwst.nasa.gov/content/about/index.html
jwst.nasa.gov/content/science/firstLight.html
jwst.nasa.gov/content/observatory/ote/mirrors/index.html
jwst.nasa.gov/content/observatory/sunshield.html

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