What will the James Webb Space Telescope look at first?

"The first images are going to be ugly."

 

As the James Webb Space Telescope begins the lengthy process of aligning its 18 primary mirror segments, a question burns in the astronomical community: What will the huge observatory look at first?

Webb soared into space successfully on Dec. 25 and successfully completed its major deployments about two weeks later while speeding toward its ultimate destination: the Earth-sun Lagrange Point 2 (L2), a gravitationally stable spot in space about 930,000 miles (1.5 million kilometers) away from our planet.

The telescope includes 18 hexagonal mirror segments that need to be gradually aligned into a single, nearly perfect light-collecting surface. A necessary part of that process is taking images of the sky to see how well the alignment is proceeding, but Jane Rigby, Webb operations project scientist, warned everyone not to expect much from the "first light" of Webb.

"The first images are going to be ugly. It is going to be blurry. We'll [have] 18 of these little images all over the sky," Rigby told reporters during a livestreamed press conference on Saturday (Jan. 8) discussing the successful deployment of Webb's 21.3-foot-wide (6.3 meters) primary mirror that day. Rigby was speaking from NASA's Goddard Space Flight Center in Maryland, where telescope operations are centered.

The James Webb Space Telescope (JWST) is a space telescope developed by NASA with contributions from the European Space Agency (ESA) and the Canadian Space Agency (CSA). The telescope is named after James E. Webb,[8] who was the administrator of NASA from 1961 to 1968 and played an integral role in the Apollo program.[9][10] It is intended to succeed the Hubble Space Telescope as NASA's flagship mission in astrophysics.[11][12] JWST was launched December 25, 2021 on Ariane flight VA256. It is designed to provide improved infrared resolution and sensitivity over Hubble, viewing objects up to 100 times fainter[13] and will enable a broad range of investigations across the fields of astronomy and cosmology, including observations up to redshift z≈20[13] of some of the oldest, most distant, events and objects in the Universe such as the first stars and formation of the first galaxies, and allowing detailed atmospheric characterization of potentially habitable exoplanets.

JWST's primary mirror, the Optical Telescope Element, consists of 18 hexagonal mirror segments made of gold-plated beryllium which combine to create a 6.5-meter (21 ft)[14] diameter mirror. This gives Webb's telescope a light collecting area about 5.6 times as large as Hubble's 2.4 m (7.9 ft) mirror (25.37 m2 collecting area to Hubble's 4.525 m2). Unlike Hubble, which observes in the near ultraviolet, visible, and near infrared (0.1–1.0 μm) spectra, JWST will observe in a lower frequency range, from long-wavelength visible light (red) through mid-infrared (0.6–28.3 μm). This will enable it to observe high-redshift objects that are too old, faint, and distant for

Hubble.[15][16] The telescope must be kept below 50 K (−223 °C; −370 °F) to observe faint signals in the infrared without interference from any other sources of warmth, so it will be deployed in space near the Sun–Earth L2 Lagrange point, a point in space about 1.5 million kilometers (930,000 mi) from Earth, where its 5 layer kite-shaped sunshield can protect it from warming by the Sun, Earth and Moon at the same time.[17][18]

The NASA Goddard Space Flight Center (GSFC) in Maryland managed the development and the Space Telescope Science Institute is operating JWST.[19] The prime contractor was Northrop Grumman.[20]

Development began in 1996 for a launch that was initially planned for 2007 with a US$500 million budget.[21] There were many delays and cost overruns, including a major redesign in 2005,[22] a ripped sunshield during a practice deployment, a recommendation from an independent review board, the COVID-19 pandemic,[23][24][25] issues with the Ariane 5 rocket[26] and the telescope itself, and communications issues between the telescope and the launch vehicle.[27] The high-stakes nature of the launch and the telescope's complexity was remarked upon by the media, and commented on by scientists and engineers.[28][29]

Construction was completed in late 2016, when an extensive testing phase began.[30][31] JWST was launched 12:20 UTC 25 December 2021[32] by an Ariane 5 launch vehicle from Kourou, French Guiana and was released from the upper stage 27 minutes later.[33] The launch was described by NASA as "flawless" and "perfect".[34] As of 8 January 2022, the telescope has been fully and successfully unfolded to its operational configuration,[35][36] and continues to travel to its target destination.[37][38][39] It will take several weeks to cool to its operational temperature, and will then undergo final testing and calibration procedures for around 5 months, potentially including its first images,[40][41] before commencing its planned research program.[42][43]

Rough plot of Earth's atmospheric transmittance (or opacity) to various wavelengths of electromagnetic radiation, including visible light

The James Webb Space Telescope has a mass about half of Hubble Space Telescope's, but a 6.5 m (21 ft)-diameter gold-coated beryllium primary mirror made of 18 hexagonal mirrors, giving it a total size over six times as large as Hubble's 2.4-metre (7.9 ft). Of this, 0.9 m2 (9.7 sq ft) is obscured by the secondary support struts,[44] making its actual light collecting area about 5.6 times larger than Hubble's 4.525 m2 (48.71 sq ft) collecting area. Beryllium is a very stiff, hard, lightweight metal often used in aerospace that is non-magnetic and keeps its shape accurately in an ultra-cold environment.[45] The gold coating provides infrared reflectivity and durability.

JWST is designed primarily for near-infrared astronomy, but can also see orange and red visible light, as well as the mid-infrared region, depending on the instrument. It can detect objects up to 100 times fainter than Hubble, and objects much earlier in the history of the universe, back to redshift z≈20 (about 180 million years cosmic time (after the Big Bang)).[13] For comparison, the earliest stars are thought to have formed between z≈30 and z≈20 (100-180 million years cosmic time),[46] the first galaxies may have formed around redshift z≈15 (about 270 million years cosmic time), and Hubble is unable to see further back than very early reionization[47][48] at about z≈11.1 (galaxy GN-z11, 400 million years cosmic time).[49][50][13]

The design emphasizes the near to mid-infrared for three main reasons:

high-redshift (very old and distant) objects have their visible emissions shifted into the infrared, and therefore their light can only be observed today via infrared astronomy;

colder objects such as debris disks and planets emit most strongly in the infrared;

these infrared bands are difficult to study from the ground or by existing space telescopes such as Hubble.

Ground-based telescopes must look through Earth's atmosphere, which is opaque in many infrared bands (see figure of atmospheric absorption). Even where the atmosphere is transparent, many of the target chemical compounds, such as water, carbon dioxide, and methane, also exist in the Earth's atmosphere, vastly complicating analysis. Existing space telescopes such as Hubble cannot study these bands since their mirrors are insufficiently cool (the Hubble mirror is maintained at about 15 °C (288 K; 59 °F)) thus the telescope itself radiates strongly in the infrared bands.[

By Elizabeth Howell