James Webb Space Telescope

The James Webb Space Telescope (JWST) is a space telescope designed to conduct infrared astronomy. It is the largest telescope in space, and is equipped with high-resolution and high-sensitivity instruments, allowing it to view objects too old, distant, or faint for the Hubble Space Telescope. This enables investigations across many fields of astronomy and cosmology, such as observation of the first stars and the formation of the first galaxies, and detailed atmospheric characterization of potentially habitable exoplanets.

Despite the Webb's mirror diameter being 2.7 times larger than that of the Hubble Space Telescope, it produces images of comparable resolution because it observes in the infrared spectrum, which has longer wavelengths than the Hubble's visible spectrum. The longer the wavelength the telescope is designed to observe, the larger the information-gathering surface (mirrors in the infrared spectrum or antenna area in the millimeter and radio ranges) required to achieve the desired resolution.

The Webb was launched on 25 December 2021 on an Ariane 5 rocket from Kourou, French Guiana. In January 2022, it arrived at its destination, a solar orbit near the Sun–Earth L₂ Lagrange point, about 1.5 million kilometers (930,000 mi) from Earth. The telescope's first image was released to the public on 11 July 2022.

The U.S. National Aeronautics and Space Administration (NASA) led Webb's design and development and partnered with two central agencies: the European Space Agency (ESA) and the Canadian Space Agency (CSA). The NASA Goddard Space Flight Center in Maryland managed telescope development, while the Space Telescope Science Institute in Baltimore on the Homewood Campus of Johns Hopkins University operates Webb. The primary contractor for the project was Northrop Grumman.

The telescope is named after James E. Webb, who was the administrator of NASA from 1961 to 1968 during the Mercury, Gemini, and Apollo programs.

Webb's primary mirror consists of 18 hexagonal mirror segments made of gold-plated beryllium, which together create a 6.5-meter-diameter (21 ft) mirror, compared with Hubble's 2.4 m (7 ft 10 in). This gives Webb a light-collecting area of about 25 m² (270 sq ft), about six times that of Hubble. Unlike Hubble, which observes in the near ultraviolet, visible, and near infrared spectra (0.1–2.5 μm), Webb observes a lower frequency range, from long-wavelength visible light (red) through mid-infrared (0.6–28.5 μm). The telescope must be kept extremely cold, below 50 K (−223 °C; −370 °F), so that the infrared radiation emitted by the telescope itself does not interfere with the collected light. Its five-layer sunshield protects it from warming by the Sun, Earth, and Moon.

Initial designs for the telescope, then named the Next Generation Space Telescope, began in 1996. Two concept studies were commissioned in 1999, for a potential launch in 2007 and a US$1 billion budget. The program saw enormous cost overruns and delays. A significant redesign was carried out in 2005, with construction completed in 2016, followed by years of exhaustive testing, at a total cost of US$10 billion.

Features

The mass of the James Webb Space Telescope (JWST) is about half that of the Hubble Space Telescope. Webb has a 6.5-meter-diameter (21-foot) gold-coated beryllium primary mirror made up of 18 separate hexagonal mirrors. The mirror has a polished area of 26.3 m² (283 ft²), of which 0.9 m² (9.7 ft²) is obscured by the secondary support struts, giving a total collecting area of 25.4 m² (273 ft²). This is over six times larger than the collecting area of Hubble's 2.4 m (7.9 ft) diameter mirror, which has a collecting area of 4.0 m² (43 ft²). The mirror has a gold coating to provide infrared reflectivity, covered by a thin layer of glass for durability.

Webb is designed primarily for near-infrared astronomy, but can also detect orange and red visible light and the mid-infrared region, depending on the instrument used. It can detect objects up to 100 times fainter than Hubble can, 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). For comparison, the earliest stars are thought to have formed between z≈30 and z≈20 (100–180 million years cosmic time), and the first galaxies may have formed around redshift z≈15 (about 270 million years cosmic time). Hubble is unable to see further back than very early reionization at about z≈11.1 (galaxy GN-z11, 400 million years cosmic time).

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

• high-redshift (very early and distant) objects have their visible emissions shifted into the infrared, and therefore their light can be observed only via infrared astronomy;
• infrared light passes more easily through dust clouds than visible light;
• 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 earlier space telescopes such as Hubble.

Ground-based telescopes must look through Earth's atmosphere, which is opaque in many infrared bands (see figure at right). Even where the atmosphere is transparent, many of the target chemical compounds, such as water, carbon dioxide, and methane, are present in the Earth's atmosphere and interfere with observations. Existing space telescopes, such as Hubble, cannot study these bands since their mirrors are at a temperature high enough to emit significant infrared radiation; for example, the Hubble mirror is maintained at about 15 °C [288 K; 59 °F], so that the telescope itself radiates strongly in the relevant infrared bands.

Webb can also observe objects in the Solar System at angles greater than 85° from the Sun and with apparent angular rate of motion less than 0.03 arc seconds per second. This includes Mars, Jupiter, Saturn, Uranus, Neptune, Pluto, their satellites, and comets, asteroids and minor planets at or beyond the orbit of Mars. Webb has sufficient near-IR and mid-IR sensitivity to be able to observe virtually all known Kuiper Belt Objects. In addition, it can observe opportunistic and unplanned targets such as supernovae and gamma ray bursts within 48 hours of a decision to do so.

Location and orbit

Webb operates in a halo orbit, circling a point in space known as the Sun–Earth L₂ Lagrange point, approximately 1,500,000 km (930,000 mi) beyond Earth's orbit around the Sun. Its actual position varies between about 250,000 and 832,000 km (155,000–517,000 mi) from L₂ as it orbits, keeping it out of both Earth and Moon's shadow. By way of comparison, Hubble orbits 550 km (340 mi) above Earth's surface, and the Moon is roughly 400,000 km (250,000 mi) from Earth. Objects near this Sun–Earth L₂ point can orbit the Sun in synchrony with the Earth, allowing the telescope to remain at a roughly constant distance with continuous orientation of its sunshield and equipment bus toward the Sun, Earth and Moon. Combined with its wide, shadow-avoiding orbit, the telescope can simultaneously block incoming heat and light from all three bodies and avoid even the most minor changes in temperature from Earth and Moon shadows that would affect the structure, while maintaining uninterrupted solar power and Earth communications on its Sun-facing side. This arrangement keeps the temperature of the spacecraft constant and below the 50 K (−223 °C; −370 °F) necessary for faint infrared observations.

Sunshield protection

To make observations in the infrared spectrum, Webb must be kept under 50 K (−223.2 °C; −369.7 °F); otherwise, infrared radiation from the telescope itself would overwhelm its instruments. Its large sunshield blocks light and heat from the Sun, Earth, and Moon, and its position near the Sun–Earth L₂ keeps all three bodies on the same side of the spacecraft at all times. Its halo orbit around the L₂ point avoids the shadow of the Earth and Moon, maintaining a constant environment for the sunshield and solar arrays. The resulting stable temperature for the structures on the dark side is critical to maintaining precise alignment of the primary mirror segments.

The sunshield consists of five layers, each approximately as thin as a human hair. Each layer is made of Kapton E film, coated with aluminum on both sides. The two outermost layers have an additional coating of doped silicon on the Sun-facing sides, to better reflect the Sun's heat into space. The sunshield has an effective sun protection factor, or SPF, of 1,000,000, compared to suntan lotion with a range of 8 to 50. Accidental tears of the delicate film structure during deployment testing in 2018 led to further delays to the telescope deployment.

The sunshield was designed to be folded twelve times so that it would fit within the Ariane 5 rocket's payload fairing, which is 4.57 m (15.0 ft) in diameter, and 16.19 m (53.1 ft) long. The shield's fully deployed dimensions were planned as 14.162 m × 21.197 m (46.46 ft × 69.54 ft).

Keeping within the shadow of the sunshield limits the field of regard of Webb at any given time. The telescope can see 40 percent of the sky from any one position, but can see all of the sky over a period of six months.