Image credit: NASA, ESA, CSA, STScI, Janice Lee (NOIRLab). Image processing: Alyssa Pagan (STScI)
Using the James Webb Space Telescope (JWST), astronomers have discovered the secrets of early galaxies that pumped the infant cosmos full of dust, which would become vital for the birth of new stars and the growth of galaxies. However, while the JWST is powerful enough to see many of these early galaxies, it is still limited when it comes to delving into them in great detail……Continue reading….
By Robert Lea
Source: Space
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Critics:
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 sq ft), of which 0.9 m2 (9.7 sq ft) is obscured by the secondary support struts, giving a total collecting area of 25.4 m2 (273 sq ft). 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 sq ft).
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 see orange and red visible light, and the mid-infrared region, depending on the instrument being 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.
- Rough plot of Earth’s atmospheric absorption (or opacity) to various wavelengths of electromagnetic radiation, including visible light
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 an angle of more than 85° from the Sun and having an 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. Webb operates in a halo orbit, circling around 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 of these bodies and avoid even the smallest changes of temperature from Earth and Moon shadows that would affect the structure, yet still maintain 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.
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 back 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. The Integrated Science Instrument Module (ISIM) is a framework that provides electrical power, computing resources, cooling capability as well as structural stability to the Webb telescope. It is made with bonded graphite-epoxy composite attached to the underside of Webb’s telescope structure. The ISIM holds the four science instruments and a guide camera.
NIRCam (Near Infrared Camera) is an infrared imager which has spectral coverage ranging from the edge of the visible (0.6 μm) through to the near infrared (5 μm). There are 10 sensors each of 4 megapixels. NIRCam serves as the observatory’s wavefront sensor, which is required for wavefront sensing and control activities, used to align and focus the main mirror segments. NIRCam was built by a team led by the University of Arizona, with principal investigator Marcia J. Rieke.
NIRSpec (Near Infrared Spectrograph) performs spectroscopy over the same wavelength range. It was built by the European Space Agency (ESA) at ESTEC in Noordwijk, Netherlands. The leading development team includes members from Airbus Defence and Space, Ottobrunn and Friedrichshafen, Germany, and the Goddard Space Flight Center; with Pierre Ferruit (École normale supérieure de Lyon) as NIRSpec project scientist.
The NIRSpec design provides three observing modes: a low-resolution mode using a prism, an R~1000 multi-object mode, and an R~2700 integral field unit or long-slit spectroscopy mode. Switching of the modes is done by operating a wavelength preselection mechanism called the Filter Wheel Assembly, and selecting a corresponding dispersive element (prism or grating) using the Grating Wheel Assembly mechanism. Both mechanisms are based on the successful ISOPHOT wheel mechanisms of the Infrared Space Observatory.
The multi-object mode relies on a complex micro-shutter mechanism to allow for simultaneous observations of hundreds of individual objects anywhere in NIRSpec’s field of view. There are two sensors, each of 4 megapixels. MIRI (Mid-Infrared Instrument) measures the mid-to-long-infrared wavelength range from 5 to 27 μm. It contains both a mid-infrared camera and an imaging spectrometer.
MIRI was developed as a collaboration between NASA and a consortium of European countries, and is led by George Rieke (University of Arizona) and Gillian Wright (UK Astronomy Technology Centre, Edinburgh, Scotland).[47] The temperature of the MIRI must not exceed 6 K (−267 °C; −449 °F): a helium gas mechanical cooler sited on the warm side of the environmental shield provides this cooling.
FGS/NIRISS (Fine Guidance Sensor and Near Infrared Imager and Slitless Spectrograph), led by the Canadian Space Agency (CSA) under project scientist John Hutchings (Herzberg Astronomy and Astrophysics Research Centre), is used to stabilize the line-of-sight of the observatory during science observations. Measurements by the FGS are used both to control the overall orientation of the spacecraft and to drive the fine steering mirror for image stabilization.
The CSA also provided a Near Infrared Imager and Slitless Spectrograph (NIRISS) module for astronomical imaging and spectroscopy in the 0.8 to 5 μm wavelength range, led by principal investigator René Doyon at the Université de Montréal. Although they are often referred together as a unit, the NIRISS and FGS serve entirely different purposes, with one being a scientific instrument and the other being a part of the observatory’s support infrastructure.
NIRCam and MIRI feature starlight-blocking coronagraphs for observation of faint targets such as extrasolar planets and circumstellar disks very close to bright stars. Webb is not intended to be serviced in space. A crewed mission to repair or upgrade the observatory, as was done for Hubble, would not be possible, and according to NASA Associate Administrator Thomas Zurbuchen, despite best efforts, an uncrewed remote mission was found to be beyond available technology at the time Webb was designed.
During the long Webb testing period, NASA officials referred to the idea of a servicing mission, but no plans were announced. Since the successful launch, NASA has stated that nevertheless limited accommodation was made to facilitate future servicing missions. These accommodations included precise guidance markers in the form of crosses on the surface of Webb, for use by remote servicing missions, as well as refillable fuel tanks, removable heat protectors, and accessible attachment points.
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