The Hubble telescope, despite its initial optics crisis, was an enormously successful undertaking that answered far more questions about the universe than originally posed. Now comes the Webb space telescope, the most powerful telescope ever put into space and a successor to Hubble. If successful, it will look farther back in time, image fainter objects and answer questions we are just now formulating.
Hubble and Webb are capable of imaging different light frequencies. Hubble is sensitive to ultraviolet, visible and near infrared light (200 – 2,400 nm). Webb focuses on the infrared spectrum (600 – 28,000 nm). It has the ability to see orange and red visible light but no other colors. By narrowing the targeted spectrum, Webb lets us look more deeply into the distant past and discover how our universe began.
Unlike Hubble, which orbits Earth, Webb will obit the sun, one million miles from earth at L2, the second Lagrange point. This is a special location where Webb’s sunshield can block both the Sun and Earth (and Moon) all the time. L2 is a semi-stable point in the gravitational potential around the Sun and Earth. The L2 point lies outside Earth’s orbit while it is going around the Sun, keeping all three in a line at all times. The combined gravitational forces of the Sun and the Earth can almost hold a spacecraft at this point, so it takes relatively little fuel to keep the spacecraft near L2.
The point of the L2 orbit is to keep the telescope cold and in a stable temperature environment to make possible the sensitive infrared observations needed. Because the telescope will be photographing faint radiation from distant objects, it must be shielded from bright, hot sources including the satellite. The telescope will be at about -225°C. Without the sunshield, the difference between the hot and cold sides of the telescope would be enormous, enough to boil water on the hot side and freeze nitrogen on the cold side.
Another benefit of positioning Webb at L2 is that the Webb will always be at the same location relative to Earth. That facilitates continuous communication with the telescope as the Earth rotates, using stationary antennas located in Australia, Spain and California.
The Webb telescope has half Hubble’s mass though it is bigger (Hubble spans 43×14 ft at its maximum dimension, Webb is 66.26×46.46 ft which is basically the size of its sunshield). Webb’s primary mirror, comprising the optical telescope elements, consists of 18 hexagonal mirror segments made of gold-plated beryllium, forming a 21-ft-diameter mirror having a 6.5 m aperture. By way of perspective, the earth-bound reflecting telescope with the largest mirror is the Hobby–Eberly Telescope in Texas which sports a 10-m (30-ft) aperture. Light-gathering capability is related to the mirror’s area rather than diameter and increases with the square of the aperture.
Unlike Hubble, which observes and photographs in near ultraviolet visible and near infrared spectra, Webb will focus in a lower frequency range, from long-wavelength visible red through mid infrared. This will permit viewing high-redshift objects too old and distant for Hubble. To observe in the infrared, the telescope must be kept cold because heat from the machinery is, after all, itself infrared. This is part of the rationale for the L2 orbit.
The sunshield, made by Dupont, is Kapton E, a polyimide. It comprises five layers. Each layer is as thin as a human hair. Membranes are coated with aluminum on both sides, and there is a layer of silicon-doped aluminum on the sunny side of the hottest layers. If you think this structure sounds delicate, you’re right. During testing in 2018, accidental tears delayed the project.
Image plane wavefront sensing through phase reversal will be used to position the mirror segments using precise micromotors. Webb’s optical design consists of three mirrors that will render objects free from optical aberrations. There is also a fine steering mirror which can adjust its position many times per second to stabilize images.
Scientific instruments reside in the integrated science instrument module, dubbed the bus, which is a framework that provides electrical power, computing resources and cooling. It consists of a bonded graphite epoxy composite, which is attached to the bottom of the telescope. It holds four science instruments and a guide camera:
NIRCam (near infrared camera) is an infrared imager that will have a spectral capability ranging from the edge of the visible (0.6 μm) through the near infrared (0.5 μm). Ten sensors, each 4 MP, will operate in conjunction with NIRCam’s wavefront sensor, required for wavefront sensing and control.
NIRSpec (near infrared spectrograph) will perform spectroscopy over the same range. Its design provides three observing modes: a low-resolution mode using a prism, an R 1,000 multi-object mode and an R 2,700 integral field unit or long-slit spectroscopy mode. A wavelength preselection mechanism switches the modes in conjunction while selecting a corresponding dispersive element (grating or prism) using a grating wheel assembly mechanism. The multi-object mode uses a micro-shutter mechanism to permit simultaneous observations of hundreds of objects.
MIRI (mid-infrared instrument) will measure the mid-to-long infrared wavelength from 0.5 to 0.27 μm. It contains both a mid-infrared camera and an imaging spectrometer.
FG3/NIRISS (fine guidance sensor and near-infrared imager and slitless spectrograph) is used to stabilize the line-of-sight during 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 spacecraft bus contains a multitude of computing, communication, electric power, propulsion and structural parts. It also is the primary support component of the Webb telescope. The spacecraft bus can rotate the telescope with a pointing precision of 1 arcsecond, and it isolates vibration down to 2 milliarcseconds.
In the central computing, memory storage and communications equipment, the processor and software direct data to and from the instruments, to the solid-state memory core and to the radio system, which can send data back to earth and receive commands. The computer also controls the pointing of the spacecraft, taking in sensor data from the gyroscopes and star tracker and sending commands to the reaction wheels or thrusters.
Webb has two pairs of rocket engines (for redundancy) to make corrections on the way to L2 and for maintaining the correct position in the halo orbit, Eight smaller thrusters are used for attitude control. These engines burn hydrazine fuel and dinitrogen tetroxide as oxidizer. One bit of good news: Four days after launch, mission control determined that Webb should have enough excess fuel to support significantly more that the expected ten-year lifetime. The precision of the Ariadne 5 launch and the first mid-course corrections were credited with limiting the use of on-board fuel.
