NASA’s Hubble Space Telescope and NASA’s James Webb Space Telescope are reflecting telescopes that observe the cosmos as partners. According to NASA, their observations complement each other and together cover a broad wavelength range. Hubble launched in 1990; NASA describes Webb as the largest, most technically advanced telescope ever built.
How each telescope is designed
Both telescopes use curved mirrors instead of lenses to collect and bend light toward their instruments, according to NASA. Light from a distant object reflects off a large primary mirror to a smaller secondary mirror, then back through a hole in the primary mirror to instruments behind that mirror. Hubble uses a Ritchey-Chrétien Cassegrain design, NASA says. Many reflecting telescopes of that kind use a large concave primary mirror and a smaller convex secondary mirror, folding the light path to fit a long focal length into a compact package. Webb is a Three-Mirror Anastigmat design, which adds a third, or tertiary, curved mirror for optical performance over a wider field, according to NASA. Hubble observes from an orbit just above Earth’s atmosphere and must keep unwanted light from the Sun, plus sunlight bouncing off Earth and the Moon, from entering the telescope, NASA says. An insulated, aluminized Teflon light shield wraps its forward assembly and creates a tube shape. Webb’s large, multi-layered sunshield looks different, but NASA says it serves the same purpose.
Wavelengths and scientific uses
NASA says Hubble can observe in near-infrared light but was optimized for shorter ultraviolet and visible wavelengths, from 0.1 microns to 2.5 microns. Webb’s deeper infrared vision runs from 0.6 microns to 28.5 microns. That difference is why NASA calls the two a pair of observatories covering a broad wavelength range. From above Earth’s light-distorting atmosphere, Hubble has mapped how galaxies, stars, nebulae, comets, the outer planets, and those planets’ moons have changed, according to NASA. NASA says Hubble’s discoveries have confirmed black holes at the centers of galaxies, measured what exoplanet atmospheres are made of, located the farthest galaxies described on the page, and confirmed that the universe’s expansion is speeding up, work NASA links to a 2011 Nobel Prize in physics. With more than one million observations, NASA says, Hubble has also been tied to the discovery of dark energy and to work on the age of the universe. NASA says Webb’s larger size and infrared views let it go beyond Hubble’s Deep Field observations and peer back over 13.5 billion years, to the first stars and galaxies forming. Comparing those early galaxies with spiral and elliptical galaxies seen later can guide understanding of how galaxies evolve, building on the work of astronomer Edwin Hubble and the telescope named for him, according to NASA. Webb’s infrared vision also cuts through dust and gas where stars and planetary systems form and goes beyond Hubble’s infrared view in studies of distant exoplanet atmospheres.
Mirrors, light, and cold
Webb’s 21.3-foot (6.5-meter) primary mirror is significantly larger than Hubble’s 7.9-foot (2.4-meter) primary mirror and gives Webb more than six times the light collecting area, according to NASA. NASA says that matters at the longer, dimmer wavelengths Webb observes. As the universe grows, light leaving distant objects is drawn out toward redder wavelengths, a change NASA calls redshift, and those objects are also dimmer. NASA says Webb’s larger mirror gathers more of that redshifted, dim light and can show objects as much as 100 times dimmer than those Hubble can detect. Even with the size difference, the detail Webb records in near-infrared light is about the same as the detail Hubble records in visible light, according to NASA. NASA treats resolution as how much detail can be told apart. It rises as the mirror grows and falls as the wavelength of the light being observed gets longer, so Webb’s mirror is proportionally wider for longer-wavelength light. Hubble’s primary mirror is one piece of Ultra-Low Expansion Glass coated with aluminum and magnesium fluoride, NASA says. The aluminum layer is 3/1,000,000th of an inch thick and provides reflectivity; magnesium fluoride protects it from oxidation and increases ultraviolet reflectivity. At 1,825 pounds (828 kg), the mirror weighs nearly one ton. NASA says a Webb-sized mirror of the same material would be too heavy to launch. Webb’s mirror is beryllium, which is strong and light, and is built from 18 six-sided pieces. Each piece measures 4.3 feet (1.32 meters) across and weighs about 46 pounds (21 kg). A microscopically thin gold layer, about 0.12 of an ounce, reflects infrared light. Each of Webb’s 18 segments, and its secondary mirror, has actuators, tiny mechanical motor assemblies that move the optics in three-dimensional space, according to NASA. Each segment also has an actuator that changes its curvature so the 18 segments can share one focal length and act as a single primary mirror. Hubble has 24 actuators on the back of its one-piece mirror, in two concentric circles; NASA says they can make small corrections for thermal distortions but cannot significantly adjust the mirror’s figure. Hubble’s design for ultraviolet and visible light means its primary mirror does not have to stay as cold as Webb’s, according to NASA. More than 200 thermal sensors and an array of heaters help hold Hubble’s instruments and the Focal Plane Structure, the optical bench behind the mirror, at 70 degrees F (21 degrees C). Webb must reach -364 degrees F or -220 degrees C to capture faint infrared light, which NASA describes as heat, and must shield itself from infrared radiation from the Sun, Earth, and the Moon.
Where they orbit
Hubble circles Earth a short way above the atmosphere, at an altitude of 300 miles, or 483 km, according to NASA. Webb does not orbit Earth. It orbits the Sun with Earth, roughly one million miles away, at the second Lagrange point, or L2; NASA also gives that distance as some 1 million miles (1.5 million kilometers) and as more than four times farther from Earth than the Moon. From Webb’s perspective, the Sun, Earth, and the Moon stay in the same part of the sky, so the sunshield can block their light and heat, NASA says. Gravitational forces of the Sun and Earth make L2 an easy place to hold an orbit, and Webb needs only occasional, modest rocket thrusts. Hubble’s closer orbit takes it through the South Atlantic Anomaly, a dent in Earth’s magnetic field that collects charged particles from the Sun and can disrupt communications and electrical systems. NASA says Hubble crosses that region on 10 orbits in a row each day, close to 15 percent of its time.
Servicing and joint observations
Because Hubble circles just above the atmosphere, astronaut servicing missions, including a possible return to Earth for a refit, were part of its long-term plan, according to NASA. The design reflects the launch and repair vehicle, and Hubble fit inside the Space Shuttle’s cargo bay. NASA says Hubble is the first space-based observatory specifically designed for servicing while in orbit, with modular components that enabled upgrades. At roughly one million miles from Earth, four times farther than the Moon, there is no servicing capability or plan for Webb, NASA says. The telescope’s design does not rely on servicing missions. NASA says Hubble’s visible and ultraviolet vision complements Webb’s infrared views. Working together, and with other space- and ground-based observatories, the two telescopes expand understanding of the universe. NASA describes that partnership as standing on earlier work, with each observatory extending what the other can see.