JWST versus Hubble: How are they different?

The wait is over. NASA finally released the James Webb Space Telescope’s first full-color science images on July 12. The initial release showcased the observatory’s abilities.

JWST built on three decades of discoveries by the iconic Hubble Space Telescope. While JWST is sometimes called Hubble’s replacement, NASA thinks of it as a successor.

What does that mean? As we celebrate the release of JWST's stunning first images, let’s compare the two telescopes and explore what to expect from JWST’s images of the cosmos.

Is JWST a successor or replacement for Hubble?

JWST was designed to pick up where Hubble left off in studying the early universe. To understand how the two telescopes relate to one another, we need to look back through Hubble’s famous deep field images.

These images are filled with countless glittering galaxies and distant objects captured by Hubble after it stared at the same patch of sky over multiple days. The oldest galaxies in the deep field images formed just 400 million years after the Big Bang.

But there are even older galaxies that escaped Hubble’s detection. These galaxies are very far away, and due to the expansion of the universe, they are moving away from us so fast that the wavelengths of their light have shifted into the infrared, far beyond Hubble’s detection capabilities.

Scientists want to see those fleeing galaxies, which is where JWST comes in. The telescope can see mid-infrared wavelengths that will allow it to spot galaxies born just 200 million years after the Big Bang. The first stars in these early galaxies produced the chemical elements of life as we know it — the very star stuff that we’re made of. By studying them, scientists will learn what conditions were like shortly after the universe was born.

Hubble won’t last forever. In 2021 the observatory experienced a string of technical glitches. When Hubble’s mission ends, JWST can step in — to a certain extent. Like Hubble, JWST is a multipurpose observatory that anyone can use.

Hubble sees ultraviolet light, visible light, and a small slice of infrared. Although JWST is optimized for infrared, it overlaps a little with Hubble and can see red, orange, and gold visible light.

JWST and Hubble also complement each other’s capabilities. Scientists will have the unprecedented opportunity to observe objects with both JWST and Hubble simultaneously, bringing the cosmos to life in a wide range of light.

When did Hubble launch? When did JWST launch?

Hubble flew into low-Earth orbit aboard Space Shuttle Discovery on Apr. 24, 1990, while JWST launched to a special spot in deep space called L2 on Dec. 25, 2021.

L2, formally known as the second Sun-Earth Lagrange point, is located 1.5 million kilometers (932,000 miles) away from Earth, where the Earth and Sun’s gravity balance. A spacecraft can orbit L2 using very little fuel and keep the Sun, Earth, and Moon at its back indefinitely. That's what JWST will do.

What challenges have Hubble and JWST faced?

Neither Hubble nor JWST have had an easy time securing their places as the world’s premier space observatories.

The first images Hubble sent back to Earth after launch in 1990 were blurry. Engineers and scientists soon discovered that the edges of Hubble’s mirror were just a fraction of the width of a human hair too flat.

Fortunately, the telescope was designed to be serviced by astronauts. In December 1993, a Space Shuttle crew installed a corrective optics package — essentially, glasses — inside the telescope to fix the problem.

JWST’s difficulties have so far been limited to the ground. NASA originally estimated that the observatory would cost $5 billion and launch in 2014. Its final price tag ended up being $9.7 billion, which includes five years of operations.

The cause? Serious project mismanagement and resource shortages during early development. Closer to launch, the mission experienced more delays and cost overruns due to technical problems and COVID-19.

How long will Hubble last? How long will JWST last?

Hubble has received NASA funding to operate through 2026 — as long as the aging telescope remains healthy until then.

JWST has a minimum mission duration of five years. However, like Hubble, it is expected to operate far beyond its original shelf life. Its main limiting factor is fuel, which the space observatory uses to maintain its orbit around L2.

Thanks to the precision of the European Space Agency’s Ariane 5 rocket that launched JWST, NASA says the observatory will have enough fuel for at least 10 years of operations. ESA and the Canadian Space Agency are partners with NASA on the JWST mission.

What are the scientific goals of Hubble vs. JWST?

At launch, Hubble’s official goals were:

• Investigate the constitution, physical characteristics, and dynamics of celestial bodies
• Determine the nature of processes occurring in stellar and galactic objects

• Study the history and evolution of the universe

• Confirm universality of physical laws

• Provide a long-term space research facility for optical astronomy

JWST’s official mission science goals are:

• Search for the first galaxies or luminous objects that formed after the Big Bang.
• Determine how galaxies evolved from their formation until the present.
• Observe the formation of stars from the first stages to the formation of planetary systems.
• Measure the physical and chemical properties of planetary systems and investigate the potential for life in those systems.

Both projects also use key science themes to categorize their work.

• Hubble’s themes are the solar system, exoplanets, stars and their environments, galaxies, and cosmology.
• JWST’s themes are the early universe, galaxies over time, star lifecycles, and other worlds.

Anyone in the world can apply for time on the telescopes. Both missions use a dual-anonymous peer review system to reduce bias. This means reviewers do not know the identities of proposers or their institutions, and proposers do not know the identities of their reviewers.

How is the engineering of JWST vs. Hubble similar and/or different?

Both JWST and Hubble are reflecting telescopes that conceptually work the same. Light reflects off a large primary mirror onto a secondary mirror, which sends it back through a hole in the primary mirror and into science instruments for analysis.

Hubble’s single mirror is 2.4 meters (7.9 feet) wide, whereas JWST’s segmented honeycomb-shaped mirror is 6.6 meters (21.7 feet) across. JWST has the largest mirror ever flown in space. Its 18 segments are made out of lightweight beryllium and coated with a thin layer of gold, making the mirror more sensitive to infrared light. Tiny actuators shape each mirror to provide a single, sharp image for the telescope’s science instruments to digest.

The two telescopes have very different cooling requirements. Hubble, which is in Earth orbit, is optimized for visible and ultraviolet wavelengths of light. It does not have as sophisticated cooling needs as JWST.

To detect infrared light, JWST needs shade from light sources like the Sun, Earth, and Moon, as well as protection from the warmth created by its own electronics. To stay cool, the observatory uses a five-layer Kapton sunshield the size of a tennis court. Orbiting L2 with the Sun, Earth, and Moon at its back, the sunshield’s “dark” side chills down to -237 degrees Celsius (-394 degrees Fahrenheit). The Sun-facing side, meanwhile, roasts in temperatures reaching 110 degrees Celsius (230 degrees Fahrenheit).

What are the imaging capabilities of JWST vs. Hubble?

JWST images will have resolutions equal or greater to those captured by Hubble.

A telescope’s resolution depends on the size of its mirror and the wavelength of light being observed. Bigger is better, except when it comes to wavelength: the longer the wavelength of light, the lower the telescope’s resolution. Infrared light, which JWST will observe, has a longer wavelength than the light Hubble observes.

If JWST and Hubble were to both observe the same object at a wavelength of 0.7 microns (near-infrared), JWST’s larger mirror will create an image that has a much higher resolution than Hubble’s. But if JWST observes that same object at a wavelength of 2 microns (almost mid-infrared), the resulting image’s resolution will only be the same as Hubble’s.

Will JWST’s images be as “good” as Hubble’s? Can we expect something in visible light that’s as impressive as the Pillars of Creation, for example?

JWST images will look as good or better than Hubble images. The human eye cannot see infrared, so JWST images will have to be artificially colored. Scientists already do this for Hubble images; the telescope must use red, green, and blue filters to create a full-color image.

Infrared light makes objects appear differently. Take the Pillars of Creation, a star-forming region inside the Eagle Nebula, for example. Hubble has captured them in both visible light and in the small slice of infrared light it can see. In visible light, the dust and gas inside the pillars are opaque, but in infrared, much of it disappears, revealing warm baby stars within.

This capability is particularly useful for studying exoplanets. One of the ways we study exoplanets is by watching them transit in front of their host stars. Changes in starlight during these transits can reveal the composition of an exoplanet’s atmosphere. These changes show up particularly well in infrared light.

How has Hubble inspired public interest in space, and how will JWST do the same?

For more than three decades Hubble has inspired the world with stunning images of the universe. JWST is expected to do the same.

Space images that reveal the wonders of the cosmos can even help rally the public to support space missions. This happened in 2004, when Hubble was desperately in need of a repair visit. Its batteries were dying, and its science instruments were falling into disrepair.

With the 2003 Columbia disaster still fresh on NASA’s mind, the agency canceled a final Hubble flight before the Space Shuttle retired, deeming it too risky. Unlike missions to the International Space Station, the shuttle had no safe harbor at Hubble in the event of a problem.

A public outcry to save the observatory followed. Planetary Society members and supporters urged their government representatives to press NASA for one more Hubble mission. The pressure worked, and the repair mission launched in 2009, with a second Space Shuttle on standby in the event of a contingency.

The repairs were a success. Astronauts gave Hubble a complete, final overhaul and installed two new science instruments — all thanks to the power of space images. JWST’s images will no doubt carry on that same legacy.