Astro Bob: What will the James Webb Telescope show us?
NASA's largest and most ambitious space telescope will take us places we've never seen or imagined before.
After a flawless Christmas Day launch, NASA's James Webb Space Telescope (JWST) now heads to its observing outpost a million miles away on the opposite side of the Earth from the sun — a convenient "parking place" called the the L2 Lagrange point . Here the gravities of the Earth and sun combine to create a stable place for a spacecraft to anchor itself in space. The JWST will still require occasional blasts from its thrusters to maintain position, but it has enough fuel to operate at L 2 for up to 10 years.
Unlike the Hubble Space Telescope, which orbits just 340 miles above the Earth and races in an out of the planet's shadow every 95 minutes, the Webb Telescope will have unfettered access to a dark, sunless sky 24/7. The telescope itself consists of 18 gold-coated,, hexagonal mirror segments that when full unfolded form a single, 21-foot-wide (6.5-meters) mirror with a surface area six times larger than Hubble's. The same way a bigger bucket collects more rain, JWST's bigger "eye" will gather more light, allowing us to see fainter objects at greater distances than any current telescope on Earth or in space.
There's a reason the mirrors are coated in a thin layer of gold instead of aluminum, the case with most telescopes. Gold is an excellent reflector of infrared , a type of light we can't see but instead feel as heat. Invisible infrared light warms your hands over a fire and warns you from touching a hot stove. Earth's atmosphere is opaque to most infrared light arriving from space, blocking it from reaching the surface and the hungry eyes of astronomers.
To see the universe in infrared, we have to loft our telescope above the atmosphere into outer space as well as remove it from the vicinity of the Earth and moon. Both radiate in the infrared and would otherwise flood the instrument with light the same way bright streetlights make it difficult to see the stars. Then there's the sun itself, which pours out a steady stream of heat across space like a firehose.
That's why the L 2 Lagrange point was selected. Not only a great parking place, it's located a million miles away from the offending Earth and moon. To insulate it from heat, the Webb sports a tennis-court-sized, five-layer sunshield that faces back toward the Earth and sun.
Each gossamer sheet conducts heat away from the telescope and into the vacuum of space. The first and outermost layer faces the brunt of solar radiation, but by the time the remaining heat reaches the fifth layer, the telescope has cooled down to -370° F (-223° C), nearly the temperature of space. Thus chilled, it can nose out even the tiniest dribbles of warmth radiated by distant suns and planets — sensitive enough to detect the heat of a bumblebee at the moon's distance.
Dust is transparent to infrared light. JWST will be able to spot embryonic planets and stars inside their dusty birth clouds (called nebulae) otherwise hidden from view in visible light. One of its main goals will be to image the first generation of stars and galaxies that formed soon after the Big Bang .
The reason infrared light is critical to uncovering the earliest stars relates to the expansion of the universe. Since the origin of space and time some 13.7 billion years ago, space itself has been expanding like a blown-up balloon. Universal expansion doesn't just increase the separation between distant galaxies over time. It stretches light, too.
The first generation of stars radiated lots of visible and ultraviolet light when they coalesced some 100 to 250 million years after the Big Bang. But as that light has traveled towards us, the ever-expanding universe has stretched it out like pulled taffy, changing its color from visible (short waves) to longer-wavelength infrared. To have any hope of detecting these extremely faint suns requires an infrared telescope with a BIG mirror.
Infrared light allows astronomers to see farther back in time compared to visible light. But it has its limitations. We can only go back as far as the first luminous objects. Beyond that lies the cosmic Dark Ages, when a fog of neutral hydrogen atoms enveloped all. The universe's version of "nothing to see here."
In addition to peering at the first stars and galaxies, the Webb will study planets orbiting other stars. As of Dec. 26, 2021, more than 4,900 have been discovered. Although these alien worlds are small and faint, they're warmed by their host suns, making them glow in the infrared. JWST will employ a special instrument to study their atmospheres in detail. On Earth, life produces copious methane and oxygen. Detecting similar gases in an exoplanet's atmosphere could indicate the possibility of life and lead one day to its discovery.
On the telescope's prime targets is the Trappist-1 planetary system located 39 light-years from Earth. Seven Earth-sized planets orbit a small red dwarf star, three of them within the so-called "Goldilocks Zone," where the temperature is just right for liquid water to pool on the surface and provide a potential haven for life. Astronomers also will use the telescope to study asteroids, planets and comets within our own solar system as well. Expectations are high. Once the Webb unfolds and chills down to operating temperature, a process that takes about six months, observing will begin.
Like any new scientific venture, astronomers expect the JWST to take us places and reveal aspects of the universe never before imagined. Such are the fruits of working hard and risking much.
"Astro" Bob King is a freelance writer for the Duluth News Tribune. Read more of his work at duluthnewstribune.com/astrobob .