Riding the Invisible Waves: Through the Looking Glass of Infrared Astronomy

 

  

Riding the Invisible Waves: Through the Looking Glass of Infrared Astronomy

by Vicky L. Oldham, March 30, 2022

 

We tend to believe what our eyes see firsthand.  Our visual perception of the material world largely determines our sense of what's real, distinguishing the tangible from the intangible, the solid and practical from the realm of imagination and dreams.  Visible light, spanning the colors of the rainbow, is but a small part of what is known as the electromagnetic spectrum.  As strange as it seems, most light is invisible and ranges from incredibly short gamma waves to ultra-long radio waves.  Within the spectrum, just beyond our ability to see, is infrared radiation, better known as the heat we feel on our skin.  Its value to astronomers is well established; infrared reveals the universe beyond visible light, its long waves piercing through dust and gas across unfathomed lightyears of interstellar space.

Despite scientific knowledge of the complete electromagnetic spectrum early in the 20th century, for a long time, astronomers were skeptical of studying the sky in other wavelengths, considering invisible radiation more relevant to the concerns of physicists (Stolte, 2021).  At first, "…established astronomers considered infrared a dead end, and "…those who did pursue infrared astronomy often had to cross professional boundaries" (Rottner, 2017, p. 21).  Thanks to groundbreaking discoveries by pioneers dedicated to exploring space through its invisible wavelengths, that situation would soon change.

The Space Age, marked by Sputnik's launch in 1957, spurred inquiry into the use of infrared for astronomy, and word of its potential began spreading throughout the scientific community.  However, the idea of exploring the sky through infrared waves was still a "hard sell."  In 1962, a panel of top U.S. astronomers and physicists advised NASA to assign it a low priority (Rottner, 2017).  Fortunately, a few visionary personalities became intrigued (later making history with their own accomplishments).  One astronomer especially interested in Frank Low's work with cryogenically cooled instruments for infrared observations was Carl Sagan.  Just three years later, the same scientists who previously dismissed infrared's importance enthusiastically embraced it.

Not Seeing is Believing: Discovering Invisible Light

Infrared exists just beyond the color red in the visible spectrum.  Its wavelengths are longer than our eyes can see, moving from short to long, transitioning to microwaves, and finally, to the longest radio waves (Britannica, T. Editors of Encyclopaedia, n.d.a).  In 1800, astronomer Fredrick William Herschel experimented with measuring the temperatures of different colors produced when light separated through a glass prism (White, 2012).  Herschel found that blue registered as the coolest hue, but when he moved his thermometer toward red and then beyond red, he recorded the warmest temperature.  From this observation, he deduced that some value of color must exist beyond red—but that it's invisible (Herschel Space Observatory, n.d.).  Herschel had discovered the infrared part of the spectrum, a type of radiation with wavelengths ranging from 1 micron to 1 millimeter in length. 

Getting Off the Ground

In 1960, Gerard Kuiper, Dutch-born astronomer, and director of the University of Chicago's Yerkes Observatory, left his post for the University of Arizona in Tucson to establish a planetary science program (Britannica, T. Editors of Encyclopaedia, n.d.b).  Named the "Lunar and Planetary Laboratory," it bucked the trend in astronomy at the time, characterized by a preference for studying stars.  Unfortunately (and unfairly), planetary research was negatively associated with science fiction.  The bias stemmed from past assertions by astronomer Percival Lowell who believed he discovered intelligent life and their engineered canals on Mars (Rottner, 2017).  Undaunted, Kuiper, determined to study objects in the Solar System, hired assistants with complementary specialties: Harold Johnson and Frank Low.  Johnson worked with infrared photometry and detectors, and Low explored cryogenic cooling of instruments. 

Night Vision, Repurposed

Surprisingly, the development of instruments for observing in the infrared began with night-vision technology built by the Germans during World War II (Hankering for History, 2013).  Astronomer Gerard Kuiper obtained a captured, formerly classified German night-vision device and, after modifying it, "…observed the atmospheres of the planets in the near-infrared and was the first to discover that the satellite Titan had an atmosphere" (Rottner, 2017, p. 6).  The idea that infrared could detect an atmosphere, let alone help spectrographically analyze it, demonstrated its vital role alongside observations in visible light.

Figure 1. Infrared Man

 

 

Note.  Adapted from NASA/IPAC, n.d., Human Infrared, Wikimedia Commons. (https://commons.wikimedia.org/wiki/File:Human-Infrared.jpg, https://commons.wikimedia.org/wiki/File:Human-Infrared.jpg#/media/File:Human-Visible.jpg). In the Public Domain.

Infrared Astronomy Grows Up

What can infrared radiation detect that our eyes cannot?  Figure 1 demonstrates how some materials, like plastic, become invisible to infrared waves.  Other materials, like eyeglasses, turn opaque, revealing distinctive properties using different forms of light.  From the 1960s to the 1970s, Kuiper, Johnson, and Low explored infrared astronomy from different perspectives (NASA, n.d.).  With an inspired interest in its potential for astronomy, they first made observations from the ground with detecting instruments adapted to a 60-inch telescope on Mt. Lemmon in Arizona's Catalina Mountains (Walker et al., 2000).  Today, as part of the University of Arizona's Steward Observatory, the Mt. Lemmon complex of mountaintop observatories is referred to as the "birthplace of infrared astronomy" (The University of Arizona, 2022).  The team soon graduated to placing detectors on high-altitude weather balloons, aircraft, and rockets to escape the absorption of infrared waves by water vapor in Earth's atmosphere.

Infrared astronomy has come a long way.  From its humble beginnings to the recently launched, game-changing ten-billion-dollar James Webb Space Telescope (JWST), the field has advanced by unprecedented leaps and bounds over five decades, thanks to the efforts of astronomers and researchers exemplified by those at the University of Arizona.  The JWST uses the NIRCam, an ultra-sensitive infrared camera designed and built by a University of Arizona team in collaboration with Lockheed Martin's Advanced Technology Center (Kelley, 2022).  The NIRCam incorporates a spectrograph and coronagraph for exploring and analyzing the atmospheres, masses, and materials of distant planets and stars.

Challenges Overcome in Space

Infrared astronomy produced in-depth studies of celestial objects that were impossible through the visible spectrum.  Still, the greatest challenge to the infrared observation of distant objects is interference by ambient heat from the telescope, its instruments, housing, surrounding objects, and atmospheric water vapor.  Astronomers realized the ideal solution lay in a cryogenically cooled telescope, far from inadvertent heat sources—in outer space. 

Early space observatories surprised everyone with their stunning discoveries.  The Infrared Astronomical Satellite (IRAS) was launched into orbit in 1983 as a joint project of the U.K., Netherlands, and NASA.  The results of its mission included discoveries of new comets, details about our galactic center, and evidence of other planetary systems outside our Solar System (NASA Jet Propulsion Laboratory, n.d.).  Following IRAS, the European Space Agency launched the Infrared Space Observatory (ISO) in 1995, operating until 1998 (Harland, n.d.).  The ISO amassed a wealth of data about planetary formation and stars during its mission.

But Wait—How Can We "See" Invisible Waves?

By comparing images recorded in different wavelengths, the advantage of observing celestial objects in the infrared becomes clear.  Still, how can we actually "see" infrared pictures if the wavelengths used to capture them are invisible?  Gradients of infrared radiation are substituted or "mapped" into visible spectrum colors, creating images we can see (Smithsonian National Air and Space Museum, 2020).   A photo of the Lagoon Nebula demonstrates how infrared radiation pierces the veil of cosmic dust (Figure 2).  The first image displays colorful interstellar gas clouds but obscures whatever lies beyond the same region.  Compare this to the infrared view, brimming with stars in different stages of formation, revealing an active stellar nursery.  Unlike visible light, infrared penetrates cosmic dust to show structure, mass, atmosphere, and objects too distant to be seen with visible light due to wavelengths longer than we can see.   

Figure 2. Lagoon Nebula (comparing the visible light image with the infrared view).

Note. NASA HUBBLESITE, 2018. Hubble Space Telescope. (https://stsci-opo.org/STScI-01EVT0MAXP28Q0W4KN9V75KX89.png). In the Public Domain.

In the 1970s, NASA planned and developed its fleet of "Great Observatories" to explore the hidden mysteries of the universe, only possible by seeing the invisible.  The "fleet" included four space telescopes: the Hubble Space Telescope, the Compton Gamma Ray Observatory, the Chandra X-ray Observatory, and the Spitzer Space Telescope, all designed to detect specific wavelengths in the electromagnetic spectrum (Canright, 2009).  Only the Hubble could record visible light, ultraviolet, and limited-range infrared light.  The Space Shuttle Discovery deployed the Hubble in 1990; the Compton and the Chandra space telescopes were deployed into Earth orbit by Space Shuttle missions in 1991 and 1999.  The Spitzer Space Telescope, designed to detect a wide range of infrared, was launched from a Delta rocket in 2003. 

The Spitzer Space Telescope Reveals New Worlds

In August 2003, the Spitzer Space Telescope, first known as the Space Infrared Telescope Facility, or SIRTF, launched from a rocket at Cape Canaveral, Florida.  The telescope was later renamed for Lyman Spitzer, champion of space-based telescopes as far back as 1946 and an avid promoter of infrared astronomy (Rottner, 2017). In less than six months after launch, it had entered its Earth-trailing orbit and began observations.

The Spitzer Space Telescope opened a window into the "old, cold, and dusty universe" (Launch Pad Astronomy, 2020, 00:00:14).  Its operation history consisted of separate missions defined by its cryogenic coolant supply.  When its coolant depleted after five and a half years, the Spitzer's "cold" mission transitioned to the "warm" mission.  The warm mission, lasting over ten years, still produced an extraordinary wealth of data.  From its Earth-trailing solar orbit, far from interfering radiation by the Earth and Moon, the Spitzer explored new avenues in astronomy, from our Solar System to deep space.

The Spitzer Space Telescope was decommissioned in 2020, having functioned much longer than initially planned.  For over 20 years in space, its mission inspired "more than 8,500 referee papers which cite Spitzer's results" (Smithsonian National Air and Space Museum, 2020, 00:04:48).  Dr. Michael Werner, Project Scientist for the Spitzer Space Telescope and Chief Scientist for Astronomy and Physics at the Jet Propulsion Laboratory, California Institute of Technology, recounts Spitzer's major discoveries and contributions (Smithsonian National Air and Space Museum, 2020). This list includes:

·       Star and galaxy formation, including finding stellar embryos shrouded in cosmic dust,

·       Remnants of dying stars and supernovae,

·       Stars with planet-forming disks and condensing clouds, comparable to our early Solar system,

·       Brown dwarf stars that fail to ignite thermonuclear fusion.  Brown dwarfs may offer a part of the solution to the mystery of dark matter,

·       Giant molecular clouds found in the interstellar medium (may become a source for new stars),

·       Locations of stellar nurseries in galaxies,

·       Ultra-luminous infrared galaxies and faraway galaxies that emit more than 90 percent of their light in the infrared,

·       Distant galaxies whose light is only detected in the infrared due to the extreme redshift; by the time their light reaches Earth, due to the expansion of the universe, they are so far away that they emit wavelengths longer than visible light, and

·       Discovery of the TRAPPIST-1 planets, a system of seven exoplanets revolving around a star.

Reflecting on Spitzer's mission, Farisa Morales, Spitzer Space Telescope Scientist, says a vast archive is waiting to be mined, but its revelations have already been "tremendous and revolutionary" (NASA Jet Propulsion Laboratory, 2020, 00:03:43).  She adds, "...only time will tell [what is] Spitzer's greatest legacy" (00:03:52).  Even though much of the data remains untapped, discovering five of the seven exoplanets in the TRAPPIST-1 system is considered one of Spitzer's most notable achievements (Greicius, 2022).  Approximately the size of Earth, several planets lie in the "habitable" zone where liquid water could exist.  The dim, red dwarf star these planets orbit is the most common type of star in the cosmos, suggesting that many similar systems may be out there (Professor Dave Explains, 2020).  All seven planets are so close to each other that standing on the surface of one enables a view of the others moving across the sky.  TRAPPIST-1-D, the smallest of the seven planets, also in the habitable zone, is reminiscent of Earth due to the amount of daylight and a similar surface temperature.  The planet TRAPPIST-1-E, also in the habitable zone, has a rocky surface and seems to be covered in a steamy atmosphere, including it among the most Earth-like worlds yet found.  TRAPPIST-1-F also hints at a thick atmosphere, and all three planets could harbor liquid water.  Interestingly, the orbital periods of the TRAPPIST-1 planets exhibit a peculiar phenomenon (speculated to function as stability in a system of close orbits), as revolutions around their parent star reflect whole number ratios.

The Hubble Space Telescope Reveals Old Worlds

The Hubble Space Telescope is credited with a long list of remarkable "firsts" and extends its observing capabilities to include infrared but never intended to match the Spitzer Space Telescope's full-range infrared detection capabilities.  Despite its limitations, from its deployment in April 1990 to over 30-plus years in orbit, Hubble has generated over 13,000 scientific studies and recorded spectacular images of the cosmos (A. Research, Innovation & Impact, 2016).  One famous example is Hubble's Ultra-Deep Field image revealing over 10,000 galaxies after "a million-second-long exposure" (Belleville, 2021).  The image combined views from Hubble's updated cameras: the Advanced Camera for Surveys and the Near Infrared Camera installed by Space Shuttle astronauts in 2002 (Garner, 2021).  Hubble's latest wide-field camera increases its infrared capacity from 65,000 to 1 million pixels, 20 times better than Hubble's original infrared detector.

By 2009, Hubble used its infrared camera to look more than 13 billion years into the past.  It found a compact galaxy of blue stars that formed shortly after the "Big Bang," the origin of the known universe.  Thanks to improved technology, astronomers have continually "upped the ante" to look even further into the past than previously thought possible.  Observing the same region using ground-based telescopes, just below the constellation Orion, results in a primarily empty sky.  By 2012, Hubble ventured even further, combining ten years of images to construct its "eXtreme Deep Field" image of the faintest galaxies ever seen.

Table 1. Comparison Between Three Infrared Detecting Space Telescopes.

space Telescopes	wavelengths observed	type of Orbit	Launch Date	Power source	mirror size
HUBBLE space telescope       SPITZER space telescope       james WEBB space telescope	Visible, ultraviolet, and infrared, 0.8 to 2.5 µm   Infrared, 3 to 160 µm; warm mission, 3.6 to 4.5 µm   Visible, infrared	Low Earth orbit       Earth trailing         L2 LaGrange Point, approx. 1 million miles from Earth	April 24, 1990       August 25, 2003       December 25, 2021	Solar         Solar         Solar	2.4 meters (7 ft., 10 in.)       85 centimeters (33 in.)     6.6 meters (21.7 ft.)

 

Note: The data are derived from NASA space missions. Wilson (Editor), 2021, NASA missions, A-Z. https://www.nasa.gov/missions. Copyright 2022 by NASA.

 

Reinventing Astronomy: The James Webb Space Telescope

The James Webb Space Telescope (JWST), launched on Christmas Day, 2021, represents the culmination of knowledge gained through the Spitzer, Hubble, and scores of other space and ground telescopes.  Infrared, having proved its immense value to astronomy to explore the furthest reaches of the universe, is the JWST's observation bandwidth of choice.  Protected by its tennis court-sized sun shield, the telescope is super-cooled to near absolute zero (-370 F) and "parked" in space at the L2 (LaGrange) point almost a million miles from Earth.  With its ultra-sensitive infrared camera, the NIRCam, and a mirror many times larger than Hubble's and Spitzer's, the JWST is projected to be 100 times more powerful, a fact almost impossible to imagine.  The NIRCam has already captured the first clear image of a single star while unintentionally capturing entire galaxies in the background.

Since the JWST also needed to fit on a rocket during its journey to space, how is it possible that its mirror and light collecting capacity is significantly larger than past space telescopes?  While Spitzer and Hubble's mirrors were limited by the dimensions of their means of transport, fitting into the Space Shuttle Bay (Hubble) or on a rocket (Spitzer), the JWST's advanced design relied on a strategic, origami-like structure that could unfold.  Once successfully launched into space, it could deploy its honeycomb-like mirror system, a plan requiring incredible precision and a dizzying number of moving components.

The JWST: Are We Ready?

Ken Sembach, Director of the Space Telescope Science Institute, wonders if astronomers are ready to embrace whatever JWST finds (Smithsonian National Air and Space Museum, 2018).  The question is partly inspired by the expectation that the JWST's deep dive into new frontiers of the cosmos could prove challenging to long-held theories in astronomy and physics.  Such apprehension reminds us of the adage, "beware of what you wish for—you may get it!"  Could the JWST's discoveries contradict current theories about gravity, dark matter, and dark energy?  What if its observations reveal the existence of life on other planets?  How do scientists react if more than microorganisms are detected?  Continuing where the Spitzer Space Telescope left off, the JWST will be able to analyze the atmospheres of planets and spectrographically detect their compositions, including signs of biological activity.  What will it find among the seven Earth-like planets in the TRAPPIST-1 system?  If no life is found in the TRAPPIST-1 system, what about the approximately 5,000 confirmed exoplanets awaiting analysis?

Captured in the Webb – The Future of Space Telescopes

NASA's Great Observatory program resulted in a treasure trove of data that generated tens of thousands of research papers, accumulating data that will take years to analyze and appreciate.  Like intrepid space travelers visiting new worlds, space telescopes like Spitzer and Hubble have inspired future missions that promise to explore even deeper reaches of space.  Still, no space telescope currently under development is designed to observe in the range of far-infrared wavelengths like Spitzer during its cold mission.  For the near term, only the SOFIA (the Stratospheric Observatory For Infrared Astronomy) can observe infrared's longest waves.  Designed to fit into a Boeing 747 aircraft, it flies above 37,000 feet to escape 99% of atmospheric water vapor (Klesman, 2020).  Since it's not super cooled, SOFIA uses a complex system of "chopping" to remove unwanted infrared background emissions (SOFIA, n.d.).  Although SOFIA is a productive project, infrared telescopes work best in the vacuum of space where optical elements are "…cooled to hundreds of degrees below freezing and still remain operational" (Fraknoi, 2016, para. 4).  Fortunately, such a cryogenically cooled space telescope is just starting to focus its infrared-enabled instruments on the old, cold, and dusty universe.

Known as the James Webb Space Telescope (JWST, or "the Webb"), its deployment defines a new era of space exploration.  University of Arizona astronomers Marcia and George Rieke reflect a passionate dedication to infrared astronomy through their decades-long accomplishments.  Both Regents Professors in the University of Arizona's Steward Observatory have been "instrumental in developing technology that will enable the [JWST] telescope to peer deeper back in time and space than any instrument before it" (Stolte, 2021, para. 1).  Remarkably, George Rieke began his foray into infrared astronomy under Frank Low, the original member of Gerard Kuiper's team in the 1960s.  From those early days, the Reikes have served as key architects in developing infrared telescopes, from instruments on ground telescopes right up to the JWST.  Now they are excited not just about what they may discover but how it will impact young researchers.  George says, "By getting twice as close to the Big Bang, we're really pushing back to the time when things SHOULD look different. But we don't know HOW they're different.  Who knows what we'll find?" (para. 18).  With its massive collecting area, cryogenically cooled instruments, ultra-sensitive infrared cameras, and location in space, far from thermal radiation sources like the Earth and Moon, it's almost sure to lift the cosmic curtain in ways we never imagined.

 

 

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