Infrared Telescopes: What Lies Beneath
What is a Wavelength?
Any wave, whether it occurs in the water so you can observe it, in the air so you can hear it, or as a string of photons making up light so you can see it, is the same. The distance between any two identical points is called the wavelength. The height of the wave above or below the “zero line” (green) represents the power of the wave and is called the amplitude.
Wavelengths of Sound
You have probably noticed that if you are distant from a music source, it is challenging to hear middle or high-range notes, but the thumping bass notes are still intensely clear. Ask any apartment-dweller with a rude neighbor.
This is because the bass notes (dark red) have a much wider (longer) wavelength, and therefore they interact with fewer air molecules as they travel towards your ear. High frequencies (purple) move rapidly up and down while only travelling a small distance away from the source. They expend much of their energy (amplitude) immediately and attenuate quickly as the air absorbs their energy.
It works the same way with brick walls. The bass notes slip through almost as if there is nothing there. That’s why the muffled boom-boom-boom penetrates shared walls so easily. Mid & high frequencies use up most or all of their energy inside the brick.
High-frequency sounds attenuate quickly as you move away from the source. Fast food restaurants like McDonalds are noisy work environments, so their deep-fryer cooking-timers are very loud. You can see customers at the counter wince when the piercing signals go off, yet people sitting in the main restaurant, barely notice.
Wavelengths of Light
Infrared telescopes are fascinating devices, and let us look at light that is not visible to our eyes. These long wavelengths lose energy very slowly and can pass right through interstellar dust that completely stops visible light.
This is fortunate because we happen to live in a very dusty area that prevents us from seeing the center of our galaxy from here. This is probably a good thing because the light of all of the Milky Way’s 300,000,000,000 stars might make for a permanent daylight glow when we faced the galactic core.
How does it work?
The prefix “infra-” means below and refers to light with wavelengths and frequencies that are slower and longer than our eyes can detect.
When we break up a beam of full-spectrum white light by passing it through a prism, it turns into the colors red, orange, yellow, green, blue, indigo, and violet. One of our more clever scientists, Sir Frederick William Herschel (1738-1822), noted that the different colored filters he was using to observe sunlight expressed varying amounts of heat. He wondered if it was an orderly progression, so he split the light with a prism, put a thermometer in each color, and recorded the temperatures.
Violet, it turned out, was the coldest, and each color got progressively hotter as it moved from indigo and blue, all the way down to red. Just out of curiosity, he also put a thermometer just outside of the red area in the "unlit" portion, and it turned out to be the hottest spot of all. He called this invisible light calorific rays because it produced calories of heat. Heat Rays sounded too much like science fiction, so now we call it Infrared, indicating it is below-red.
We used to observe from the Earth’s surface in IR, but the moisture in our atmosphere makes that problematic because water readily absorbs infrared light. We built observatories in high and dry climates (on mountaintops) to get better results. We used balloons to loft IR telescopes up to 65 km (40 miles), which is well above most of the atmosphere. Still, it could have been better.
Your friendly author participated briefly in the 2003 Spitzer IR Space Telescope project, in association with NASA, JPL, and the SSS (SIRTF Science Center). This space-based IR Telescope observed in the short 3–180 μm (micrometer) band.
To validate the idea of exo-atmospheric observation, there was a marginally successful experiment on a 1983 shuttle flight. The atmosphere in the vicinity of the shuttle was still moisture-laden due to OMS (Orbital Maneuvering System) engine firings as well as wastewater dumps. It made a good case for a space telescope, though, so seven years later, we launched the Hubble Space Telescope in 1990. Spitzer followed in 2003, and was the first dedicated IR telescope; it taught us a great deal about our galaxy and the universe.
What Spitzer Saw
You may think you’re seeing everything there is to see when you look at a famous constellation like Orion the Hunter. After all, stars are light, and darkness is the absence of “stuff” because there is no light, right? Spitzer, however, begs to differ.
Here is an image from Spitzer that shows Orion in visible light and infrared light—suddenly, there is a lot more to see. Entirely aside from the fact that there is a massive amount of dust that is abruptly visible, there are also dozens of new objects that were not in the previous image.
The interstellar medium is composed of dust—lots and lots of dust. Once we start looking at it in the infrared band, we see that there is a lot more going on there than you would imagine.
Seeing (infra) Red
To “see” heat, however, we have to create a very cold, solid-state photodetector so that the differences stand out. Spitzer’s primary mirror was cooled to just 5.5 K (-267.5° Celsius) above absolute zero. This, of course, requires a cryogenic coolant (liquid helium in this case), which was supplied in a quantity to power the primary mission for a minimum of 2½ years. Due to excellent management, the cryogenic-fluid was stretched out to May of 2009, or just a little less than 6 years.
Since then, without coolant, there are just two instruments left functioning on Spitzer. It is due to be retired on Jan 30th of this year (2020), after a long and stunningly productive career.
Don’t worry, though, because the James Webb Space Telescope is planned for launch in March of 2021, replacing the Hubble and the Spitzer capabilities for IR observation. JWST is even more capable and will see distant objects beyond the capabilities of any of its predecessors.
What Can We See?
By being sensitive to photons arriving with such low levels of energy, it becomes possible to image gas clouds (nebulae), dusty galaxies, which are just starting to form new stars, and other such stellar nurseries. We can also trace the development or origin of life-forming molecules, such as water.
We Photographed a Planet
One of the most exciting achievements occurred in August of 2019 when Spitzer managed to image the surface of an exoplanet (a planet circling another star). It’s 1.3 times the size of Earth, and has a surface more reminiscent of the Moon or Mercury, with no appreciable atmosphere. Uninhabitable, sure, but just getting a look at another world is very rewarding. The fact that it is ridiculously close to its parent star, orbiting in just eleven hours, and is tidally locked (the same side always faces its star) means there is virtually no chance of life…
The photo was only possible because this planet (LHS 3844b) circles a relatively cool M-class Dwarf star. That tight orbit makes its surface incredibly hot, so it radiates a lot of IR. The combination of a cool star and a hot planet gives enough contrast for images to be compiled.
Up until the 1960s, Infrared Photometry was considered more of an eccentric hobby for physicists than an actual pursuit for astronomers. Luckily, Radio Astronomy was showing some intriguing results at that time. It was as a direct result that optical astronomers finally admitted there was a lot to be learned using frequencies outside of visible light.
We have come to acknowledge that virtually every frequency in the electromagnetic spectrum has something to tell us. It might be radio waves, microwaves, IR, ordinary light, ultraviolet, x-rays, gamma rays, or cosmic rays, but except for some IR, visible light, and some UV, most of that information is best gathered by telescopes/receivers well outside our atmosphere.
The bulk of the Universe’s light and radiation is hidden from us by our atmosphere. Infrared lets us see back in time to how the universe evolved. We see multi-billion-year-old starlight as it arrives here; we’re looking billions of years into the past. Seeing what happened to them informs us of what is in store for us locally.
More important, however, is IR tracking of local comets and asteroids that could destroy our planet one day. We’re the first intelligent life on Earth capable of deflecting an asteroid; this is part of how we will know about it in time to fix it!