by Lee Phillips
Since Galileo first discovered the moons of Jupiter and the phases of
Venus, telescopes have gotten larger, more accurate, and more powerful.
They're now installed all around the world from mountaintop
observatories to suburban backyards. And over those 350 years, all of
them have battled the same enemy: our Earth’s atmosphere.
We get information about our terrestrial environment in several forms via personal sensors that correspond to most of them: sound through our ears; pressure and temperature through our skin; gravity through the semicircular canals in our middle ears; small molecules through smell and taste receptors. Light is perceived through our eyes.
The thin layer of nitrogen, oxygen, and carbon dioxide that
makes life possible on our planet makes observation of everything beyond
the planet maddeningly difficult. The atmosphere absorbs a great deal
of the light outside the visible part of the spectrum, blocking or
severely attenuating information about the cosmos. Its turbulent motions
distort what does get through. It scatters our own light back down into
our eyes and instruments, making the night more like a slightly darker
day, washing out all but the brightest celestial objects in a haze of
light pollution.
We can reduce this atmospheric obfuscation slightly by situating our
observatories at high altitudes, far from population centers. But it is
not enough. If only we could open a hole in our gaseous shroud and peer
through it...
...but obviously, we can't. As such, researchers can do the
next best thing. By 1990, we had years of experience putting machines
into orbit around the Earth, outside of the troublesome atmosphere. Some
of these machines were even telescopes of a sort, but they pointed
toward the Earth in the service of intelligence agencies rather than
science. Putting an astronomical telescope in space was a natural
solution, and it followed directly—and, according to some, very
directly—from the spy satellite program.
The Hubble Space Telescope was launched in that year, and
it continues to return images even today. After a repair mission to
correct its notorious optical defect, it has surpassed expectations,
gifting us with inspiring images of galaxies, gas clouds, colorful
nebulae, and planets in formation.
The Hubble takes advantage of its position 360 miles above
the surface to gather information that would be absorbed by the
atmosphere. It sees mainly in the visible part of the spectrum, extended
slightly into the near infrared and ultraviolet. But there is much
information about the Universe that is invisible even to the Hubble
Space Telescope—and that's where NASA's much hyped,
two-decades-in-the-making, $8.8 billion-plus James Webb Telescope comes
in.
The Universe in infrared
Given its limitations, the Hubble is part of a system of four telescopes in space collectively known as NASA's Great Observatories Program. The other devices are Compton Gamma-Ray Observatory (now retired), the Chandra X-ray Observatory, and the Spitzer Space Telescope. The Spitzer sees in infrared light, the part of the electromagnetic spectrum commonly known as heat. The others observe the most energetic wavelengths of light.We get information about our terrestrial environment in several forms via personal sensors that correspond to most of them: sound through our ears; pressure and temperature through our skin; gravity through the semicircular canals in our middle ears; small molecules through smell and taste receptors. Light is perceived through our eyes.
In space, no one can hear you scream, and there's precious
little to taste or feel. We get some data from the flux of various
particles, but most of what we know about the cosmos comes in the form
of light. Some of this light falls in the visible part of the spectrum
and forms the images brought to us by optical telescopes. These range in
quality from nearly all amateur hardware to the orbiting Hubble.
The Compton Gamma-Ray Observatory made many unique observations, including the discovery of an antimatter fountain in the Milky Way. X-ray observation is a central part of astronomy, and the Chandra Observatory continues to advance this field.
Past achievements include the discoveries of black holes, galactic
winds, and enormous X-ray-producing jets hundreds of thousands of light
years long.
The infrared part of the spectrum, just as the others,
carries unique types of information. We’ve known for a long time that
our Universe is expanding and that the farther away something is, the
faster it is receding from us. The velocity of light in a vacuum is
always the same, so its color is shifted if there is a relative velocity
between us and its source. If we are moving closer together, the
spectrum is shifted toward smaller wavelengths, a so-called blue shift.
And if the source of light is moving away from us, its spectrum is
shifted toward longer wavelengths: a red shift.
The farthest (and therefore, oldest) objects in the Universe are
receding the fastest, so they have the largest red shift. In fact, their
spectrum is shifted so far that the visible light (and some of the UV)
emitted by the oldest stars and galaxies reaches us as infrared light.
In this essence, the Webb Telescope being built by NASA and
its partners is a more direct successor to the Spitzer Telescope rather
than the Hubble. In short, the Webb will open up a whole new world of
infrared astronomy when it launches in 2018. The telescope will be able
to capture images of the very first stars and galaxies, formed only 200
million years after the Big Bang.
The beautiful shapes of modern galaxies take billions of years
to evolve, as the stars of which they are composed arrange themselves
under the influence of their mutual gravitational interaction. That’s a
bit too long for us to wait if we want to study the evolution of a
single galaxy. However, just as our view of the Sun shows us our star as
it was eight minutes ago, our views of distant galaxies show us how
they looked billions of years in the past. This allows us to study them
in all stages of growth, from the early proto-galaxies to the mature
spirals and ellipses in our galactic neighborhood.
The James Webb Space Telescope will be a major advance over all previous infrared observatories. Its primary mirror will be 50 times the area
of the Spitzer Space Telescope, and its infrared images will have eight
times the resolution (about the same resolution in the near-infrared as
the Hubble has in the visible spectrum). This will allow the device to
capture images of the structure of the first galaxies in the Universe
with unprecedented detail. The wavelengths that the Webb will image can
not be seen at all from the surface of the Earth because of our
atmosphere.
But being able to see like a boss in the infrared will tell us much more than what the earliest galaxies looked like.
Much of the information about stars and their births is
obscured by interstellar dust, which absorbs visible light. The Webb
Telescope will be able to see through this dust, which is relatively
transparent to infrared light. This is particularly useful in studying
the creation of stars, as young stars tend to form in dusty
environments.
Infrared imaging will also provide information about the
formation of planetary systems around these young stars, as the material
from which planets are formed is heated by the star and glows in the
infrared.
All this is made possible by the extreme infrared
sensitivity of the new observatory. This was made vivid by John Mather,
the physics Nobelist who is guiding the Webb Telescope toward
completion. He joined the project in 1995 after accepting an invitation
from NASA. And at the most recent AAAS meeting
in Washington, DC, he astounded the audience by remarking that the
telescope would be able to detect the heat generated by a bumblebee a
quarter-million miles away—the distance from the Earth to the Moon.
If some form of life is eventually discovered on an
exoplanet, the first sign will likely be the presence of signal
molecules (such as methane and oxygen) in its atmosphere. One of the
main purposes of the James Webb Telescope is the study of the
composition of exoplanet atmospheres.
The telescope will carry sophisticated spectrometers to
measure the characteristic absorption lines produced by the atmospheres
of other worlds as they pass in front of their stars. This means that
they will measure the intensity of the starlight at different
wavelengths as the light passes through the exoplanet atmosphere,
searching for dips in this graph where the light is absorbed by a
chemical in the atmosphere. Most molecules of interest have key
absorption features in the infrared part of the spectrum, so the Webb is
well-suited for this mission.
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