Since its launch on Christmas day, astronomers have eagerly followed the
complex deployment and unfurling of NASA's James Webb Space Telescope—the
largest to ever take to the skies.
Right around the time this article is published, it's expected Webb will
have reached a place called the Earth-Sun second Lagrange point, or L2. This
is a point in space about 1.5 million kilometers away from Earth (in the
opposite direction from the Sun) where the gravity from both the Sun and
Earth help to keep an orbiting satellite balanced in motion.
Now the astronomical community—including my team of Swinburne University
astronomers—is preparing for a new epoch of major discoveries.
30 years and US$10 billion
In 2012, I wrote an article for The Conversation looking forward to the
launch of Webb, and reminiscing about the amazing early days of its
predecessor, the Hubble Space Telescope.
Back then, Webb's planned launch date was in 2018. And when the project was
originally conceived in the 1990s, the goal was to launch before 2010. Why
did it take nearly 30 years, and more than US$10 billion (roughly A$14
billion), to get Webb off the ground?
First, it's the largest telescope ever put into space, with a gold-coated
mirror 6.5m in diameter (compared with Hubble's 2.4m mirror). With size
comes complexity, as the entire structure needed to be folded to fit inside
the nose cone of an Ariane rocket.
Second, there were two major engineering marvels to accomplish with Webb.
For a large telescope to produce the sharpest images possible, the mirror's
surface needs to be aligned along a curve with extreme precision. For Webb
this means unfolding and positioning the 18 hexagonal segments of the
primary mirror, plus a secondary mirror, to a precision of 25 billionths of
a meter.
Also, Webb will be observing infrared light, so it must be kept incredibly
cold (roughly -233℃) to maximize its sensitivity. This means it must be kept
far away from Earth, which is a source of heat and light. It must also be
completely protected from the Sun—achieved by a 20m multilayered reflective
sunshield.
All of Webb's major spacecraft deployments, including the unfurling of the
primary mirror and sunshield, were completed on January 8. The entire
process involved more than 300 single points of failure (mechanisms that had
only once chance to work). The remaining tiny motions will take place over
the next few months.
The main mission
Webb's primary mission will be to witness the birth of the first stars and
galaxies in the early Universe. As the light from these very faint galaxies
travels across the vast gulf of space, and 13.8 billion years of time, it
gets stretched by the overall expansion of the Universe in a process we call
"cosmological redshift".
This stretching means what started out as extremely energetic ultraviolet
radiation from young, hot and massive stars will be received by Webb as
infrared light. This is why its mirrors are coated in gold: compared with
silver or aluminum, gold is a better reflector of infrared light and red
light.
Webb will see much farther into the infrared than Hubble could. It's also up
to 1 million times more sensitive than ground-based telescopes, where the
light from distant galaxies is drowned out by the infrared emission of
Earth's own hot atmosphere.
Because of these previous technological limitations, the first billion years
of cosmic history has barely been explored. We don't know when or how the
first stars formed. This is a complex question as stars produce heavy
elements when they die. These elements pollute the interstellar gas in
galaxies and change how this gas behaves and collapses to form later
generations of stars.
All current star formation we can observe, such as in the Milky Way, is from
enriched interstellar gas. We haven't yet seen how stars form in pristine
gas, which is without any heavy elements—as such a state hasn't existed for
more than 13 billion years.
But we think formation from pristine gas likely had a large effect on the
properties of the first stellar populations.
A deep space observatory
In addition to studying the early Universe, Webb will be a NASA "Great
Observatory" and will support a diversity of other projects.
It will allow scientists to peer into regions obscured by dust, such as the
centers of galaxies where supermassive black holes lurk, or regions of
intense star formation in our galaxy and others. Webb will also be sensitive
to the coldest objects, including very low mass stars, and planets orbiting
other stars within the Milky Way.
One big improvement on Hubble is that Webb will be well-equipped for
spectroscopy, dissecting light into its component wavelengths. This will let
us measure the cosmic redshift of galaxies precisely, and figure out what
elements they're made of.
Closer to home, Webb will help us find molecules such as water, ammonia,
carbon dioxide (and many others) within the solar system, the Milky Way and
nearby galaxies. It will be able to see these in the atmospheres of planets
around nearby stars, which is particularly exciting for the search for
extraterrestrial life.
Astronomers await with anticipation for the first data to be collected in
the next few months. While the most dramatic and risky mechanical motions
have been completed, the telescope continues to move, and the mirror
segments are making tiny nanometre-sized motions to bring it into a focus.
This will take many weeks as the telescope cools to its operating
temperature.
For myself, perhaps the most exciting aspect to look forward to is the
completely unknown. With Webb, we'll be observing a previously murky cosmic
era, when physical conditions were very different to those in the modern
Universe.
The history of astronomy suggests we can expect paradigm-shifting
discoveries.
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Space & Astrophysics