January 13, 2011
Scientific Role of the James Webb Space Telescope
James Webb Space Telescope
The James Webb Space Telescope (Webb) was the highest priority for large space
missions in the 2001 decadal survey2. An infrared-optimized telescope, Webb is a
successor to both the Hubble Space Telescope and the Spitzer Space Telescope. Webb’s 6.5-m segmented mirror is more than twice the diameter of the Hubble mirror and seven
times that of Spitzer, making Webb the largest space telescope in construction. Webb is
due for launch in 2014, and will be operated for 5 years with enough fuel to allow an
extension to ten years.
In April 2007, all ten of Webb’s enabling technologies were ready for flight development.
The project passed its Preliminary Design Review in March 2008; in April 2010, Webb
passed its Critical Design Review (CDR), indicating approval to proceed with
construction. Many of the most critical observatory components have advanced well
beyond CDR status. For example, all the science instruments will be delivered in the
summer of 2011, and the telescope optics are on a similar schedule. All of the components
meet their performance requirements and will enable the revolutionary observations
envisioned in NWNH.
“Astronomers are on the threshold of finding the root of our cosmic origins by revealing
the very first objects to form in the history of the universe. This step will conclude a quest
that is akin to that of an anthropologist in search of our most ancient human ancestors.”
NWNH, Page 7-5.
Webb plays a critical role for this science theme, often termed ‘first light.’ The “first stars”
themselves are too faint to observe individually, but they should form protogalaxies - the
collapsing clumps of gas that are the small building blocks of future galaxies like our
Milky Way. Webb will provide unparalleled sensitivity to light emitted by the first
galaxies and pinpoint the formation sites of the first stars (NWNH Page 7-5). Webb will
also obtain spectra to determine the state of these early stellar populations and the relative
abundances of the elements in them, both of which are key measurements to understand
their formation and evolution. After Webb, the proposed next generation of giant groundbased
optical/infrared telescopes would investigate these primitive objects in more detail
(i.e., measure their masses, more detailed chemical compositions, and ages).
Webb will be a crucial tool in tracing the trail from cosmic dawn to the light of today: how
and why stellar birth rates grew, peaked when the universe was a few billion years old, and
subsequently declined. Simulations suggest that the first galaxies were likely relatively
small and that the giant galaxies observed today grew by successive mergers. Webb “will
provide observations on the assembly of galaxies over cosmic time” through this merger
sequence (NWNH Page 7-14). These observations will be complemented with ALMA: a
“powerful synergy between [Webb] and ALMA applies not only to [the] first objects in the
universe, but also to the generations of stars that followed them” (NWNH Page 7-5).
The “fossil record” of how our own Milky Way galaxy was assembled can be traced by
studying galactic stellar populations with Webb (NWNH Page 7-14), along with LSST and
adaptive optics capability on GSMT. Our understanding of star formation under a wide
variety of physical conditions will benefit from surveys of the giant molecular clouds
within which stars form. Complementary studies of the young stars spawned in these
molecular regions will require infrared surveys with high angular resolution both in our
galaxy and in the neighboring galaxies the Magellanic Clouds, using Webb in space and
GSMT equipped with adaptive optics on the ground (NWNH 7-14).
“Astronomers are now ready to embark on the next stage in the quest for life beyond the
solar system—to search for nearby, habitable, rocky or terrestrial planets with liquid
water and oxygen… The observational challenge is great, but armed with new
technologies and advances in understanding of the architectures of nearby planetary
systems, astronomers are poised to rise to it.” NWNH, Page 1-3
The search for life around other stars is a multi-stage process. Webb will “take the first
steps” along this path (NWNH Page 2-2), laying critical groundwork for the more complex
and specialized instrumentation of a longer-term program. Webb, “with its superb midinfrared
capability, will also use imaging and spectroscopy transit techniques to study the
atmospheres of exoplanets” (NWNH Page 7-9).
The currently operating Spitzer Space Telescope has already demonstrated the capability
of seeing objects roughly twice the size of Earth around small stars. Webb’s much larger
collecting area will take the science to an entirely new level. Webb will be “a premier tool
for studying planets orbiting stars that are smaller and cooler than the Sun” (NWNH Page
7-9). The goal is detecting water on an Earth-sized planet in the habitable zone around
another star, and this goal is within the reach of Webb. “The era of study of … cousins of
the Earth … is underway” (NWNH 2-4).
Knowledge of young circumstellar disks—from which planets eventually form—enriches
and complements observations of mature exoplanets. Webb, along with ground-based
adaptive-optics infrared telescopes, will provide spatially resolved multi-wavelength
images and spectra of light scattered from these disks with spatial resolution comparable to
that of ALMA (NWNH 7-8). The study of these nascent planetary systems will benefit
greatly from the high spatial resolution of GSMT, fitted with high-contrast instrumentation
so that the faint disks do not get lost in the glare of their parent stars; this would
complement the wavelength coverage of Webb and ALMA (NWNH 7-15).
Physics of the Universe
“We can now say that there is a ubiquitous and ethereal substance called “dark energy”
that is expanding the fabric of space between the galaxies at ever faster speeds and
accounts for 75 percent of the mass-energy of the universe today. The effects are so tiny
on the scale of an experiment on Earth that the only way forward is to use the universe at
large as a giant laboratory.” NWNH Page 7-10
One of the most remarkable advances in astrophysics throughout the past decade has been
confirmation of cosmic acceleration, and the concomitant theory of dark energy as its
explanation. Some doubt lingers, however, about whether there is something missing in
our fundamental understanding of physics. “Comparing the expansion history of the
universe with the history of the growth of structure will in principle enable us to test
whether dark energy or modifications of general relativity are responsible for cosmic
acceleration” (NWNH Page 2-27). Webb will excel in exploring the evolutionary pathway
from “first light” to the galaxies of today (NWNH Pages 7-5 and 7-6), and thus may break
the degeneracy between dark energy and fundamental physics. In particular, Webb’s large
aperture has the potential to vastly improve the calibration of the distance scale for the
earliest supernovae that are the signposts of acceleration. In doing so, Webb will help
refine our understanding of dark energy.
Another frequent focus of fundamental
physics is the study of black holes, due
to their extreme nature. Two of the
major goals of the coming decade for
these exciting and enigmatic objects are:
first, to understand the cosmic evolution
of black hole “ecosystems” (i.e., the
intense interplay between the black
holes and their environments); and
second, to figure out how these
extremely powerful “engines” function.
Black hole masses will be measured by
Webb and ground-based optical and
radio telescopes (NWNH 2-18).
Are the supermassive black holes we can now detect only the ‘tip of the iceberg,’ i.e., the
most noticeable members of a vast but undetected population? Deep imaging surveys in
the near-infrared and X-ray regimes, with follow-up spectroscopy by Webb and groundbased
extremely-large telescopes, will detect and study the growth of the less massive
objects through the capture of gas and accompanying emission of electromagnetic
radiation (NWNH 2-14).
Space Studies Board, National Research Council, New Worlds, New Horizons in Astronomy and Astrophysics, National Academy Press, Washington, D.C., 2010.
2. Space Studies Board, National Research Council, Astronomy and Astrophysics in the New Millennium, National Academy Press, Washington, D.C., 2001.
To see pictures, please visit http://www.jwst.nasa.gov/publicpublications.html
and click "Scientific Role of the James Webb Space Telescope in 'New Worlds, New Horizons' "
I was very excited to see this article because I learned about the JWST this past summer but didn't realize all that it was going to be used for.
Posted by emmatula at January 13, 2011 11:17 AM