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OCR for page 22
magine yourself lying in an open
field on a warm summer evening,
watching the sky as it darkens.
Suddenly, a streak of light traces across
the sky. As you continue to watch
you see another, and then another.
Over the course of the evening, you
lose count of the number of streaks.
The meteor shower you are watching
offers evidence that the solar system is
replete with small pieces of cosmic
debris. Most of this material is too
small to detect too small, that is,
until it collides with Earth's atmos-
phere and provides the celestial light
show you are witnessing.
Meteor showers are only one type
of evidence of these bits of wandering
rock. While you are counting the
meteor trails, the Moon rises into the
sky. Studying it with binoculars, you
pick out some of the craters that cover
the lunar surface craters caused by
meteoroids hitting a surface without a
protective atmosphere like Earth's.
It rarely happens, but sometimes
these objects do descend through
Earth's atmosphere, impact the surface,
and form craters like those seen on the
Moon. Though not as numerous as
lunar craters, the scars of such
impacts are still evident on Earth's
surface (see page 14~. Scientifically,
the history of impacts on Earth is
vital for understanding how the plan-
et evolved and how life arose. For
example, it has been suggested that
most of the water on this planet was
delivered by comet impacts (see the
Kuiper Belt Objects section, pages 12-
13~. A better-known example of the
role of impacts is the Cretaceous-
Tertiary event 65 million years ago
that led to global mass extinctions,
including that of the dinosaurs.
Impacts are not now as numerous as
they were in the first billion years of
the solar system's history, but the
potential for a major impact is still
there. A close examination of Earth's
history shows that there is a 1 percent
chance in the next century that Earth
will be struck by an object large
enough (greater than 300 meters in
diameter) to cause significant damage.
Current telescopic surveys have
identified an estimated 50 percent of
near-Earth objects (NEOs; asteroids
and comets whose orbits cross that of
Earth) that have a diameter of 1 kilo-
meter or greater, and approximately
10 to 15 percent of objects between
0.5 and 1 km. NASA's current goal is
to finish cataloging the objects larger
than 1 km by 2008, but the agency
has no formal plans to extend the
search to smaller objects.
Searching for NEOs demands an
exacting observational strategy. To
locate NEOs as small as 300 meters
requires a survey down to 24th magni-
tude (sensitive enough to detect
objects 16 million times fainter than
the feeblest stars that are visible to the
The 19-km-long asteroid 951 Gaspra
(above] and the 33-km-long 433 Eros
(right] as seen, respectively, by Galileo
from a distance of 5,300 km while en
route to Jupiter and by the NEAR
Shoemaker orbiter from a distance of
about 200 km.
Faze {~: ~~ ~~f~ ~;f5~:ffff~ ~f/~'f:~
OCR for page 23
large Synoptic Survey Telescope
A simulation of a design for the Large
Synoptic Survey Telescope. This ground-
based telescope will survey the visible
sky once each week.
Profile
Large Synoptic Survey Telescope
Mission Type: Ground-based Facility
Cost Class: Small
Priority Measurements:
· SurveytheKuiperBelt.
· Survey the population of near-Earth
objects down to 300 km in diameter.
naked eye). Images have to be taken
every 10 seconds to allow complete
coverage of the sky in a reasonable
amount of time, a necessary capability
that is almost 100 times greater than
that of existing survey telescopes.
Furthermore, NEOs spend only a frac-
tion of each orbit in Earth's neighbor-
hood where they are most easily seen.
Repeated observations over a decade
would be required to explore the full
volume of space populated by these
objects. Such a survey would identify
Guiding Themes Addressed Important Planetary Science Questions Addressed
several hundred NEOs per night and
obtain astrometric (positional) meas-
urements on the much larger (and
growing) number of NEOs already cat-
alogued. Precise astrometry is needed
to determine the orbits of the NEOs
and to assign a hazard assessment to
each object. Astrometry at monthly
intervals would ensure against losing
track of these fast-moving objects in
the months and years after discovery.
In its most recent decadal survey
(gastronomy arid Astrophysics ir' the New
Miller~r~ium, National Academy Press,
Washington, D.C., 2001), the astrono-
my and astrophysics community sin-
gled out the proposed Large Synoptic
Survey Telescope (LSST) as one of its
highest-priority ground-based instru-
ments. The SSE Survey echoed this
finding and named the LSST as the
solar system exploration community's
top-ranked ground-based facility.
Instruments like the Hubble Space
Telescope and the Keck telescopes in
Hawaii are designed to study selected,
localized regions of the sky with very
high sensitivity. Another type of tele-
scope is needed to survey the entire
sky relatively quickly, so that periodic
maps can be constructed that display
how objects change in position and/or
appearance from week to week.
The LSST is a 6.5-m-effective-diam-
eter, very wide field (~3 degrees) tele-
scope that will produce a digital map
of the visible sky every week. For this
type of survey observation, the LSST
will be a hundred times more capable
than the Keck telescopes, the world's
largest at present. Not only will LSST
carry out an optical survey of the sky
far deeper than any previous survey,
but also and just as importantly it
will add the dimension of time and
thereby open up a new realm of dis-
covery. By surveying the sky each
month for over a decade, LSST would
revolutionize our understanding of
various topics in astronomy concern-
ing objects whose brightnesses vary
on time scales of days to years.
NEOs, which drift across a largely
unchanging sky, are easily identified.
The LSST could locate 90 percent of all
near-Earth objects down to 300 m in
size, enable computations of their
orbits, and permit assessment of their
threat to Earth. In addition, this facili-
ty could be used to discover and track
objects in the Kuiper Belt, the largely
unexplored, primordial component of
our solar system. Beyond the solar sys-
tem, it would discover and monitor a
wide variety of variable objects, such
as the optical afterglows of cosmic
gamma-ray bursts. In addition, it
would find approximately 100,000
supernovae per year and be useful for
many other cosmological observations.
At this time, NASA has no system-
atic survey capability to discover the
population distribution of solar system
bodies. The LSST would enable the
compilation of a systematic inventory
of near-Earth objects that is crucial to
an improved understanding of Earth's
cosmic environment, especially to the
prediction of future hazards posed to
our species. Many of the targets are as
yet undiscovered, and construction of
the LSST provides a necessary first step
toward a rational spacecraft explo-
ration program for these bodies.
~3
OCR for page 24
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Representative terms from entire chapter:
synoptic survey
