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Pioneer...Voyager...Magellan. As the roll-call of 'firsts'
in imaging our planetary neighbors draws to a close, the search
for new solar systems to explore will loom as an ever more
tantalizing prospect. Believe it or not, much of the ground
work for this next, great adventure has already been lain.
Long before NASA celebrates its 100th anniversary, using instruments
now on the drawing boards, astronomers may well have cataloged
all of the Jupiter-sized planets orbiting stars within 50
light years of the earth. Even the bright stars we admire
in the night sky will have had their sunspots and prominences
scrutinized. All this will come to pass, not because humans
will travel to these distant worlds in starships, but because
we will have taken remote sensing to its ultimate, natural
limit.
To understand
how these discoveries might come about is to rediscover the
basic physical principles that allow us to see our universe
as clearly as we do. Every amateur astronomer is familiar
with the simple relationship A = 20 W/D describing how well
a lens with a diameter of D centimeters will let you resolve
features subtending A arcseconds, working at a wavelength
of W microns. For the human eye, we can discern features about
60 arcseconds across; good enough to see large craters and
mare on the moon. The Hubble Telescope with its 4-meter mirror
can do much better than this, and has an acuity of 0.07 arc
seconds. With it, we can obtain photos of Jupiter and Saturn
nearly as good as those taken by the Voyager spacecraft.
But to distingish
even a big planet like Jupiter orbiting the nearest star requires
a resolving power of 0.0002 arc seconds, and at optical wavelengths
implies a mirror 500 meters in diameter. At the much longer
wavelengths used by radio astronomers, often measured in centimeters,
most radio telescopes have dishes between 10 to 100 meters
across providing a resolution at the shorter wavelengths that
seldom exceed a few tens of arc seconds. For example, the
140-foot dish at Green Bank has a resolution at wavelengths
of 1 centimeter of only one arcminute. At 1 millimeter wavelengths,
the 12-meter radio telescope at Kitt Peak has a resolution
of only 30 arcseconds. Even the 1000-foot Arecibo telescope
operating at 21-centimeter has a reslution of only about 2
arcminutes: twice as poor as the human eye!
To acheive the
highest resolutions, clearly the vastly shorter wavelengths
of optical technology provide major advantages over radio
wavelengths, yet it is impossible to build a single, optically
perfect mirror 500 meters across to resolve distant extra-solar
planets. Fortunately, astronomers have found a clever way
around this problem: Two smaller mirrors separated by 500
meters can acheive the same resolving power as a single mirror
of the same diameter.
INTERFEROMETRY
Interferometry,
unlike natural vision, is a new way of seeing unanticipated
by nature. No known organism sees its world in quite this
way. If human eyes used this physical principle, our vision
would be limited, not by the diameter of the lenses in our
eyes, but by their 3 inch separation. Our acuity would be
25 times better than it is, at least in the horizontal plane.
Astronomical
interferometers form an image in much the same way that a
holograph is used to record the 3-dimensional image of an
object on a piece of photographic film. The light reaching
each pair of telescopes is combined to produce an interference
pattern of the image of a celectial object. This interference
pattern, or fringe, is a measure of just how much structure
the wavefront from the source has along the orienation angle
of the telescopes, and at the resolution scale set by their
separations. When enough of these fringes have been collected
from a variety of telescope orientations and separations,
a complete interferogram is created containing all the information
in the wavefront from the object. Just as for a hologram,
this interferogram can be processed to reconstitute the image
of the source itself.
All of this
sounds very complicated... and it is. When you look into the
eyepiece of your telescope, the optical surfaces of the mirrors
and lenses bring the light from the object to a common focus
to form an image which is itself a collection of bright and
dark regions. Each pair of diametrically opposite spots on
your mirror act as mineature telescopes in an interferometer
and transmit their interference pattern to a common focal
point. Uncounted trillions of these pairs work together to
build-up a complex pattern at the focal point. In-phase components
appear as bright spots, and out-of- phase pairs cancil to
form blackness or shadows. This combination of light and dark
is what forms what our brain interprets as an image. Radio
astronomers were the first to use this technique in the 1950's.
Since then, optical and infrared astronomers have begun to
extend this technique to even shorter wavelengths.
If it were possible
to build interferometers of any size, there would be much
for them to see. The world of high resolution astronomy is
a mathematical world bounded by distance, size and angular
scale (Figure 1). The human eye and most single aperture telescopes
crowd the lower reaches of this mathematical universe. Radio
and optical interferometers now in use have begun to explore
the vast middle ground extending to 0.0001 arcseconds. The
high frontier, however, is vacant and portends a staggering
view of the universe we can only dimly comprehend today. Consider
just a few of the possibilites.
MEGAVISION
: One Micro Arc Second
The disks of
planets like Jupiter would be resolvable as far away as 3000
light years. Earth-sized planets orbiting nearby stars would
be discerned as clearly as we see Jupiter with a 6" telescope.
The motions of distant galaxies would be detectable in a single
human lifetime as would the motions of individual stars in
nearby galaxies.
SUPERVISION
: One Nano Arc Second
The surfaces
of planets orbiting Alpha Centauri are seen as clearly as
Magellan now shows us Venus. We begin to resolve planets like
Jupiter within the Andromeda galaxy, and the surface markings
on individal red giant stars out to the Virgo Cluster. The
motions of distant quasars would be discerned within a few
years.
HYPERVISION
: One Pico Arc Second
Features one
foot across on the surfaces of planets orbiting nearby stars
could be seen. Magellan-like images of all planets anywhere
in the Local Group of galaxies would be possible. Jupiter-sized
planets within distant quasars could be studied. Sunspots
and prominences on stars anywhere in the visible universe
could be studied in detail.
Beyond Hypervision,
the mind reels at even more stupendous possibilities. Unfortunatly,
the laws of physics present us with three formidible obstacles
to overcome in making these possibilities a scientific reality:
High resolution necessarily means having to build large costly
instruments; Not all surface details are bright enough to
be seen at every conceivable wavelength, distance and resolution;
and finally, since everything in the universe is in motion,
we have only a limited amount of time to form a sharp image
before the objects move and cause image blurring.
A
QUESTION OF MAGNITUDE
To acheive the
highest resolutions with the smallest instruments, we must
form our images using light of the shortest wavelengths. An
optical interferometer 100 km across, not much larger than
the Very Large Array in Socorro, New Mexico used by radio
astronomers, will certainly let us acheive Megavision, but
a Hypervision optical interferometer will have to be as big
as the earth's orbit! At radio wavelengths measured in centimeters,
even a modest Supervision interferometer would be as big as
the solar system. These are not limitations to be transcended
by some clever, future superscience. These limitations are
set by the quantum nature of light itself. It is a curious
parallel that to explore the sub-nuclear world, we must also
build enormous machines such as the Superconducting Super
Collider which are themselves nearly 100 kilometers in diameter.
For each 10-fold
improvement in instrument size have come a host of new technological
problems to be reconed with. Single telescopes require individual
mirrors or lenses to be fabricated with surfaces figured to
1/8 wave tolerances or better to insure that the images they
form are distortion-free. Arrays of telescopes acting as interferometers
also require accurate surfaces, but there are in addition
entirely new issues to be delt with. Not the least of these
is the accurate timing of the incoming light wavefronts to
insure proper fringe formation. Radio telescopes use highly
accurate maser clocks, while optical and infrared interferometers
under construction use complex combinations of lasers and
optical modulation techniques to count and track the interference
fringes. A precise knowledge of the exact geometry of the
array is also needed. Compensation for the earth's ionosphere
and atmosphere must also be performed to suppress 'twinkeling';
a pernicious form of wavefront distortion easily seen with
the unaided eye.
STAR
LIGHT...STAR BRIGHT
A part from
the shear size of the instruments involved, there is an even
ugglier problem waiting in the wings like a mugger on a dark
night. Having built these gargantuan, solar system girding
interferometers, and having developed new technologies to
get them to work, there may not be anything for them to see!
Among the brightest
and most numerous visual objects in the universe are the lowly
stars. A patch of the sun 100 km on a side subtends 0.1 arc
second from the earth. It emits 6 trillion trillion trillion
photons into space every second in blinding, optical radiance.
But, after traveling only 8 parsecs, these photons expand
to become such a tenuous hemispherical shell of particles,
no more than 100 pass through each square meter of your detector
every second.
An optical interferometer
consisting of two Hubble telescopes separated by 1400 kilometers
would just be able to resolve this distant star patch with
an resolution of 0.0000001 arcseconds. Their combined collecting
area would gather 2500 photons each second, just enough light
to form a single recognizable interference fringe. But just
as you cannot throw away 99 percent of the information in
a hologram and expect to see a high definition image afterwards,
a single interferogram cannot recover all of the information
in the image of the star at 100 km resolution. Thousands of
additional interferograms from many orientations and separations
of the telescope mirrors must be gathered and combined. This
of course takes time. The situation is far worse for viewing
the surfaces of planets beyond our solar system.
Although reflected
sunlight from a planet is lost in the glare of the star's
light, their contrast in the infrared can be hundreds of times
greater because planets emit most of their light in the infrared.
Even so, a 100 km patch of a planet like the earth is about
100,000 times fainter in the infrared than a similar sized
piece of a star's surface. At 8 parsecs, only one photon from
such a surface will pass through a one meter square collecting
area every 20 minutes. It would take one month to accumulate
a single interferogram; and a thousand times that long to
complete a full image. For even more distant planets or stars,
photons from even continent-sized areas of their surfaces
will arrive so infrequently, markings at sub-planetary scales
would be virtually invisible.
At other wavelengths,
we are left with an even narrower selection of objects bright
enough to study. Planets emit virtually no radio waves. Only
violent storms on stellar surfaces, supernovae, pulsars, stellar
nurseries, maser spots, or galactic cores produce radio waves
in detectable quantities. X-rays and ultraviolet light are
the preferred spectral regions for studying young supernovae,
neutron stars and the mysterious inner regions of black hole
accretion disks. The universe seen with Supervision begins
to thin out rapidly, with fewer and fewer interesting things
to see for each additional dollar invested. A disturbing parallel
to modern high-energy physics, where billions will be spent
to find a 'missing' quark and the Higgs boson. But at the
scale of stars and galaxies, do there still exist new types
of phenomena that we haven't already seen? .
A
MOVING EXPERIENCE
If we had all
the time in the world, even miniscule rates like one photon
per minute, with a sufficiently long exposure time, could
be amplified to give useable numbers of photons from which
to form an image. But virtually everything in the universe
is in motion relative to the earth. At each resolution we
wish to acheive, image motion sets a maximum limit to how
long we can take to accumulate a complete image. A Megavision
telescope for studying the surface of nearby stars 8 parsecs
distant which move at 10's of kilometers per second, will
have no more than a minute or so to complete the image. Supervision
interferometers which have a thousand times the resolution
will require correspondingly shorter exposure times to avoid
image blurring. Even extragalactic studies of galaxies that
typically move 100's of km/sec will demand progressively shorter
integration times, just as the need for longer exposures to
offset the faintness of these sources becomes critical.
One solution
to this problem is to greatly increase the collecting area
of each telescope in the interferometer array. Every doubling
of telescope aperture quadruples the number of photons detected
per unit time. In addition, the more telescopes you have in
the array, the more interferograms you can form simultaneously.
For instance, 10 telescopes provide information for (10x9/2
= ) 45 interferograms while 30 telescopes give (30x31/2=)
435 interferograms during each integration period. These telescopes
can also be strategically placed to optimize the array's sensitivity
for particular features. A planet like the earth, for example,
may only have interesting structure between 100 km and its
full diameter, so that at 8 parsecs, telescope separations
between 14 and 1400 km are all that are needed to form an
acceptable optical image. At the longer infrared wavelengths
where planets are brightest, these separations will, of course,
have to be 10 to 20 times longer to acheive the same resolution.
BACKGROUND
NOISE
Potentially,
the most serious limitation to any of these "photon-counting"
interferometers is the large number of photons entering the
telescopes caused by naturally occurring sources of light
unrelated to the object you are trying to study. In the visual
and infrared, the earth's atmosphere is the worst of these
offenders, producing millions of background photons for every
photon of interest from the object. Even from a space-based
platform, interplanetary dust and the "zodiacal"
light they scatter towards earth from the sun, though a million
times less that the atmospheric background, still pose severe
problems for studying faint sources. At radio wavelengths,
interplanetary and interstellar charged particles form a lumpy
haze causing distant radio sources to twinkle, and become
indistinct at sufficiently high resolution and long wavelength.
The telescope itself can also be a major source of background
light for infrared imaging unless the telescope is cryogenically
cooled to reduce its heat radiation. Then of course there
is the background due to the Big Bang fireball which interferes
with observations in the millimeter wavelength band.
So long as your
measurements are not limited by the brightness of these diffuse
backgrounds, every doubling of your observing time, doubles
the number of photons you gather from the distant surface
you are studying. But if this is not the case, and you are
trying to detect light levels against the background sky limit,
a far worse state of affairs takes control. For example, if
the background level contributes 10000 photons during each
integration period, and your source contributes only 1 photon
during the same time, the random fluctuations in the detecyted
number of background photons will be equal to the square root
of the background level or plus or minus 100 photons. These
will completely drown out the 1 photon signal you are trying
to detect. The solution from purely statistical considerations
is that you will have to accumulate at least 10,000 images
for each fringe measurement so that the background noise averages
out to plus or minus 1 photon. Eventually, even space-based
observations run up against the limit of the interplanetary,
interstellar and cosmic background light, necessitating integration
times vastly greater than the limit set by the motions inherant
in the objects being studied.
THROUGH
A LOOKING GLASS...BRIGHTLY
You might think,
after all of this, that the situation is pretty bleak. Compliments
of Nature's quantum limitations on instrument size, background
noise, and surface brightness, most of the regions in our
mathematical universe of resolution appear as unreachable
as the inside of a black hole. Even so, there is still much
that could be done. To a distance of 3000 light years there
would be over a billion stars to investigate, and potentially
as many solar systems. Continents and weather systems on the
planets orbiting the thousand or so nearby stars could under
favorable circumstances be mapped and monitored for changes.
More significantly, if there is enough light to form such
an image, it could also be used to feed a spectroscope from
which the planet's atmospheric chemistry could be deduced.
The presence of free oxygen would immediatly indicate living
organisms.
Long before
the first space probes are launched out of our solar system
to discover earth-like planets around other stars, astronomers
will already have identified all such worlds out to hundreds
of light years using Megavision technology, and at orders
of magnitude lower costs. Finally, accurate proper motion
studies of galaxies will complete a detailed picture of the
motion of galaxies in the local universe. The dynamics of
individual galaxies will be revealed for the first time, as
will the subtelities of the expansion of the universe.
What the final
outcome of reaching the Megavision or Supervision frontiers
will be is unimaginable, however, it seems unlikely that astronomy
as we know it will survive as an open-ended subject. Having
more than likely settled the question of the destiny of the
universe, and having received weather reports from planets
orbiting nearby stars, it is hard to imagine that urgent new
questions will remain. Fortunately for the continued vitality
of astronomical research, the Megavision frontier is technologically
within reach at most wavelengths, and what we discover there
will keep us occupied for centuries.
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