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When I first
joined the Center for Astrophysics/ University of Arizona
infrared program some 5 years ago, I had no firsthand experience
with 'ballooning' and only the vaguist notion of how it was
done. This situation did not really change until I traveled
with the rest of the 'balloon crew' to a quiet little town
in eastern Texas called Palestine. A casual glance at this
community wouldn't show it to be much different in geography
or temperment from its neighbors, but during the prime months
for scientific ballooning between May and October the situation
changes dramatically. You might hear the motels and restaurants
echoing with conversations of 'scintillation counters', 'bolometers',
'cryogenics' and 'ground loops', spoken in crisp tongues very
different than the leasurly Texan inflections.
The center of the scientific activity is a small group of
buildings just off route 287 outside of Palestine called the
National Scientific Balloon Facility or the 'balloon base'
for short. This is the workplace of some 61 engineers, technicians,
secretaries and administrators who oversee the ballooning
needs of the scientists who come from all parts of the world
just to use these facilities.
My first impression
of the balloon base was muted by its low-key operation. The
real drama of ballooning occurs only during those 2 or 3 times
a week that a launch is in progress. The rest of the time,
the scientists are busy, sometimes working 16 hours a day,
tending to their instruments while the NSBF staff attend to
the details of maintaining and operating the base.
Why do astronomers
use balloons? Very simply, we want our instruments to get
as high as possible above the screening affect of the earths
atmosphere. To do this, some astronomers have outfitted a
C-141 jet, the Kuiper Airborne Observatory, with sensitive
infrared detectors and fly at altitudes of nearly 8 miles.
Satellites like IRAS have also been launched from Cape Canaveral.
Infrared astronomers also use balloon-borne platforms, carrying
large telescopes and sensitive infrared sensors.
Until recently,
the astronomers could only look at the granduer of the universe
through a few windows in the electromagnetic spectrum, such
as the optical or radio, where the atmosphere was more or
less transparent. Occasionally, we could find a high mountain
top where the 'seeing' through one of the other windows, such
as the ultraviolet or infrared, was only partly obscurred.
Twenty five years of space exploration have shown us what
an unobstructed view looks like but only at great expense.
Also, the instruments that must be used in satellites such
as IRAS are at least 5 years out of date by the time they
are flown.
By flying instruments
on balloon-borne platforms, not only can we beat the staggering
costs of multi-million dollar rocket launches but the instruments
can also be rapidly up-graded to keep up with developments
in detector technology. The 'cycle time' can be very short
so that several flights can be performed every year provided
that the instruments are not severly damaged.
THE
TELESCOPE
The far-infrared
instruments we use are similar to those that optical astronomers
use. A large mirror gives us the light gathering ability to
see faint objects and to resolve them in detail. Our 102-cm,
F/13.7 cassegrain reflector in an altitude-azimuth mounting
serves this purpose admirably and is the largest far-infrared
telescope being used in this way in the world. The mirror
itself is actually made of an aluminum aloy called TENZALLOY
and is about 8 inches thick at the center but only 1/2 inch
thick at its circumference. The entire surface is nickle coated
and figured to a spherical shape. Anyone who has ever used
a telescope knows that because of the earths rotation, a star
will drift out of the eyepiece field unless the telescope
is driven mechanically to keep up with the stars diurnal motion.
The gyros on the telescope keep it in 'inertial space' so
that the effects of star field motion due to earth rotation
are automatically suppressed.
To find out
where the telescope is pointing, we use a wide field TV camera
strapped to the telescope and a 35 mm film camera that photographs
the region of the sky where our telescope is pointing. A bright
planet like mars is a very strong infrared source and provides
us with information about where the detectors are pointing
and their sensitivity to a known source of radiation.
The telescope
is normally operated in either the 'raster' mode or more recently
the 'point-and-integrate' mode. The former has been our workhorse
over the last decade and with it we can map areas of the sky,
4 times as large as the full moon in as little as 20 minutes.
With point-and-integrate mode, we can see objects 10-20 times
fainter than ever before since staring at a single part of
the sky for a longer time and lets us gather more far-infrared
radiation.
All of the instructions
that we send to the payload are transmitted 'up-link' to a
device called a command decoder that communicates with the
other electrical sub-systems onboard to activate motors, change
the pointing direction, turn on amplifiers and so on. The
digitized data sent 'down-link' are recorded on magnetic tape
and particular system functions, such as the battery current,
are displayed during the flight on strip-chart recorders.
The down-link telemetry rate is 40,960 bits per second so
that during a typical 8 hour flight we gather over 1.2 billion
bits of data. This is about the same amount of information
that is stored in a 24 volumn set of the Encyclopedia Britanica
and occupies about 20, 2500' magnetic tapes.
DETECTORS
The detectors
are the astronomers retinae which measure the intensity of
the far-infrared radiation. These are usually chips of a special
material that change their resistence depending on how hot
the incoming radiation makes them. The resulting change in
voltage, when amplified 1000s of times, tells us how bright
the object is that is heating the detector. Since the detector
only operates when it is very cold, it must be cooled in a
cryogenic dewar to liquid helium temperatures of 1.8 K. The
detectors are so sensitive that after 1 minute of staring
at a particular part of the sky, we can detect 10(-14) watts/m2.
To understand how little this is, in principle, we could 'see'
the heat given off by a person sitting on a snow bank from
a distance of nearly 40 miles! In astronomical terms, we can
also see the most luminous, dust obscurred stars that are
formed in dense clouds anywhere in the Galaxy.
'THERE'S
NO SUCH THING AS A FREE LUNCH'
As you might
well imagine, astronomical research is an expensive undertaking.
Our payload was designed and assembled by engineers at the
Center for Astrophysics between 1971 and 1973 using in-house
research and development funds. Since then, $100,000 has been
obtained on a yearly basis to maintain, upgrade and launch
the payload and to pay the salaries of the scientists and
engineers. A typical observing campaign to Palestine can cost
$20,000 for the balloon, $10,000 for the helium to fill it
and between $10,000 and $15,000 for food, lodging and transportation
for the 10 scientists and enginers during a 3 week observing
session. The detectors themselves change every few flights
and are maintained by separate funds provided by the scientists
or by the NASA research grant. A detector chip, its amplifiers
and cryogenic dewar can cost between $4000 and $10,000 for
a simple, 1-element bolometer system to as much as $500,000
for a two band, far-infrared spectrometer system. The money
is obtained through a formal proposal submitted to NASA by
Dr. Giovanni Fazio during the year prior to the intended year
of the observing 'campaign'.
In the face
of the recent IRAS successes, our group will have to re-evaluate
its goals and strike-out in directions other than those that
originally established our program during the infant years
of infrared astronomy. Although we continue to have the advantage
of higher angular resolution, the next generation of our experiments
must rely on much more that this.
LAUNCH
DAY
After 3 weeks
of troubleshooting finiky electronics and double checking
the telemetry to make sure there's no electrical interference,
we have the payload more or less 'buttoned-up' and ready for
flight. The meteorologist at the 11:00 AM briefing has told
us that the weather looks promising; the surface winds will
be about 3 mph at launch and the sky is clear at the balloon
base and at the recovery site. The 250 mile track that the
balloon is expected to take will be a gentil arc from here
to somewhere near Lubock, Texas. The launch vehicle, 'Tiny
Tim' a 10 ton, diesel-powered, behemoth with wheels 8 feet
tall, picks up our gondola at 1 PM and drives the 1/2 mile
stretch down to the launch pad at the pace of a brisk walk.
After one last check-out on the launch pad, the balloon is
brought out in a large wooden crate on the back of a truck;
the 'tie down' and helium tank vehicles are brought out as
well. One hour before the flight, our checkout is complete
and the balloon and parachute have been carefully unfolded.
Although it weighs 2 tons, the balloon is exquisitely delicate:
Made entirely of tough, thin plastic, the nylon straps or
'gores' support the full, 2 ton weight of our payload. If
human fingers apply too much pressure to the fabric when it
is unfolded, a tear will eminate from that point and shread
the entire balloon once the full weight of the payload bears
down on it. In seconds a balloon will dissolve into an undulating
sea of plastic covering an area the size of a football field.
The unlucky, multi -million dollar payload that was released
before this happens would be dragged across the launch pad
into the surrounding forest and may have to be completely
re-built.
Just before
launch, two men carrying snake-like hoses from the helium
truck begin the inflation of the balloon through plastic filling
tubes attached to the top of the balloon. During inflation,
the upper 1/3 of the balloon is held down securely by the
tie-down vehicle. Inflation usually takes about 45 minutes
and sounds like a small jet reving up for takeoff as the helium
gas is transfered from the tank truck into the cavernous volume
of the balloon. Following inflation, the roar of the helium
through the inflation hoses ceases, the two men tie-off the
filling tubes with a few overhand knots and back away from
the balloon that rises over 100 feet above their heads. There
is complete silence. The balloon sways gently in the twilight,
everyone crosses their fingers and when all is clear, the
launch command is given. The tie-down vehicle releases the
balloon; it rises, fluttering almost noislessly in the wind
and ascends quickly until it is almost above the payload.
Tiny Tim moves with amazing speed and agility for a few moments
to get the payload directly under the balloon and then releases
it. The payload, at the end of its balloon and parachute train
swings to and fro like a giant pendulum and is slowly lost
in the encroaching night sky. It shines briefly like a bright
star as it catches the dwindeling rays of the setting sun
high above the earth which, to us ground dwellers, had already
set long ago. The NSBF launch crew return to their homes to
have dinner, play with their children and watch TV while the
scientists and NSBF tracking crew remain behind, their day
has just begun and the universe this night is about to reveal
yet another of its secrets.
What follows
is a 2 hour lull during which the balloon continues to ascend
until it reaches its appointed 'float' altitude some 95,000'
above the earth. At this altitude, the atmosphere provides
100 times less pressure than at sea level and the temperature
is a 'nippy' -90 F. We actually have to supply heating pads
to some of our equipment to get them to work at all! At float,
the telescope is 'un-stowed' and we move to the first object
on our schedule, hopefully a planet, for that all-important
calibration. The schedule is planned well in advance. Unlike
the romantic notion of scientists who somehow accidentally
discover some new magical chemical or a 'new star' in the
sky, astronomers embark on premeditated journeys of exploration.
To a large degree, we have to know what we are looking for
before we find it. The sky is much too vast for us to waste
our limited time on regions that will not bear the kind of
fruit we are looking for. After all, you don't look for whales
in the middle of the desert! So, we look at regions of the
sky like the Milky Way where we know star forming regions
are found in great quantity. We look at regions that radio
astronomers say are filled with hot molecular gas. We look
at near-by galaxies that are known to be dusty and luminous
from optical studies by other astronomers. If we are lucky,
we see something that no one has ever seen before, or have
probably not seen clearly enough to understand what it was
that they were looking at.
As the night
unfolds and we step through our list of 10 to 15 targets,
we see how each one is different from the other. Some are
just single, diffuse blobs of invisible warm dust that our
sensitive detectors say is suspended between the stars we
see on our TV screen. Others may show individual hot spots
scattered over an area as large as the full moon. We watch
the squiggles of the strip chart pens as they rise to announce
some new enigma just discovered and fall to tell us there's
nothing more to be seen here! Sometimes, just when we think
there are no more new sources to be found during one raster
through a region, the pens move suggestively once again! A
mixture of excitement and pain accompanies our realization
that we must move on to the next target, a move we have already
delayed twice in the last 5 minutes.
After 8 to 10
hours of observing, we have accumulated 20 magnetic tapes
of data; the parachute is 'cut-down' from the balloon and
unfolds to break the descent of our 2 ton payload. There are
an unending number of stories about where payloads have landed:
in the alligator-infested swamps of Louisiana, the backyards
of expensive homes, in lakes or dense forests. In fact, there
are even stories about how rural farmers have actually shot
at balloons and their fragile payloads, no doubt thinking
that they were sent by aliens from outer space! In spite of
the many hazards, natural and man-made, our payload always
seems to make it back in one piece, ready for the next launch.
Recovering the
flight film from the star field camera completes the data
taking operation of a successful flight and, after packing
up the flight control electronics and 'moth balling' the gondola
until next time, we return to our respective institutions
to study the scientific data, make our maps and write-up our
discoveries. After a year, we are ready and anxious to do
it all over again with a new set of targets!
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