In the movie GoldenEye, Commander James Bond takes a sharp jab to the
chin and tumbles backwards off a steel catwalk. He catches himself in mid
drop and clings perilously to the bottom of the line feed receiver suspended
high above the aluminum dish of the Arecibo radio telescope. The evil
traitor Alec looms above leering with his boot resting on Bond's fingers. Alec
snarls wickedly then raises his boot and stomps...
Set on a 118 acre site in the limestone hills, of northwestern Puerto
Rico, the Arecibo Observatory is the home of the world's largest and most
powerful radio telescope. The aluminum clad antenna dish is 305 meters
in diameter and 51 meters deep. It covers 8 hectares, about 26 football
fields, with it's 38778 aluminum panels supported on a
network of cables over a huge natural sinkhole. The 630 metric ton
triangular receiver platform is suspended 137 meters above the dish on 18 cables
strung from three concrete towers that hold the receivers in position to within
a fraction of a centimeter. The receiver frame includes the 93 meter long
bow-shaped azimuth arm on which the receivers are mounted and a 42 meter
circular track that can rotate the entire azimuth arm. This allows the
receivers to focus as much as 20 degrees on either side of the center of the
receiver dish.
The idea for the Arecibo Observatory originated in the late 1950's when
William E. Gordon, a professor at Cornell University proposed the
construction of the Arecibo telescope for the purpose of studying the properties
of the Earth's upper atmosphere. He calculated that an antenna roughly 300
meters in diameter would be necessary for his studies and hit on the idea of
utilizing a natural depression in the Earth's surface as an economical platform
for the telescope. The karst geology around Arecibo in northwestern
Puerto Rico proved ideal for the project and construction of the telescope began
in 1960. The telescope was completed in 1963 and Gordon's original project
was completed successfully soon thereafter.
The Observatory is operated by a staff of 140 at the site and another 15 at
the National Astronomy and Ionosphere Center1 (NAIC)
headquarters at Cornell University in Ithaca, New York. It originally
cost $9.3 million dollars to construct in 1963 and another $31 million has been
invested in upgrades of the years. The most recent upgrade was the
Gregorian Dome that was installed in 1996. Arecibo's annual operating
budget is about $10 million.
How it works
Radio astronomy is the study of space through the use of radio waves
outside the visible spectrum. Radio waves discovered in 1888 and
by 1901 were used by Guglielmo Marconi to transmit the first transatlantic
radio message. Following this breakthrough, a myriad of practical uses for
radio signals quickly followed. In 1931, Karl Jansky a scientist at
Bell Labs detected radio waves from space using a rotating antenna that he
dubbed the "merry-go-round." In 1965, Arno Penzias and
Robert Wilson were awarded the Nobel Prize for physics in 1978 for their
discoveries of radio signals from the beginning of the universe, the Big Bang.
The receivers at Arecibo Observatory process signals in the frequency range
between 50 and 10,000 megahertz. This corresponds to wavelengths
between 6 meters and 3 centimeters. The telescope and observe signals from
as close as three kilometers above the Earth's surface to the most distant parts
of the universe, 10 billion light years away.
The primary difficulty with radio astronomy is the weakness of the signals
received from space. To put this in perspective, consider that the
combined strength of all the radio signals ever received by all the radio
telescopes on Earth is roughly equivalent to the total force of a few raindrops
striking the surface of a pond. Visitors to the Angel Ramos visitor center are
asked to turn off their cell phones with the admonition that the signal from
each cell phone is billions of times stronger than those being received by the
telescope.
Sphere versus Parabola.
Most radio telescopes use a parabolic antenna to focus the energy received
to a single point. The antenna is then pointed at the object of interest using a
flexible mount. In the case of Arecibo, this approach would have severely
limited the telescope's usefulness because, with the antenna fixed in the
ground, the telescope would only be able to look straight up. The ingenious
solution that Gordon proposed to solve this problem was to use a spherical
reflector that is essentially looking equally in all directions at once, and a
movable collector that would allow different parts of the visible sky to be
observed. Arecibo's moveable receiver poised above the antenna dish,
combined with the rotation of the Earth, and the 600+ million mile journey that
our Earth makes around the Sun each year provides and almost endless series of
targets to observe. An inherent advantage to radio telescopes is that they
can operate day and night, and are not affected by cloud cover or other
atmospheric conditions, however the constant increase in radio devices creates a
similar radio wave pollution that clutters the "view" from Arecibo.
The downside of this flexibility is that the much weaker reflected signals
must be concentrated after being received. Two methods are currently in
use for this purpose, the 29 meter long Line Feed antenna, from which James Bond
dangled in the movie Golden Eye, and the Gregorian feed system which uses two
smaller dish reflectors to focus the received energy. The Gregorian Dome at
Arecibo was named after James Gregory, one of the foremost mathematicians of
the 17th century. Gregory devised the first practical reflecting telescope
which uses a parabolic primary mirror to collect light and a smaller secondary
mirror to focus it for viewing. This is the same basic method used to
focus the weak radio signals received at Arecibo.
Areas of study
Like most expensive scientific research platforms, every available minute at
the Arecibo Observatory is carefully scheduled to accommodate the many competing
requests for telescope time. About 80 percent of the operating time at Arecibo
observatory is allocated to radio astronomy. Radar astronomy represents 5
percent, and atmospheric studies take up the remaining 15 percent of the time.
The search for extra terrestrial radio signals piggybacks on other studies and
is in progress almost continuously. Some additional information on these areas
of study is provided below.
Most of the observing time at Arecibo is devoted to the reception, detection,
processing and recording of electromagnetic radio wave signals from
space. Radio astronomy, is a very powerful and versatile technique that
has contributed significantly to our knowledge of the universe. Some of the
areas of study addressed include:
Pulsars. Pulsars are rapidly rotating neutron stars that emit
strong pulsating beams of radio waves that are as visible to a radio telescope
as a lighthouse beacon on a dark night. The first pulsar in a binary star was
discovered at Arecibo by Russell Hulse and Joseph Taylor in 1974. In 1991,
astronomers at Arecibo discovered the first planetary system outside our solar
system. The rotating pulsar B 1257+12 was found to have three planets in orbit
around it.
Quasars. Quasars are thought to be the central regions of young
galaxies. They appear as relatively small objects that are releasing enormous
amounts of energy, up to 100 times the energy of an entire galaxy. The
quasars observed so far are at enormous distances from Earth and appear to be
receding at speeds up to 95 percent of the speed of light. The radio
waves received at Arecibo from quasars may have been en route for over ten
billion years.
Galactic studies. Radio telescopes like Arecibo are important
tools for studying the creation and lifecycle of the huge clusters of stars
known as galaxies. In the late 1970's, Arecibo contributed to the
discovery that in addition to the millions of stars we can see, galaxies
contain another component called, "dark matter," that astronomers
now believe comprises the bulk of all matter in the universe. Dark
matter cannot be observed directly but its presence and characteristics and be
inferred by the effects it has on observable stars and gas.
Atmospheric scientists focus much of their attention on the layer of the
Earth's atmosphere 50 kilometers above the surface called the
ionosphere. The ionosphere contains mostly neutral gases and ionized
oxygen, hydrogen and helium. These electrically charged particles
are affected by the Earth's electrical and magnetic fields as well as
geomagnetic events on the sun and metallic particles from falling meteors.
The influence of events in the ionosphere on the lower atmosphere and the
Earth's climatology are still not entirely understood.
Radar astronomy, also a focus of study at the Arecibo Observatory, measures
reflected radio signals to detect the size and motion of planets, comets and
asteroids. Radar signals are transmitted from the observatory into space,
then the small fraction of reflected energy is measured by the telescope. Radar
astronomy is a very flexible tool that is used for a wide variety of
investigations. Recent upgrades at Arecibo allow the telescope to transmit up to
one million watts of energy into space for the study of planets, asteroids and
other objects in our solar system. By bouncing radar pulses off these
objects, we can create very detailed surface maps, like those used to help
NASA determine where on Mars to land the Viking mission, obtain
information about the movements of near space objects like meteors and even
determine the chemical composition of distant planets. Radar waves
penetrate cloud cover and have made it possible to study the surface of planets
with cloud cover such as Venus and Jupiter. In 1964, Arecibo's radar
capabilities allowed astronomers to measure the rotation of Mercury for the
first time.
The Arecibo telescope provides many megabytes of data every day that is used
in the search for other life in the universe. There are Search for Extra
Terrestrial Intelligence (SETI) projects active at the privately funded SETI-Institute/Project
Phoenix2
in California, as well as at Harvard and Berkley universities. The SETI@Home
project managed by the Space Sciences Laboratory at the University of
California, Berkeley makes use of hundreds of thousands of volunteers to
perform the massive amount of computations necessary to scan the universe for
signals. To reduce the cost of the project, the SETI@Home antenna is
piggybacked on the main azimuth arm at Arecibo so that it collects information
from whatever part of the sky is being observed.
In the movie Contact, Jodi Foster plays the SETI astronomer Ellie
Arroway.
Ellie smiles broadly as she looks across the gaping spherical valley sheathed in
shiny aluminum panels below her. Above, the graceful azimuth arm of the
huge Arecibo radio telescope slowly and silently glides into the next viewing
position above her. Somewhere out there a signal from another world is
coming her way.
===================&===================
1 National Astronomy and Ionosphere Center (NAIC):
http://www.naic.edu/
2 Arecibo journal by a SETI@Arecibo scientist:
http://www.space.com/searchforlife/phoenix_diary_five_010319.html