Goodbye Arecibo, your Spirit is now in FAST
On the island of Puerto Rico, nestled in the hills surrounding the city of Arecibo, stands
the most famous radio telescope in the world, which over the years has become a Pop icon
of scientific research.
Built in the early sixties exploiting a natural depression of the ground, for almost sixty
years was the main instrument of the Arecibo Observatory, thanks to which many things have
been discovered in every field of astronomy, from the planetary one with measurements of
all kinds and the production of the first radar images of asteroids, to the astrophysical
one with the confirmation of the existence of neutron stars and the discovery in 1992
of the first extrasolar planets.
Without forgetting the strong emotional drive that the great antenna has always represented
in the world for SETI research.
In its approximately 60 years of existence, the large parabola 305 meters wide has withstood
storms and hurricanes that sometimes affect Puerto Rico, and even several earthquakes.
All adversities were overcome without serious consequences, although in September 2017 it
was necessary to close the structure for a short period due to Hurricane Irma, and in
January 2020 the activity was stopped as a result of the seismic swarm that occurred
during the earthquake on January 7.
Last August, however, the first signs of catastrophe: one of the support cables slipped from its
anchorage and fell on the antenna destroying dozens of panels.
The restoration work should have started a few weeks later, but before they started,
on November 7 another cable collapsed, causing even more damage to the antenna.
At this point, on November 19, the National Science Foundation, the agency that manages
the observatory, was forced to announce that none of us would ever want to hear: "The damage
to the structure is too severe to be repaired.
The large parabola with the entire aerial platform will be dismantled in the coming
months...".
A decision probably not entirely unrelated to the sharp decline in public funding, which
over the years has become completely insufficient to ensure the full operation of the structure.
It remains an immense void, therefore, for the loss of an observatory so loved by all,
but perhaps not everyone knows that...
...that this gap, at least from a technical point of view, has already been abundantly
filled!
Let's see how and by whom.
China has been chasing the West on the road to technological transformation for many years
now, but never before - with Chinese astronauts in Earth orbit, automatic probes on the Moon,
space stations, supercomputers and satellites of the latest generation - has its determination
to invest much of its growing economic exuberance in basic research and space exploration been
felt so strongly.
But the news that more than any other seems to us able to give the measure of this extraordinary
acceleration is that after just five years of work China has completed the construction
of FAST - that is what with its 519 meters in diameter must be considered today the largest
single antenna radio telescope in the world - thus appropriating a record that for more
than half a century was held by the radio telescope of Arecibo with its parabola of
305 meters!
The idea to build FAST (we will see later the meaning of this name) was born from a
previous adhesion of China to the troubled Square Kilometer Array (SKA) project, when
it was still assumed that the radio telescope wanted by the international scientific community
could be built in the Asian country.
In the end, it was decided that SKA would be built in two separate sites, in South Africa
and Australia, and that it would use not a single mirror but a large expanse of smaller
radio antennas.
So, in 2006, China decided to set up on its own and take advantage of the experience gained
to build its own radio observatory, extraordinarily large not only for reasons of political prestige,
but also because of the need to impose a new dimensional standard to the scientific research
of radio waves.
But let's go in order, and enjoy step by step the stages of this extraordinary realization,
starting from the description of the place where it was erected, i.e.
Pingtang County, in the southern province of Guizhou.
This part of the territory is formed by a continuous expanse of low mountains separated
by cavities of karstic origin, mostly of almost circular shape.
Just inside one of these, the "Dawodang depression", 800 meters wide and chosen among many others
scattered throughout China, it was decided in 2007 to house the antenna of a radio telescope
similar to that of Arecibo, whose parabola, as we said at the beginning, was also built
in a natural hollow.
The area is definitely sparsely populated.
The nearest cities are Anshun, more than 130 km northwest, and the great Guiyang, 150 km
north, and therefore few are the artificial radio signals that could interfere with the
observations.
The obligation to respect the radio silence, also turning off cell phones, will have to
be observed about ten kilometers before the arrival at the site, so that the Chinese government
had to move to another location (not without controversy) the about 9 thousand people who
lived less than 5 km from the installation.
The staff of resident scientists and technicians, about seventy people in all, will move for
emergencies with two helicopters always available on the square of the control center, while
the visitors, who are expected to be very numerous, will almost all arrive by bus, since
the area is forbidden to private traffic.
And the first thing they will see on the horizon will be the six large metal towers each 150
meters high: pylons that support by means of tie rods, as in a suspension bridge, a
wire mesh covered with a giant puzzle of about 4500 triangular-shaped panels.
It will be a really exciting spectacle that they will be able to enjoy when they manage
to have an overall view of the instrument going up the surrounding hills!
But at this point, making the eye run on the imposing structure from one of the panoramic
shovels, many people will ask themselves a question: if the radio telescope is so heavily
anchored to the ground, how will it be able to receive signals from objects that instead
rotate following the apparent motion of the celestial sphere?
In fact, both FAST and Arecibo are static structures; in the sense that for obvious
mechanical reasons (we are talking about weights of thousands of tons) their parabola remains
always directed to the zenith, without any possibility to move to follow the stars as
it happens instead in optical telescopes or radio telescopes of smaller dimensions.
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But if the parabola cannot move, a minimum solution exists anyway: to move in its place
the signal reception cabin located in the focus of the instrument; and here lies one
of the most important qualitative differences of FAST compared to Arecibo: the greater mobility
of the receiver, which in the Chinese radio telescope can be moved up to 40° from the
zenith, while in Arecibo's radio telescope only 20°; which translates, for FAST, into
the possibility of chasing a celestial object for a period of almost 6 hours, compared to
2.7 hours in Arecibo.
A really considerable gain, such as to allow a higher speed in data acquisition, especially
considering that the receiver has also the possibility to observe 19 regions of the sky
at the same time and different wavelengths!
And it is certainly this characteristic that the Chinese National Academy of Sciences wanted
to emphasize with the choice of the name FAST, which is the acronym of "Five-hundred-meter
Aperture Spherical radio Telescope".
but also the English word for "fast".
Another huge advantage for FAST is the ability to modify the curvature of a part of the reflector,
transforming from spherical to parabolic a portion of about 300 meters in diameter: every
single panel, in fact, can vary its inclination, with the double advantage of supporting the
movement of the receiver, making it always work in axis with the reflector, and to correct
the spherical aberration of the latter by better focusing the signal.
In the first case, the difference - in positive - is between following an object at the edge
of the field of view with the naked eye and having it directly in front of it... in the
second case, it is a bit like correcting a defect in vision by modifying the curvature
of the crystalline lens.
The technology, in the latter case, is similar to what is called "active optics" in mirror
telescopes, and that has contributed so much in recent years to the extraordinary progress
of astronomical photography.
But what will be the fields of research where FAST will be able to make use of its much-praised
capabilities?
And will the possible scientific discoveries really be such as to justify an economic investment
of almost 250 million dollars?
Well, in the meantime, it must be said that, in the recent past, the greatest discoveries
in this field, such as pulsars, quasars, cosmic background radiation and organic interstellar
molecules, have arrived thanks the use of increasingly large and sophisticated radio
telescopes, which suggests that it will still be the large antennas (at least until the
next generation of optical space telescopes, enormously more favored than Hubble from larger
diameters) to bring the most significant changes in astronomical research.
Investing in radio waves will therefore be scientifically convenient also in the near
future.
And to quantify the superiority of FAST over its predecessors, it will be enough to remember
that the Australian Parkes radio telescope (single antenna 65 meters in diameter) would
be able to detect a hypothetical alien signal of 1 gigawatt power only if it came from a
star less than 4.5 light-years away, which would restrict the research to a single star
system, that of Alpha Centauri...
The Arecibo Observatory, with its opening of 305 meters, would extend the range up to
a distance of 18 light-years, to include a dozen stars.
But FAST could pick up such a signal to thirty light years, keeping more than 1500 stars
under control!
And to better understand the magnitudes at stake, remember that an emission power of
1 gigawatt (one billion watts) is very little on a galactic scale, almost a whisper: in
1974 the famous message sent by Arecibo to the globular cluster M13 had an omnidirectional
energy of 20 terawatts: therefore, 20 thousand times more powerful!
In the search for extraterrestrial intelligence, FAST would therefore have more than one hundred
times the probability of Arecibo being able to pick up a possible signal with the same
sample of stars examined.
But although the titles of the popular articles on the radio telescope have focused only or
almost on this application aspect, FAST will be at the forefront in many other things.
Designed to detect signals in wavelengths between 0.1 and 4 meters, it will be able
for example to trace with great accuracy the distribution of neutral hydrogen, the most
abundant element in the galaxy.
This will allow, among other things, to get more reliable data on the acceleration of
the expansion of the universe and therefore on the real nature of the so-called dark energy,
the mysterious force that according to the most popular cosmological models is the primary
cause.
The only way to accurately measure the amount of acceleration is in fact to establish the
distance of very remote galaxies, which becomes possible only by detecting if between the
galaxy and the observer there is or not a hydrogen cloud capable of "damping" its brightness.
And for this type of measurement, at the required accuracy levels, can provide only a radio
telescope of FAST diameter and sensitivity.
The great Chinese parabola will also be able to give a decisive impulse to the discovery
of new pulsars, objects whose casuistry needs to be expanded to provide important indications
on all that we still do not know about black holes and gravitational waves.
The theory tells us that there should be about 60 thousand pulsars observable in our galaxy,
but so far in half a century have been discovered less than two thousand.
With FAST we hope to find at least another four thousand, and about fifty even in the
Andromeda Galaxy.
It is also expected that its sensitivity is high enough to identify the radio emissions
of gas giants like our Jupiter orbiting distant stars, thus increasing the case history on
extrasolar planets.
And that here on Earth is able to take the leadership of Very Long Baseline Interferometry
systems, that is the observation of the same portion of the sky with several radio telescopes
scattered around the world, coordinated with each other to obtain radio images with resolutions
in the order of thousandths of a second of arc.
Combining in fact the signals acquired by two or more antennas distant from each other,
it is possible to obtain a resolving power equivalent to that achievable with a single
antenna having a diameter equivalent to the distance between the combined instruments.
And to be able to implement the already existing network with an antenna like the Chinese one
would mean to increase the efficiency of the whole system.
For the moment the telescope has been managed only by Chinese technicians and scientists,
who, in addition to starting the planned research, have identified and solved the problems typically
related to the beginning of such complex structures.
Only recently has been given access to researchers from all over the world; thanks also to the
possibility to remotely manage the radio telescope and its instruments.
On September 26, 2017, there was the "first light", the first official observation made
by the instrument.
And it seems quite understandable, and also very symbolic, that the choice of the object
to be targeted fell on the pulsar at the center of the Crab Nebula, which in 1054 AD was observed
in China as a brilliant galactic supernova, and instead completely ignored in Europe by
Western astronomers ... So we can rest assured, the old Arecibo is
unfortunately gone, but FAST has all the numbers not to make him regret too much...