Largest telescope in the world puerto rico: World’s Largest Radio Telescope | HOSLAC
Legendary Arecibo telescope will close forever — scientists are reeling
One of astronomy’s most renowned telescopes — the 305-metre-wide radio telescope at Arecibo, Puerto Rico — is closing permanently. Engineers cannot find a safe way to repair it after two cables supporting the structure broke suddenly and catastrophically, one in August and one in early November.
It is the end of one of the most iconic and scientifically productive telescopes in the history of astronomy — and scientists are mourning its loss.
“I don’t know what to say,” says Robert Kerr, a former director of the observatory. “It’s just unbelievable.”
Arecibo telescope wins reprieve from US government
“I am totally devastated,” says Abel Méndez, an astrobiologist at the University of Puerto Rico at Arecibo who uses the observatory.
The Arecibo telescope, which was built in 1963, was the world’s largest radio telescope for decades and has historical and modern importance in astronomy. It was the site from which astronomers sent an interstellar radio message in 1974, in the hope that any extraterrestrials might hear it, and where the first confirmed extrasolar planet was discovered, in 1992.
It has also done pioneering work in exploring many phenomena, including near-Earth asteroids and the puzzling celestial blasts known as fast radio bursts. All those lines of investigation have now been shut down for good, although limited science will continue at some smaller facilities on the Arecibo site.
Assessing the damage
The cables that broke helped to support a 900-tonne platform of scientific instruments, which hangs above the main telescope dish. The first cable slipped out of its socket and smashed panels at the edge of the dish, but the second broke in half and tore huge gashes in a central portion of the dish.
A high-resolution satellite image, produced at Nature’s request by Planet, an Earth-observation company based in San Francisco, California, shows the extent of the damage wrought by the second cable: the green of the vegetation below shows through large holes in the dish. A second photograph, released this week by observatory officials, also reveals the destruction. These are some of the only public glimpses of the damage so far.
A high-resolution satellite image of the Arecibo dish shows gashes in the main dish through which green vegetation below is visible.Credit: Planet Labs, Inc.
If any more cables fail — which could happen at any time — the entire platform could crash into the dish below. The US National Science Foundation (NSF), which owns the Arecibo Observatory, is working on plans to lower the platform in a safe, controlled fashion.
But those plans will take weeks to develop, and there’s no telling whether the platform might crash down uncontrollably in the meantime. “Even attempts at stabilization or at testing the cables could result in accelerating the catastrophic failure,” said Ralph Gaume, director of the NSF’s astronomy division, at a 19 November media briefing.
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So the NSF decided to close the Arecibo dish permanently. “This decision is not an easy one to make, but safety is the number-one priority,” said Sean Jones, head of the NSF’s mathematical and physical sciences directorate.
The closure comes as a shock to the wider astronomical community. A social-media campaign with the hashtag #WhatAreciboMeansToMe sprung up almost immediately, with astronomers, engineers and other scientists — many from Puerto Rico — sharing stories of how the observatory had shaped their careers. “Losing the Arecibo Observatory would be a big loss for science, for planetary defence and for Puerto Rico,” said Desireé Cotto-Figueroa, an astronomer at the University of Puerto Rico Humacao, in an e-mail before the closure was announced.
NSF officials insist that the cable failures came as a surprise. After the first, engineering teams spotted a handful of broken wires on the second cable, which was more crucial to holding up the platform, but they did not see it as a major problem because the weight it was carrying was well within its design capacity. “It was not seen as an immediate threat,” says Ashley Zauderer, programme director for Arecibo at the NSF.
The main cable that failed experienced wire breaks (shown) before its sudden and unexpected collapse.Credit: University of Central Florida/Arecibo Observatory
But that main cable, which was installed in the early 1960s, had apparently degraded over time. Over the years, external review committees have highlighted the ongoing need to maintain the ageing cables. Zauderer said that maintenance in recent years had been completed according to schedule.
Before this year, the last major cable problems at the observatory were in January 2014, when a magnitude-6.4 earthquake caused damage to another of the main cables, which engineers repaired. The ageing structure has suffered other shocks in recent years, including damage to an antenna and the dish caused by Hurricane Maria in 2017.
There is no estimate yet for the cost of decommissioning the telescope.
A legendary site
The science that has ground to a halt includes Arecibo’s world-leading asteroid studies. The telescope pinged radio waves at near-Earth asteroids to reveal the shape and spin of these threatening space rocks. Not having it “will be a big loss”, says Alan Harris, an asteroid scientist in La Canada, California. (China’s 500-meter Aperture Spherical Telescope (FAST), which opened in 2016, does not currently have the ability to do such radar studies.)
Some of the observatory’s scientific projects could be transferred to other facilities, Gaume said — and he expects scientists to propose where to move their research. Much of the work conducted at Arecibo, however, could be done only with its unique array of astronomical instrumentation. “The Arecibo Telescope is irreplaceable,” said a statement from two major US radio-astronomy organizations, the National Radio Astronomy Observatory in Charlottesville, Virginia, and the Green Bank Observatory in West Virginia.
Small amounts of science will continue at other portions of the Arecibo observatory, which encompasses more than the 305-metre dish. For instance, two lidar facilities shoot lasers into the atmosphere to study atmospheric phenomena.
Gigantic Chinese telescope opens to astronomers worldwide
The Arecibo telescope had been upgraded regularly, with several new instruments slated for installation in the coming years. “The telescope is in no way obsolete,” says Christopher Salter, an astronomer at the Green Bank Observatory, who worked at Arecibo for years.
Planned upgrades are now presumably on hold, including a US$5.8-million antenna that was being developed for the telescope’s platform and would have massively increased its sensitivity. Brian Jeffs, an engineer at Brigham Young University in Provo, Utah, who heads the project, says his team expects to discuss options for its future with the NSF eventually. “Our greatest concerns are for the wonderful scientific, technical, management and support staff” of the observatory, he says.
The observatory is a major centre for science education in Puerto Rico, where it has fostered the careers of many astronomers and engineers. And it has become a part of the pop-culture lexicon, featuring in major movies such as Contact (1997), which was based on a novel by astronomer Carl Sagan, and the 1995 James Bond film GoldenEye.
The most recent major radio-telescope disaster happened in 1988, when a 300-foot-wide antenna at the Green Bank Observatory collapsed one night, owing to structural failure.
Arecibo Observatory Telescope Collapses, Ending Era Of World-Class Research : NPR
Video: Arecibo Observatory Telescope Collapses, Ending Era Of World-Class Research Astronomers compare losing the observatory in Puerto Rico to losing a big brother. It was once the world’s largest single-dish radio telescope.
The Arecibo Observatory’s mammoth telescope collapsed overnight. It’s seen here in November, after a cable damaged its dish.
University of Central Florida
University of Central Florida
The Arecibo Observatory’s mammoth telescope collapsed overnight. It’s seen here in November, after a cable damaged its dish.
University of Central Florida
Updated on Dec. 4 at 11:30 a.m. ET
The Arecibo Observatory in Puerto Rico has collapsed, after weeks of concern from scientists over the fate of what was once the world’s largest single-dish radio telescope. Arecibo’s 900-ton equipment platform, suspended some 500 feet above the dish, fell overnight after the last of its healthy support cables failed to keep it in place.
No injuries were reported, according to the National Science Foundation, which oversees the renowned research facility.
«NSF is saddened by this development,» the agency said. «As we move forward, we will be looking for ways to assist the scientific community and maintain our strong relationship with the people of Puerto Rico.»
The dramatic collapse was captured on video, showing the mammoth structure falling in the early morning, after the last of its cables unraveled. The NSF released footage taken from two cameras, one on the ground and another mounted on a drone.
The Arecibo Observatory had been slated last month to be withdrawn from service, with the NSF citing the risk of an «uncontrolled collapse» because of failures in the cables that suspended the platform and its huge Gregorian dome above the 1,000-foot-wide reflector dish.
Ángel Vázquez, the observatory’s director of telescope operations, says he was in the control room area when the equipment began to plummet to the ground. In an interview that was posted to Twitter by scientist Wilbert Andrés Ruperto, Vázquez says he and other staff members had been in the process of removing valuable equipment from the facility when they heard a loud bang outside.
«When we looked outside the control room, we started to see the eventual downfall of the observatory,» Vázquez said. He added that strands of the remaining three cables had been unraveling in recent days, increasing the strain. And because two of the support towers maintained tension as the collapse occurred, some of the falling equipment was yanked across the side of the dish rather than falling straight down through its focal point.
Ángel Vázquez explains the collapse of the Arecibo Observatory @SaveTheAO. 1/2 pic.twitter.com/7VCZNCFsA4
— Wilbert Andrés Ruperto (@ruperto1023) December 1, 2020
«This whole process took 30 seconds,» Vázquez said, «and an unfortunate icon in radio astronomy was done. «
Vázquez said he has worked at the facility for 43 years, starting soon after college.
The massive reflector dish is made up of perforated aluminum panels, leaving an expanse of greenery underneath. But many of those panels have now fallen to the earth.
Here are the images. 😔#AreciboObservatory https://t.co/oUpUkxtcGB pic.twitter.com/1BYh4w2gAf
— Prof. Abel Méndez 👽 (@ProfAbelMendez) December 1, 2020
The telescope’s trademark dish, nestled amid thick tropical forest, was left with a huge gash in August after a cable fell and slashed through its panels. After a main cable snapped in early November, officials said they saw no way to safely preserve the unstable structure.
Instead, they were hoping to keep the visitors center and other buildings operational. But they also noted it would take weeks to work out the technical details of a plan.
A record of discovery
In Arecibo’s nearly 60 years of operation, the observatory’s powerful capabilities made it a popular choice for researchers chasing breakthroughs in radio astronomy and atmospheric science. It was used for projects from sniffing out gravitational waves in space to tracking down potentially habitable planets far from Earth.
Arecibo’s legacy includes the detection of the first binary pulsar in 1974 — a discovery that bolstered a key idea in Einstein’s general theory of relativity and that earned two physicists the 1993 Nobel Prize in physics.
The observatory has been an inspiration to many. For its neighbors in Puerto Rico and for people worldwide, it has been a literal link between the terrestrial and the extraterrestrial. And in movies and art, it has been depicted as both Earth’s doorbell and its peephole into outer space.
Pierce Brosnan clambered around its ladders in the James Bond film GoldenEye. Jodie Foster marveled at its otherworldly promise in Contact. And in 1974, it was used to beam a «Hello» message into space.
The Arecibo Observatory collapsed when its 900-ton receiver platform fell hundreds of feet, smashing through the radio dish below.
Ricardo Arduengo/AFP via Getty Images
Ricardo Arduengo/AFP via Getty Images
The Arecibo Observatory collapsed when its 900-ton receiver platform fell hundreds of feet, smashing through the radio dish below.
Ricardo Arduengo/AFP via Getty Images
Researchers have been mourning the telescope’s loss since the NSF announced its looming demise last month. Astronomer Seth Shostak of the SETI Institute compared it to learning your high school has burned down or to losing a big brother. Doing research at the facility was like going to a wonderful summer camp, he wrote in a recent farewell message to Arecibo.
«While life will continue, something powerful and profoundly wonderful is gone,» Shostak said.
Here’s how planetary scientist Ed Rivera-Valentin described one aspect of Arecibo’s importance earlier this year, on NPR’s Short Wave podcast:
«One of the really neat things about the Arecibo Observatory is that it’s a very versatile scientific instrument. Most telescopes, most radio telescopes, don’t have the ability to send out light. They only capture light. At the observatory, we can send and capture light. When an asteroid’s coming by, we are pretty much a flashlight that we turn on. We send radar out to it, and that radar comes back. … We can tell you how far these objects are down to a few meters.
«And we care about where these asteroids are going to be because what if, one day, this thing comes around and gets too close to Earth? But if we can let people know this is going to happen next year, we can actually prepare for it. Like, the dinosaurs — they didn’t have a space program, so they didn’t get to prepare for anything. «
The idea for the observatory was conceived in the late 1950s by Cornell University professor William E. Gordon, who was looking to build a huge tool to explore the Earth’s atmosphere and the composition of nearby planets and moons.
The site in Puerto Rico was chosen «to take advantage of the vicinity to the equator and of the topography of the terrain, which provided a nearly spherical valley and minimized excavation,» according to a lecture by longtime Cornell astronomy professor Martha Haynes.
The telescope underwent major upgrades in the 1970s and 1990s, allowing researchers to expand its role. Built with federal funds, Arecibo was managed for decades by Cornell before the University of Central Florida took up that role.
Arecibo and Puerto Rico have withstood natural calamities in recent years, including Hurricane Maria in 2017 and a series of earthquakes this year.
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Arecibo telescope collapses in Puerto Rico.
Arecibo telescope collapsed in Puerto Rico. Video — BBC News Russian ServiceBBC News, Russian Service
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The telescope at the Arecibo Observatory in Puerto Rico collapsed after 57 years of service.
He stopped working in August of this year. Then the cable broke for the first time. And on December 2, it collapsed completely.
It has been in operation since 1963 and has been used for research in radio astronomy, atmospheric physics and observations of solar system objects. Because of its spectacular design, the telescope has been popular in movies. He has appeared in the Bond movies, Species and Contact, and the X-Files TV series. nine0003
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Articles of Vokrug Sveta magazine
Modern radio telescopes make it possible to explore the Universe in such detail that until recently was beyond the limits of what was possible not only in the radio range, but also in traditional visible light astronomy. United in a single network of instruments located on different continents, allow you to look into the very core of radio galaxies, quasars, young star clusters, forming planetary systems. Radio interferometers with extra-long baselines surpassed the largest optical telescopes in terms of «vigilance» by thousands of times. With their help, one can not only track the movement of spacecraft in the vicinity of distant planets, but also study the movements of the crust of our own planet, including directly «feel» the drift of the continents. Next in line are space radio interferometers, which will allow even deeper insight into the mysteries of the universe. nine0003
The Earth’s atmosphere is not transparent to all types of electromagnetic radiation coming from outer space. It has only two wide «windows of transparency». The center of one of them falls on the optical region, in which the maximum radiation of the Sun lies. As a result of evolution, it was to him that the human eye adapted in terms of sensitivity, which perceives light waves with a length of 350 to 700 nanometers. (In fact, this transparency window is even slightly wider — from about 300 to 1,000 nm, that is, it captures the near ultraviolet and infrared ranges). However, the rainbow streak of visible light is only a small fraction of the richness of the «colors» of the Universe. In the second half of the 20th century, astronomy became truly all-wave. Advances in technology have allowed astronomers to make observations in new ranges of the spectrum. On the short wavelength side of visible light lie the ultraviolet, x-ray and gamma ranges. On the other side are infrared, submillimeter and radio bands. For each of these ranges, there are astronomical objects that manifest themselves most clearly in it, although in optical radiation they may not represent anything outstanding, so astronomers simply did not notice them until recently. nine0003
One of the most interesting and informative ranges of the spectrum for astronomy is radio waves. The radiation recorded by ground-based radio astronomy passes through a second and much wider transparency window of the earth’s atmosphere in the wavelength range from 1 mm to 30 m. 30 m. At waves shorter than 1 mm, cosmic radiation is completely “eaten up” by atmospheric molecules (mainly oxygen and water vapor). nine0003
The main characteristic of a radio telescope is its radiation pattern. It shows the sensitivity of the instrument to signals coming from different directions in space. For a «classical» parabolic antenna, the radiation pattern consists of the main lobe, which has the form of a cone oriented along the axis of the paraboloid, and several much (by orders of magnitude) weaker side lobes. The «vigilance» of a radio telescope, that is, its angular resolution, is determined by the width of the main lobe of the radiation pattern. Two sources in the sky, which together fall into the solution of this petal, merge into one for the radio telescope. Therefore, the width of the radiation pattern determines the size of the smallest details of the celestial radio source, which can still be distinguished individually. nine0003
A universal rule for telescope construction states that the resolution of an antenna is determined by the ratio of the wavelength to the diameter of the telescope’s mirror. Therefore, in order to increase «vigilance», the telescope must be larger, and the wavelength smaller. But as luck would have it, radio telescopes work with the longest wavelengths of the electromagnetic spectrum. Because of this, even the huge size of the mirrors does not allow achieving high resolution. Not the largest modern optical telescope with a mirror diameter of 5 m can distinguish stars at a distance of only 0.02 arcseconds. Details about one minute of arc are visible to the naked eye. And a radio telescope with a diameter of 20 m at a wavelength of 2 cm gives a resolution even three times worse, about 3 arc minutes. A snapshot of a section of the sky taken by an amateur camera contains more details than a radio emission map of the same area obtained by a single radio telescope. nine0003
A wide radiation pattern limits not only the visual acuity of the telescope, but also the accuracy of determining the coordinates of the observed objects. Meanwhile, exact coordinates are needed to compare observations of an object in different ranges of electromagnetic radiation, which is an indispensable requirement of modern astrophysical research. Therefore, radio astronomers have always strived to create the largest possible antennas. And, surprisingly, radio astronomy ended up far ahead of optical in resolution. nine0003
At the telescope of the Arecibo Observatory in Puerto Rico the world’s largest fixed solid mirror with a diameter of 305 m. A structure with receiving equipment weighing 800 tons hangs on cables above the spherical bowl. Along the perimeter, the mirror is surrounded by a metal mesh that protects the telescope from the radio emission of the earth’s surface
Record holders in singles
Fully rotatable parabolic antennas analogues of optical reflecting telescopes turned out to be the most flexible in operation from the entire variety of radio astronomy antennas. They can be directed to any point in the sky, follow the radio source “accumulate the signal”, as radio astronomers say, and thereby increase the sensitivity of the telescope, its ability to distinguish much weaker signals from cosmic sources against the background of all kinds of noise. The first large full-revolving paraboloid with a diameter of 76 m was built in 1957 at the British observatory Jodrell Bank. And today, the plate of the world’s largest mobile antenna at the Green Bank Observatory (USA) has dimensions of 100 by 110 m. And this is practically the limit for single mobile radio telescopes. The increase in diameter has three important consequences: two good and one bad. First, the most important thing for us is that the angular resolution increases in proportion to the diameter. Secondly, the sensitivity grows, and much faster, in proportion to the area of the mirror, that is, the square of the diameter. And, thirdly, the cost increases even faster, which in the case of a mirror telescope (both optical and radio) is approximately proportional to the cube of the diameter of its main mirror. nine0003
The main difficulties are associated with the deformation of the mirror under the action of gravity. In order for the telescope mirror to clearly focus radio waves, the deviations of the surface from an ideal parabolic surface should not exceed one tenth of the wavelength. Such accuracy is easily achieved for wavelengths of several meters or decimeters. But at short centimeter and millimeter wavelengths, the required accuracy is already tenths of a millimeter. Due to structural deformations under its own weight and wind loads, it is almost impossible to create a full-rotation parabolic telescope with a diameter of more than 150 m. The largest fixed dish with a diameter of 305 m was built at the Arecibo Observatory, Puerto Rico. But on the whole, the era of gigantomania in the construction of radio telescopes has come to an end. In Mexico, on the Sierra Negra mountain, at an altitude of 4,600 meters, construction of a 50-meter antenna for millimeter wave operation is being completed. Perhaps this is the last large single antenna created in the world. nine0003
In order to see the details of the structure of radio sources, we need other approaches, which we have to figure out.
How it works
The world’s largest fully rotatable parabolic antenna at the Green Bank Observatory (West Virginia, USA). The 100×110 m mirror was built after a 90 m full-revolving antenna collapsed under its own weight in 1988.
Radio waves emitted by the observed object propagate in space, generating periodic changes in the electric and magnetic fields. A parabolic antenna collects the radio waves that fall on it at one point focus. When several electromagnetic waves pass through one point, they interfere, that is, their fields add up. If the waves arrive in phase they amplify each other, in antiphase they weaken, up to complete zero. The peculiarity of a parabolic mirror is precisely that all waves from one source come into focus in one phase and amplify each other as much as possible! The functioning of all mirror telescopes is based on this idea. nine0003
A bright spot appears at the focus, and a receiver is usually placed here, which measures the total intensity of radiation captured within the telescope’s radiation pattern. Unlike optical astronomy, a radio telescope cannot take a photograph of a section of the sky. At each moment, it detects radiation coming from only one direction. Roughly speaking, a radio telescope works like a single-pixel camera. To build an image, one has to scan the radio source point by point. (However, the millimeter radio telescope under construction in Mexico has a radiometer array in focus and is no longer a “single-pixel” telescope.)
However, there is another way to do it. Instead of bringing all the rays to one point, we can measure and record the electric field oscillations generated by each of them on the surface of the mirror (or at another point through which the same beam passes), and then «add» these records in a computer device processing, taking into account the phase shift corresponding to the distance that each of the waves had to travel to the imaginary focus of the antenna. A device operating according to this principle is called an interferometer, in our case, a radio interferometer. nine0003
Interferometers eliminate the need to build huge one-piece antennas. Instead, dozens, hundreds or even thousands of antennas can be placed next to each other and the signals received by them combined. Such telescopes are called in-phase arrays. However, they still do not solve the problem of «vigilance» for this one needs to take one more step.
As you remember, as the size of a radio telescope grows, its sensitivity grows much faster than its resolution. Therefore, we quickly find ourselves in a situation where the power of the recorded signal is more than enough, and the angular resolution is sorely lacking. And then the question arises: “Why do we need a solid array of antennas? Can’t it be thinned out?» It turned out that it is possible! This idea is called “aperture synthesis”, since a much larger diameter mirror is “synthesized” from several separate independent antennas placed over a large area. The resolution of such a «synthetic» instrument is determined not by the diameter of individual antennas, but by the distance between them — the base of the radio interferometer. Of course, there must be at least three antennas, and they should not be located along one straight line. Otherwise, the resolution of the radio interferometer will be extremely inhomogeneous. It will be high only in the direction along which the antennas are spaced. In the transverse direction, the resolution will still be determined by the size of the individual antennas. nine0003
Radio astronomy began to develop along this path as early as the 1970s. During this time, a number of large multi-antenna interferometers were created. Some of them have fixed antennas, while others can move along the surface of the earth to make observations in different «configurations». Such interferometers build «synthesized» maps of radio sources with a much higher resolution than single radio telescopes: at centimeter waves it reaches 1 arc second, and this is already comparable to the resolution of optical telescopes when observing through the Earth’s atmosphere. nine0003
The most famous system of this type, the Very Large Array (VLA), was built in 1980 at the US National Radio Astronomy Observatory. Its 27 parabolic antennas, each with a diameter of 25 m and weighing 209 tons, move along three radial rail tracks and can move away from the center of the interferometer at a distance of up to 21 km.
Other systems are in operation today: Westerbork in the Netherlands (14 antennas 25 m in diameter), ATCA in Australia (6 antennas 22 m each), MERLIN in the UK. The latest system, along with 6 other instruments scattered throughout the country, includes the famous 76-meter telescope. In Russia (in Buryatia) the Siberian Solar Radio Interferometer has been created, a special system of antennas for the operational study of the Sun in the radio range. nine0003
The size of the globe
A plate with a diameter of 25 meters and a weight of 240 tons in the Owens Valley, USA, one of the 10 instruments of the American VLBI network
In 1965, Soviet scientists L.I. Matveenko, N.S. Kardashev, G.B. Sholomitsky proposed to independently record data on each interferometer antenna, and then process them jointly, as if simulating the phenomenon of interference on a computer. This allows the antennas to be spread over arbitrarily long distances. Therefore, the method was called very long baseline radio interferometry (VLBI) and has been successfully used since the beginning of 1970s. The record base length achieved in experiments is 12.2 thousand km, and the resolution at a wavelength of about 3 mm reaches 0.00008’’ , three orders of magnitude higher than that of large optical telescopes. It is unlikely that this result will be significantly improved on Earth, since the size of the base is limited by the diameter of our planet.
Currently, systematic observations are carried out by several networks of intercontinental radio interferometers. In the United States, a system has been created that includes 10 radio telescopes with an average diameter of 25 m, located in the continental part of the country, on the Hawaiian and Virgin Islands. In Europe, for VLBI experiments, the 100-meter Bonn telescope and the 32-meter telescope in Medicina (Italy), MERLIN interferometers, Westerbork, and other instruments are regularly combined. This system is called EVN. There is also a global network of radio telescopes for astrometry and geodesy IVS. And recently, Russia began to operate its own interferometric network «Kvazar» of three 32-meter antennas located in the Leningrad region, the North Caucasus and Buryatia. It is important to note that telescopes are not rigidly attached to VLBI networks. They can be used standalone or switched between networks. nine0003
Very long baseline interferometry requires very high measurement accuracy: it is necessary to fix the spatial distribution of the maxima and minima of electromagnetic fields with an accuracy of a fraction of a wavelength, that is, for short waves to fractions of a centimeter. And with the highest accuracy, note the time points at which measurements were taken on each antenna. Atomic frequency standards are used as ultraprecise clocks in VLBI experiments.
But don’t think that radio interferometers don’t have flaws. In contrast to a solid parabolic antenna, the directivity pattern of an interferometer has hundreds and thousands of narrow lobes of comparable size instead of one main lobe. Building a source map with such a radiation pattern is like touching a computer keyboard with outstretched fingers. Image restoration is a complex and, moreover, “incorrect” (that is, unstable to small changes in measurement results) problem, which, however, radio astronomers have learned to solve. nine0003
Achievements of radio interferometry
Radio interferometers with an angular resolution of thousandths of an arc second “peered” into the innermost regions of the most powerful “radio beacons” of the Universe radio galaxies and quasars, which radiate in the radio range tens of millions of times more intense than ordinary galaxies. It was possible to «see» how plasma clouds are ejected from the nuclei of galaxies and quasars, to measure the speed of their movement, which turned out to be close to the speed of light.
Many interesting things were also discovered in our Galaxy. In the vicinity of young stars, sources of maser radio emission (a maser is an analogue of an optical laser, but in the radio range) have been found in the spectral lines of water, hydroxyl (OH), and methanol (Ch4OH) molecules. On a cosmic scale, the sources are very small — smaller than the solar system. Separate bright spots on the radio maps obtained by interferometers may be the embryos of planets. nine0003
Such masers have also been found in other galaxies. The change in the positions of maser spots over several years, observed in the neighboring galaxy M33 in the constellation Triangulum, for the first time made it possible to directly estimate the speed of its rotation and movement across the sky. The measured displacements are negligible, their speed is many thousands of times less than the speed of a snail crawling along the surface of Mars, visible to an earthly observer. Such an experiment is still far beyond the capabilities of optical astronomy: it is simply beyond its power to notice the proper movements of individual objects at intergalactic distances. nine0003
Finally, interferometric observations have provided new evidence for the existence of supermassive black holes. Around the core of the active galaxy NGC 4258, clumps of matter were discovered that move in orbits with a radius of no more than three light years, while their speeds reach thousands of kilometers per second. This means that the mass of the central body of the galaxy is at least a billion solar masses, and it cannot be anything other than a black hole.
A number of interesting results have been obtained by the VLBI method during observations in the solar system. Let’s start with the most accurate quantitative test of general relativity to date. The interferometer measured the deviation of radio waves in the gravitational field of the Sun with an accuracy of a hundredth of a percent. This is two orders of magnitude more accurate than optical observations allow. nine0003
Global radio interferometers are also used to track the movement of spacecraft that study other planets. The first time such an experiment was carried out in 1985, when the Soviet vehicles «Vega-1» and «-2» dropped balloons into the atmosphere of Venus. Observations confirmed the rapid circulation of the planet’s atmosphere at a speed of about 70 m/s, that is, one revolution around the planet in 6 days. This is an amazing fact that is yet to be explained.
Last year, similar observations involving a network of 18 radio telescopes on different continents accompanied the landing of the Huygens spacecraft on Saturn’s moon Titan. From a distance of 1.2 billion km, they tracked how the device moves in the atmosphere of Titan with an accuracy of tens of kilometers! It is not widely known that almost half of the scientific information was lost during the Huygens landing. The probe relayed data through the Cassini station, which took it to Saturn. For reliability, two redundant data transmission channels were provided. However, shortly before landing, it was decided to transmit different information on them. But at the most crucial moment, due to an as yet unexplained failure, one of the receivers on the Cassini did not turn on, and half of the pictures disappeared. And along with them, the data on the wind speed in the atmosphere of Titan, which were transmitted just over the disconnected channel, also disappeared. Fortunately, NASA managed to play it safe — the descent of the Huygens was observed from Earth by a global radio interferometer. This, apparently, will save the missing data on the dynamics of the atmosphere of Titan. The results of this experiment are still being processed at the European Joint Radio Interferometric Institute, and, by the way, our compatriots Leonid Gurvits and Sergey Pogrebenko are doing this. nine0003
VLBI for the earth
The method of radio interferometry also has purely practical applications not in vain, for example, in St. Petersburg, the Institute of Applied Astronomy of the Russian Academy of Sciences deals with this topic. VLBI observations make it possible not only to determine the coordinates of radio sources with an accuracy of ten thousandths of an arc second, but also to measure the positions of the radio telescopes themselves on Earth with an accuracy of better than one millimeter. This, in turn, makes it possible to track variations in the Earth’s rotation and movements of the Earth’s crust with the highest accuracy. nine0003
For example, it was with the use of VLBI that the motion of the continents was experimentally confirmed. Today, the registration of such movements has already become a routine matter. Interferometric observations of distant radio galaxies have firmly entered the arsenal of geophysics along with seismic sounding of the Earth. Thanks to them, periodic displacements of stations relative to each other, caused by deformations of the earth’s crust, are reliably recorded. Moreover, not only long-term measured solid-state tides (for the first time recorded by the VLBI method) are noted, but also deflections that occur under the influence of changes in atmospheric pressure, the weight of water in the ocean, and the weight of groundwater. nine0003
To determine the parameters of the Earth’s rotation in the world, daily observations of celestial radio sources are carried out, coordinated by the International VLBI Service for Astrometry and Geodesy IVS. The data obtained are used, in particular, to detect the drift of the orbital planes of satellites of the global positioning system GPS. Without the introduction of appropriate corrections obtained from VLBI observations, the error in determining longitude in the GPS system would be orders of magnitude larger than it is now. In a sense, VLBI plays the same role for GPS navigation that accurate marine chronometers played for star navigation in the 18th century. Accurate knowledge of the parameters of the Earth’s rotation is also necessary for the successful navigation of interplanetary space stations. nine0003
Leonid Petrov, Space Flight Center. Goddard, NASA
Instruments of the future
At least in the next half century, the general line of development of radio astronomy will be the creation of ever larger aperture synthesis systems all large instruments being designed are interferometers. Thus, on the Chajnantor Plateau in Chile, joint efforts of a number of European and American countries began the construction of the ALMA (Atacama Large Millimeter Array) millimeter-wave antenna system. In total, there will be 64 antennas with a diameter of 12 meters with an operating wavelength range from 0.35 to 10 mm. The longest distance between ALMA antennas will be 14 km. Due to the very dry climate and high altitude (5100 m), the system will be able to observe at waves shorter than a millimeter. In other places and at a lower altitude, this is not possible due to the absorption of such radiation by water vapor in the air. Construction of ALMA will be completed by 2011. nine0003
The European LOFAR aperture synthesis system will operate at much longer wavelengths, from 1.2 to 10 m. It will be operational within the next three years. This is a very interesting project: in order to keep the cost down, it uses the simplest fixed antennas — pyramids of metal rods about 1.5 m high with a signal amplifier. But there will be 25 thousand such antennas in the system. They will be united in groups that will be placed throughout Holland along the rays of a “curved five-pointed star” with a diameter of about 350 km. Each antenna will receive signals from the entire visible sky, but their joint computer processing will make it possible to single out those that came from directions of interest to scientists. In this case, a directivity pattern of the interferometer is formed by purely computational means, the width of which at the shortest wavelength will be 1 arc second. The operation of the system will require a huge amount of calculations, but for today’s computers this is quite a feasible task. To solve this problem, Europe’s most powerful supercomputer IBM Blue Gene/L with 12,288 processors was installed last year in Holland. Moreover, with appropriate signal processing (requiring even more computer power), LOFAR will be able to simultaneously observe several and even many objects! nine0003
But the most ambitious project in the near future is the SKA (Square Kilometer Array). The total area of its antennas will be about 1 km2, and the cost of the instrument is estimated at one billion dollars. The SKA project is still at an early stage of development. The main design option under discussion is thousands of antennas with a diameter of several meters, operating in the range from 3 mm to 5 m. Moreover, half of them are planned to be installed in a section with a diameter of 5 km, and the rest to be spread over considerable distances. Chinese scientists proposed an alternative scheme — 8 fixed mirrors with a diameter of 500 m each, similar to the Arecibo telescope. Suitable dry lakes have even been proposed to house them. However, in September, China dropped out of the list of countries pretenders to host a giant telescope. Now the main struggle will unfold between Australia and South Africa. nine0003
And the whole world is not enough
8-meter space-deployable antenna of the Japanese satellite HALCA of the first space VLBI node
The possibilities of increasing the base of ground-based interferometers are practically exhausted. The future is the launch of interferometer antennas into space, where there are no restrictions associated with the size of our planet. Such an experiment has already been carried out. In February 1997, the Japanese satellite HALCA was launched, which worked until November 2003 and completed the first stage in the development of the international project VSOP (VLBI Space Observatory Program Space Observatory Program VLBI). The satellite carried an umbrella-shaped antenna 8 m in diameter and operated in an elliptical Earth orbit that provided a base three times the diameter of the Earth. Images of many extragalactic radio sources were obtained with a resolution of thousandths of an arc second. The next phase of the space interferometry experiment, VSOP-2, is planned to start in 20112012. Another instrument of this type is being created within the framework of the Radioastron project by the Astrospace Center of the Physical Institute. P.N. Lebedev RAS together with scientists from other countries. The Radioastron satellite will have a parabolic mirror with a diameter of 10 m. During launch, it will be in a folded state, and after entering orbit, it will turn around. Radioastron will be equipped with receivers for several wavelengths from 1.2 to 92 cm. Radio telescopes in Pushchino (Russia), Canberra (Australia) and Green Bank (USA) will be used as ground-based antennas of the space interferometer. The satellite’s orbit will be very elongated, with an apogee of 350,000 km. With such an interferometer base at the shortest wavelength, it will be possible to obtain images of radio sources and measure their coordinates with an accuracy of 8 millionths of an arc second.