TY - JOUR
AU - Gunn,, Alastair
AB - Abstract Alastair Gunn looks back at the scientific and cultural legacy of an iconic instrument: the 305m Arecibo radio telescope The Arecibo radio dish, officially the William E Gordon Telescope, has collapsed. After a series of ever-worsening mechanical failures through autumn, the instrument platform of the second-largest single-aperture telescope in the world finally succumbed and fell onto the dish below at precisely 7.55 a.m. local time on 1 December 2020. This marks the sad end of an impressive 57-year career of what was, for most of its existence, a unique instrument that made significant contributions in the atmospheric and ionospheric sciences, planetary sciences, the search for extraterrestrial intelligence (SETI), radio astronomy and radar astronomy. It has starred in several movies, searched the skies for alien intelligence, and beamed coded messages to distant star systems. The sheer size of the reflector at Arecibo (figure 1) was one of its main advantages over other radio and radar instruments. It remained the world's largest collecting area until 2016 saw the completion of a 500 m radio telescope of similar design near Guizhou, China: FAST, the Five-hundred-meter Aperture Spherical radio Telescope (Li & Pan 2016). But size is not everything; the inventiveness, adaptability and audacity of the many engineers, scientists and students who have employed the Arecibo facility – for nigh-on six decades – also contributed to its unparalleled scientific legacy. Like many innovative scientific instruments before it, and those that will follow, the facility made discoveries that could never have been envisaged at the time of its design, and it pursued research areas completely unknown to its early protagonists. 1 Open in new tabDownload slide The devastation after the collapse of the instrument platform onto the dish on 1 December 2020. (Juan R Costa/Noticel) 1 Open in new tabDownload slide The devastation after the collapse of the instrument platform onto the dish on 1 December 2020. (Juan R Costa/Noticel) Ionospheric origins The Arecibo project was initially conceived in the late 1950s by William E Gordon (figure 3), a professor of electrical engineering at Cornell University who had been exploring the concept of a spherical incoherent scatter radar (ISR) for ionospheric research (Gordon 1958). At this time the US Department of Defence was also keen to better understand the dynamics of the ionosphere and, more broadly, the solar–terrestrial connection and its impact on long-range radar systems, rocket launches and military communications. The US Air Force Cambridge Research Laboratories (AFCRL) had done similar work on spherical reflectors (Spencer et al. 1951) and had become interested in Gordon's ISR concept. In 1959, AFCRL officials signed a contract with engineers at Cornell to explore and develop the idea of an ISR facility for both military and civilian use. The project was initially funded by the Advanced Research Projects Agency (ARPA). 2 Open in new tabDownload slide Typical diurnal variation of the maximum ionization height (solid line) and electron density (colour) observed above Arecibo. (Reproduced with permission from Vlasov et al. 2003) 2 Open in new tabDownload slide Typical diurnal variation of the maximum ionization height (solid line) and electron density (colour) observed above Arecibo. (Reproduced with permission from Vlasov et al. 2003) 3 Open in new tabDownload slide William E Gordon (1918–2010), facing the camera, talking with engineer Domingo Albino during construction of the radio dish, in April 1961. (Courtesy Arecibo Observatory) 3 Open in new tabDownload slide William E Gordon (1918–2010), facing the camera, talking with engineer Domingo Albino during construction of the radio dish, in April 1961. (Courtesy Arecibo Observatory) The site of the facility, among the banana plants south of the town of Arecibo, Puerto Rico, was chosen for three reasons: it was a US unincorporated territory; it was near the equator, maximizing access to the sky, particularly to the ecliptic for planetary research; and the terrain favoured construction of a bowl-shaped depression for the wire-mesh reflector surface. It was fortuitous that the site was equatorial; other, complementary, ISR facilities were later built at mid- or high-latitude. Construction began in 1960. By autumn 1963, at a cost of more than $9 million, the Arecibo Ionospheric Observatory (AIO) began operations with Gordon as director (Gordon & LaLonde 1961, Gordon 1964). After the AIO had completed mapping the ionosphere through a full solar cycle, its main military goal had been achieved and ARPA then shared the AIO with ionospheric physicists, and radio and radar astronomers. It was transferred to the National Science Foundation (NSF) in 1971, but continued to be operated by Cornell University. In 2011, running of the observatory was taken over by SRI International, the Universities Space Research Association and Universidad Metropolitana, a private university in San Juan, Puerto Rico. In 2018, management of the facility, on behalf of the NSF, was handed to the University of Central Florida. Although one of the first, Arecibo used a unique design for an ISR system. With a fixed spheroidal reflector, the instrument was “pointed” by moving the transmitter (or receiver) along the radius of a spherical focal plane. Hence, the 900-ton instrument platform, suspended 137 m above the 305 m diameter reflector, acted as the instrument's tracking mechanism, meaning its view of the sky was restricted to a narrow window of observation up to 20° either side of the zenith. For ISR studies the telescope was usually used as a transit device. The instrument underwent several major upgrades. In 1974, the original steel wire-mesh reflector was replaced by 38 778 perforated aluminium panels, and the suspended structure above was strengthened, allowing the original frequency coverage (20–611 MHz) to increase up to 4830 MHz (LaLonde 1974). A second major upgrade, in 1997, saw the installation of a Gregorian sub-reflector dome to replace the original line feeds at the spherical focus, increasing the maximum operating frequency again, this time to 10 GHz (Goldsmith 1996). The resulting instrument was, depending on frequency, up to 10 times more sensitive than its original design. Finally, in 2004, the Arecibo L-band Feed Array (ALFA) was installed, allowing observations between 1225 and 1525 MHz (containing, among other spectral lines, the hydrogen 21 cm line). This seven-pixel cooled receiver array significantly increased the capabilities of the instrument and was revolutionary for many fields of radio astronomy, most notably for pulsar research and H I surveys. Aeronomy Almost the entire first decade of Arecibo's existence was spent on mostly classified surveys of the ionosphere, the research field for which it was originally intended. After transfer to the NSF, the facility became more accessible to ionospheric physicists and immediately had an enormous impact on knowledge of the Earth's upper atmosphere (Walker 1978, Mathews 2013). Much of what we know about the tropical off-equatorial ionosphere comes from Arecibo data. The great strength of Gordon's ISR technique is that it can determine several properties of the ionospheric plasma simultaneously over a wide range of altitudes: data such as density, electron temperature, ion temperature, ionic composition and the mean drift velocity of the ions. Figure 2, for example, shows the diurnal variation in the ionosphere above Arecibo (Vlasov et al. 2003). Arecibo helped in the development of ionospheric heating techniques (Gordon et al. 1971) and was the first facility to simultaneously heat both the D- and E-regions of the ionosphere, demonstrating the value of the technique (Showen 1972). Informative heating experiments continued at Arecibo throughout its lifetime (Carlson et al. 1972, Gordon & Carlson 1974, Kuo 2004). An experiment by Sulzer et al. (1982) is the only known example of UHF (430 MHz) heating of the D-region ionosphere. Arecibo also monitored the “holes” punched through the ionosphere by rocket launches, including the Saturn V launch of Skylab on 14 May 1973 (Mendillo 1975a, b) and the Atlas-Centaur launch of the HEAO-C satellite (Mendillo et al. 1979). Arecibo data showed that the ionospheric electron column density was typically reduced by 50% after launch, thereby heightening awareness of the environmental impact of space launches. In 2015, Arecibo demonstrated, for mid-latitudes, the production of an artificial ionosphere comparable to that produced by solar EUV radiation (Carlson et al. 2017). Other highlights include high-resolution images of artificial airglow above Arecibo (Bernhardt et al. 1988), the first detection of an ionized helium layer in the ionosphere (Noto et al. 1998), studies of geomagnetic storms (Nelson & Cooger 1971, Burnside et al. 1991, Buonsanto et al. 1999, Gong et al. 2013) and the discovery of ionospheric perturbations associated with tropical storm Odette (Bishop et al. 2006). Probing the planets As a planetary radar, Arecibo was unmatched in its lifetime. It proved invaluable in characterizing the planets, their moons and the minor bodies of the solar system, and particularly in the post-discovery observation of near-Earth asteroids (NEAs). It provided not only accurate velocity and range information, but, depending on the signal return strength, could reveal rotation rates and, through inversion techniques, rough target shapes. Another obvious advantage was that radar could penetrate thick atmospheres, such as those of Venus and Titan. Carefully chosen frequencies could even penetrate and reflect from several wavelengths below planetary regoliths and thus provide clues to both geology and chemistry. But perhaps its greatest legacy is as a direct radar imaging device. Arecibo's imaging resolution greatly exceeded that available from any optical telescope on the ground, or in Earth orbit. In the 1960s, the 430 MHz radar transmitter at Arecibo was used to determine the rotation periods of Mercury (Pettengill & Dyce 1965) and Venus (Dyce et al. 1967) and to refine the value of the astronomical unit (Ash et al. 1967), the basis of the cosmic distance ladder. Arecibo data on Mercury also revealed features consistent with volume scattering from a cold-trapped volatile such as water ice (Harmon et al. 1994a, Harmon et al. 2001). More recently, Arecibo data have been among several lines of evidence suggesting water in shaded regions of the Moon's poles (Patterson et al. 2017). Venus and Mars The installation of the S-band (2.38 GHz) transmitter in 1971 in the first major upgrade at Arecibo saw the radar piercing the clouds of Venus long before the Soviet Venera probes imaged the surface from orbit. Arecibo produced the first high-resolution (2 km) imaging of the venusian surface (Campbell et al. 1976, Campbell et al. 1979, Campbell & Burns 1980). Further radar images obtained in 1988 with Arecibo alone (Campbell et al. 1989) and in 2012 with Arecibo transmitting and the Green Bank Telescope receiving, were later combined to produce the spectacular radar image of Venus – with a surprisingly complex surface – in figure 4. Early radar observations of Mars were performed by Simpson et al. (1977, 1978). Later studies by Harmon et al. (1992) and Harmon & Nolan (2017) revealed radar features associated with eroded crater rims, ejecta flows, impact melts and terrain-softened plains. Arecibo was more recently used in an assessment of the InSight landing site (Putzig et al. 2016). 4 Open in new tabDownload slide Earth-based radar image mosaic of Venus from observations in 2012. (Courtesy Smithsonian Institution/NSF/Arecibo Observatory) 4 Open in new tabDownload slide Earth-based radar image mosaic of Venus from observations in 2012. (Courtesy Smithsonian Institution/NSF/Arecibo Observatory) The facility has also provided precise line-of-sight ranging measurements of the Galilean satellites of Jupiter (Harmon et al. 1994b, Brozović et al. 2020) and studied the decametric emission of Jupiter itself (Dulk 1970). Black et al. (2007) presented radar reflectance properties of the mid-size satellites of Saturn, Rhea, Dione, Tethys and Enceladus, while radar imaging suggested remnants of hydrocarbon lakes on the surface of Titan (Campbell et al. 2003, Black et al. 2011, Hofgartner et al. 2020). Observations also helped constrain the size and spatial distribution of particles in Saturn's rings (Goldstein & Morris 1973, Ostro et al. 1982, Nicholson et al. 2005). Arecibo has been an important instrument in the characterization of NEAs and main-belt asteroids (MBAs). The 2.38 GHz radar system has investigated the physical and dynamical properties of up to 120 NEAs annually (up to half of them newly discovered objects, on average). The facility's imaging capabilities directly revealed the shape, surface morphology, spin and binarity of asteroids. Highlights include imaging the unusual dog-bone shape of MBA 216 Kleopatra (Ostro et al. 2000), discovering that asteroids 2020 BX12 and 2016 AZ8 are binary (Virkki et al. 2019) and detecting craters and hills on the surfaces of (52768) 1998 OR2 and (68950) 2002 QF15 (Shepard et al. 2008). Arecibo data also provided detailed characterization of binary asteroid (66391) 1999 KW4 (Ostro et al. 2006) and discovered the first triple NEA, (153591) 2001 SN263 (Nolan et al. 2008). Figure 5 shows spectacular Arecibo+ Goldstone radar images of asteroid 1999 JM8 observed by Benner et al. (2002). 5 Open in new tabDownload slide A sequence of 8650 MHz radar images of asteroid (53319) 1999 JM8, using data from both the Arecibo and Goldstone telescopes. (Courtesy NASA/JPL-Caltech/GSSR/NSF/Arecibo Observatory) 5 Open in new tabDownload slide A sequence of 8650 MHz radar images of asteroid (53319) 1999 JM8, using data from both the Arecibo and Goldstone telescopes. (Courtesy NASA/JPL-Caltech/GSSR/NSF/Arecibo Observatory) Arecibo observations have been useful in characterizing the shape models and rotation parameters for the targets of several asteroid space missions. Radar data were used to derive the shape model of the primitive asteroid 101955 Bennu (Nolan et al. 2013), the target of the OSIRIS-REx mission (described in more detail by Kerri Donaldson Hanna on page 1.14 of this issue). Asteroid 3200 Phaethon, the source of the Geminid meteor stream, was observed by Taylor et al. (2019) and will be visited by JAXA's DESTINY+ probe, due for launch in 2024. Asteroid 65803 Didymos (Naidu et al. 2020) is the target of NASA's DART mission, launching this summer and designed to conduct the first kinetic impactor test for asteroid deflection technology. Radar data from Arecibo also provided the first detection of the Yarkovsky effect, which alters asteroid trajectories (Chesley et al. 2003, Giorgini et al. 2002), and the YORP effect was first demonstrated by Arecibo (Taylor et al. 2007). The dish has proved to be a powerful tool for rapidly improving the accuracy of the orbits of newly discovered objects and thus for characterizing their potential threat to Earth. The best example of this is the Arecibo radar observation of the 300 m diameter asteroid 99942 Apophis (Giorgini et al. 2008, Brozović et al. 2018). This work reduced the statistical uncertainty of its 2029 approach to Earth, shifting the predicted encounter 4.4 Earth radii closer, to only 5.6 Earth radii above the surface. Although cometary targets rarely fall in range to give strong radar returns, useful data on the size and spin period of cometary nuclei have been obtained at Arecibo. Notable examples include comet Halley (Campbell & Harmon 1989, Cordes et al. 1990), comet 8P/Tuttle (Harmon et al. 2010), comet 2P/Encke (Harmon & Nolan 2005), observations of comet 9P/Tempel 1 that were coordinated with NASA's Deep Impact mission (Howell et al. 2007), and the detection of grains in the coma of comet C/2001 A2 (LINEAR) by Nolan et al. (2006). Radio astronomy Although originally designed as an active radar, the legacy of Arecibo in the field of passive radio astronomy is both diverse and unrivalled (Salter 1998, Altschuler & Salter 2013). Arecibo began observing pulsars soon after their discovery in 1967, concentrating initially on discovery surveys (Craft et al. 1968, Hulse & Taylor 1974), pulsar timing observations (Zeissig & Richards 1969, Richards et al. 1970, Rankin et al. 1971) and interstellar scintillation (Rickett & Lang 1973). It was Arecibo's measurement of the Crab pulsar's 33 ms pulse period (Lovelace et al. 1968) that discounted a suggestion that pulsars might be white dwarf stars. Much of the early work at Arecibo helped inform the eventual realization that pulsars are fast-spinning neutron stars. It was during the Arecibo pulsar survey of the early 1970s that the first binary pulsar, PSR 1913+16, was discovered, through systematic variations in the pulse arrival times (Taylor & Hulse 1974). Subsequently, the decay in the pulsar orbit over time revealed a precise agreement with the loss of energy expected from the emission of gravitational waves (Taylor & McCulloch 1980, Taylor & Weisberg 1982). This first indirect observation of gravitational waves resulted in award of the 1993 Nobel Prize in Physics to Russell A Hulse and Joseph H Taylor. Arecibo also discovered the binary pulsar PSR J1906+0746 during the precursor survey observations with the Arecibo L-band Feed Array (Lorimer et al. 2006). Arecibo observations of this object, and others, have also been used to test general relativity (Desvignes et al. 2019, Archibald et al. 2018). The use of pulsars to study gravitational waves led, in 2007, to the establishment of NANOGrav (North American Nanohertz Observatory for Gravitational Waves) with Arecibo as a crucial member. Using the high precision of pulsar clocks, much like a cosmic GPS system, NANOGrav attempts to detect gravitational waves independently of the laser interferometry network that includes LIGO and Virgo. Although no detections are yet forthcoming, Arecibo data were crucial in setting limits to detectability, as well as providing the long series of pulsar timing measurements required by these studies (Arzoumanian et al. 2015, 2018). In 1982, Arecibo's discovery of a 1.5 ms pulsar, PSR 1937+21, led to the realization that, in addition to young pulsars like the Crab, born in recent supernovae, there also exists a population of older, recycled pulsars, which have been spun up by the accretion of material from a companion star (Backer et al. 1982). Arecibo also discovered the interesting millisecond binary pulsar PSR 1957+20, known as the Black Widow Pulsar, in which the radiative blast from the pulsar is ablating and may ultimately destroy its brown dwarf companion (Fruchter et al. 1988). Exoplanet Arecibo pulsar timing observations inadvertently led to the first detection of an exoplanet, three years before the optical discovery of exoplanets around 51 Pegasi (Mayor & Queloz 1995). Wolszczan & Frail (1992) found anomalies in the period of PSR B1257+12 that indicated the presence of two orbiting planets (a third was found later). Only four such pulsar–planet systems have so far been discovered. A long-term 1.4 GHz pulsar survey of the galactic plane using the Arecibo L-band Feed Array, called P-ALFA, was designed to find long-period and millisecond pulsars at large distances and high dispersion measures, where previous Arecibo pulsar surveys had not been sensitive. It discovered 203 pulsars in total (Cordes et al. 2006, Lazarus et al. 2015). Some highlights of that research include the discovery of PSR J1946+2052, a binary neutron star system that is perilously close to merging (Stovall et al. 2018), and the first discovery of a pulsar (J2007+2722) by the distributed-computing project Einstein@Home (Knispel et al. 2010). Fast radio bursts (FRBs), first discovered in 2007 (Lorimer et al. 2007), are transient pulses of radio emission, lasting between a fraction of a millisecond up to a few milliseconds, now thought to be associated with magnetars (see review in this issue by Shami Chatterjee on page 1.29). Originally thought to be one-off events, some of these high-intensity blasts of radio emission repeat. Arecibo was the second instrument to detect an FRB and the first to detect a repeating FRB (FRB 121102; Spitler et al. 2014, 2016; Scholz et al. 2016, 2017), a detection that immediately discounted many of the cataclysmic mechanisms proposed for FRBs. Arecibo later added sensitivity to European VLBI Network observations of FRB 121102 and the data suggested the bursts originated within a low-metallicity star-forming dwarf galaxy (Marcote et al. 2017). Using the ALFA instrumentation, Arecibo had been commensally searching for FRBs since 2015, but made no detections during its first two-year period (Foster et al. 2017). Since its earliest days, Arecibo has observed thermal continuum emission, and line emission, from galactic objects such as UV Ceti stars, open clusters, emission nebulae and H II regions (e.g. Parrish 1972, Spangler & Shawhan 1976). Observations typically provided flux densities, brightness temperatures, spectra and radio contour maps, but occasionally astrometric results too. For example, Arecibo was a crucial component of VLBI parallax observations that settled the distance to the Pleiades cluster (Melis et al. 2014). The arrival of the ALFA receiver in 2004 saw the emergence of H I surveys. The Arecibo Legacy Fast ALFA (ALFALFA) survey (Giovanelli et al. 2005) probed the faint end of the H I mass function in the local universe. It found some 31 500 extragalactic H I line sources out to a redshift of 0.06 (Haynes et al. 2018). The Arecibo Galaxy Environment Survey (AGES) investigated galactic environments to high sensitivity, high velocity and spatial resolution (Auld et al. 2006). Other ALFA studies discovered beautiful H I filamentary structure in our galaxy (figure 6; Peek et al. 2011) and revealed structural intricacies up to 22 kpc from the core of spiral galaxy M33 (Putman et al. 2009). 6 Open in new tabDownload slide Intricate structure in the ISM shown in different velocity bands derived from Arecibo's GALFA-HI survey Data Release 2. (Image by J Peek, STScI/JHU) 6 Open in new tabDownload slide Intricate structure in the ISM shown in different velocity bands derived from Arecibo's GALFA-HI survey Data Release 2. (Image by J Peek, STScI/JHU) Some of the telescope's greatest contributions have been in the field of radio spectroscopy. Early work concentrated on galactic star-forming regions such as W51 (Parrish et al. 1972) and W49A (Pankonin et al. 1973), but also featured extragalactic targets such as M33 (Terzian & Pankonin 1972), Stephan's Quintet (Kaftan-Kassim & Sulentic 1974) and Makarian and Zwicky galaxies (Bieging et al. 1977). Arecibo provided the first detection of OH emission from another galaxy (Baan et al. 1982), revealing that Arp 220, the closest ultra-luminous IR galaxy (ULIRG), was pumping OH masers over a million times stronger than star-forming regions in the Milky Way. A later survey (Darling & Giovanelli 2002) discovered 50 new OH megamasers in ULIRGs, half of all those known. Arecibo also provided the most sensitive survey to date for extragalactic OH methanol masers (Pandian et al. 2007) and discovered cosmic traces of prebiotic chemicals methanimine (CH2NH) and hydrogen cyanide, also in Arp 220 (Salter et al. 2008). SETI Arecibo was involved in SETI research even before the Berkeley SERENDIP programme began in the late 1970s. The telescope was used in searches for narrowband radio emission at frequencies surrounding the 21 cm H I line and/or the 18 cm OH lines, usually from nearby Sun-like stars (Horowitz 1978, Tarter et al. 1983). The third generation SERENDIP system, SERENDIP III (Werthimer et al. 1996), was a 4 million channel spectrum analyser based on fast Fourier transforms, that was fed 430 MHz data from Arecibo between 1992 and 1996. The upgraded SERENDIP IV system (Werthimer et al. 2000), operating at 1420 MHz, ran between 1998 and 2003. The latest incarnation of the programme, SERENDIP VI, used the ALFA receiver (Chennamangalam et al. 2017). No confirmed extraterrestrial signals were found in any of these studies. On 16 November 1974, as part of the inauguration of the instrument upgrade of the 1970s, the Arecibo dish was used to transmit a coded message to the stars (Staff at NAIC 1975). It was designed by Frank Drake with the help of Carl Sagan and consisted of 1679 bits of binary code detailing a counting scheme, representations of the H, C, O, N and P atoms, the generic form of several DNA bases along with the double-helix DNA structure, and representations of a human being, the solar system and the Arecibo observatory (figure 7). This transmission, the most powerful signal ever sent from Earth, was beamed at 2380 MHz towards the M13 globular cluster. No response has yet been received. 7 Open in new tabDownload slide Representation of the message beamed towards M13 in 1974. (Adapted from image by Arne Nordmann, CC BY-SA 3.0) 7 Open in new tabDownload slide Representation of the message beamed towards M13 in 1974. (Adapted from image by Arne Nordmann, CC BY-SA 3.0) The end The Arecibo facility has endured hurricanes, tropical storms and earthquakes since 1963. In fact, the damage caused by Hurricane Maria in September 2017 (Rivera-Valentín & Schmelz 2018) was still being repaired as the instrument's final demise started in a series of failures beginning in August 2020. On 10 August 2020, one of the auxiliary cables that helps to secure the 900-ton instrument platform above the reflector dish slipped out of its anchor point. When the 10 cm thick cable fell, it damaged panels in the Gregorian dome and twisted the platform used to access it. It also caused a 30 m long gash through the reflector's panels. Operations stopped immediately while engineers reviewed the damage and assessed the extent of repairs needed to bring the telescope back online. But then, on 6 November 2020, one of the main cables that supports the instrument platform snapped. It also fell onto the reflector dish, causing additional destruction, as well as damaging other nearby cables. Officials at the observatory couldn't determine why the main cable broke, but suspected it was related to the extra load the remaining cables had been carrying since the August incident. A team had been monitoring all the cables and platform while engineers were studying ways to reduce the tension in the existing cables. The second cable was thought to be structurally sound; its failure led engineers to conclude that the remaining cables were likely to be weaker than their original specification. After the engineering team found evidence of further regular breaks in the compound support cables, they deemed the entire structure unsafe. NSF announced, on 19 November, that it planned to decommission and dismantle the instrument. But less than a fortnight later, on 1 December, disaster struck. The main support cables failed, bringing the entire instrument platform and Gregorian dome crashing onto the dish below, shearing off the top sections of all three support towers in the process. Unfortunately, it was not only the telescope structure that was affected, although nobody was hurt. The falling cables significantly damaged the observatory's 12 m radio telescope and the LIDAR facility used for stratospheric, mesospheric and meteor studies. The Arecibo telescope is famous for more than the science it made possible. It appeared in the climax of the James Bond movie Golden Eye (1995) and in Contact (1997) with Jodie Foster. It also appeared in the science-fiction horror movie Species (1995) and in an episode of the hit TV show The X-Files. It is one of only four telescopes or observatories to have been featured as a Google doodle in the search-engine logo. These public appearances (and its association with the hunt for alien intelligence) have made it one of the most recognizable scientific instruments ever constructed, second only to the Hubble Space Telescope and Large Hadron Collider. Its loss is a severe blow to the scientific outreach efforts of astronomers everywhere, and removes an important source of inspiration the world over. A future for Arecibo? But the demise of the Arecibo facility is a greater blow to the aeronomical and astronomical communities. It was a unique instrument in many ways. Even the FAST instrument cannot fully make up for both its sensitivity and wide range of both passive and active radio investigative capabilities. The loss is a particular blow to NEA and PHO (potentially hazardous object) detection, characterization and mitigation. The only other radar system capable of such work, albeit with much less sensitivity, is NASA's Deep Space Network 70 m antenna at Goldstone. Many scientists in the field will be feeling uneasy now that Arecibo is no longer watching the skies for imminent threat. Despite its near six decade history, Arecibo was far from being redundant. In 2019, a white paper submitted to the Astro2020 decadal survey listed potential innovative and exciting plans for the next decade of Arecibo's life (Anish Roshi et al. 2019). In 2018, the NSF awarded the facility $5.8 million to build an advanced 40-beam cryogenic phased array receiver that would have revolutionized the instrument's output yet again. Following the decision of the NSF to decommission Arecibo last November, but prior to the complete collapse of the structure, there had been calls to save the observatory. Indeed, remedial work would probably have cost less than demolition. But things changed on 1 December 2020. Soon after the collapse, the NSF tweeted: “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.” It is unclear what this means for the scientists, engineers, staff and students of Arecibo, and it may simply be alluding to NSF's promise to return the site to its original condition if it ever closed the facility. There is unlikely to be a way back to Arecibo's heyday, despite the appearance of a petition to rebuild the facility, and despite the NSF insisting the doors are open to a reconstruction proposal. Other facilities However, there are other facilities based at the site to consider, including the 12 m radio telescope, the LIDAR facility and the award-winning visitor centre. Encouragingly, NSF remains adamant that the Arecibo Observatory is not being closed and has already instructed contractors to repair damage to the other facilities on the site. Even if there is no way back, Arecibo's legacy is assured. It was planned and built by audacious people of spirit, vision and innovation, and it was employed by scientists of passion, creativity and renown. It achieved many firsts – few instruments can claim the breadth and depth of contributions it made to the body of scientific knowledge. AUTHOR Open in new tabDownload slide Open in new tabDownload slide Dr Alastair Gunn is associate editor of A&G and a radio astronomer at the University of Manchester's Jodrell Bank Observatory REFERENCES Altschuler D R & Salter C J 2013 Phys. Today 66 43 Crossref Search ADS Anish Roshi D 2019 Astro2020: Arecibo Observatory in the Next Decade arXiv:1907.06052 Archibald A M et al. . 2018 Nature 559 73 Crossref Search ADS PubMed Arzoumanian Z et al. . 2015 Astrophys. J. 810 150 Crossref Search ADS Arzoumanian Z et al. . 2018 Astrophys. J. Supp. 235 37 Crossref Search ADS Ash M E et al. . 1967 Astron. 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TI - End of an era
JF - Astronomy & Geophysics
DO - 10.1093/astrogeo/atab040
DA - 2021-02-01
UR - https://www.deepdyve.com/lp/oxford-university-press/end-of-an-era-wkjNihoOT1
SP - 1.20
EP - 1.25
VL - 62
IS - 1
DP - DeepDyve
ER -