Legacy Discoveries

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The timeline below shows a brief description of the most important discoveries of the Arecibo Observatory.




  • 2019

    Arecibo Observations of a Binary Pulsar, J1906+0746, Test the Theory of General Relativity.

    -The observations show the relativistic spin precession of strongly self-gravitating bodies, likely caused by the misalignment of the pulsar’s spin vector with respect to the binary system’s total angular momentum due to an asymmetric supernova explosion (Desvignes et al. 2019, Science, Vol. 365, Issue 6457, pp. 1013-1017)

  • 2019

    Observation record under the NASA planetary defense program

    -As a key collaborator of the NASA Planetary Defense Office, Arecibo radar system had an observation record of 126 NEAs (77% rate of success), of which 78 were recently discovered objects. It also observed 1 comet, Mercury, and the Moon.

  • 2018

    Arecibo discovered the first ever repeating Fast Radio Burst.

    -FRBs are millisec- duration radio pulses that appear to be extragalactic. The repeater demonstrates that its source survives the bursts and rules out a class of models requiring catastrophic explosions. (Spitler, L. G et al. 2016, Nature, Volume 531, Issue 7593) Fundamental Physics -- constraints on the variation of the fine structure constant with the age of the Universe.

  • 2018

    Ionospheric O+ Momentum Balance Through Charge Exchange With Thermospheric O Atoms.

    -This study was possible thanks to the study of an unprecedented 27‐year baseline of ISR and Fabry-Perot interferometer data acquired at Arecibo. The study finds evidence for local time, seasonal, and solar cycle variation in derived F estimates, with the best agreement between data, theory, and model predictions observed near midnight at solar maximum. Joshi, P. P., et al. 2018, Journal of Geophysical Research: Space Physics, Volume 123, 9743– 9761)

  • 2018

    Constraining the variation of fine structure constant with the age of the Universe.

    -Arecibo weighed in on the controversy surrounding the possible variability of the fine structure constant, alpha, which describes the electromagnetic interaction between elementary charged particles. 150 hours of observing time at Arecibo revealed that alpha has not changed by more than 1.0 parts in a million in these three billion years (Kanekar et al. 2018 Physical Review Letters, Volume 120, Issue 6, id.06130).

  • 2017

    Arecibo discovered two extremely strange pulsars that undergo a “cosmic vanishing act;” sometimes they are there, and then for very long periods of time, they are not.

    -This has upended the widely held view that all pulsars are the orderly ticking clocks of the universe.

  • 2016

    Arecibo weighed in on the controversy surrounding the possible variability of the fine structure constant, a, which describes the electromagnetic interaction between elementary charged particles.

    -150 hours of observing time at Arecibo revealed that a has not changed by more than 1.3 parts in a million in these three billion years.

  • 2016

    Arecibo-RadioAstron VLBI observations of quasar 3C273 revealed a brightness temperature greater than 1013 K.

    -This temperature is so high that it forces astronomers to question the traditional model - synchrotron radiation - for the emission emanating from the vicinity of the supermassive black hole.

  • 2016

    Arecibo discovered the first ever repeating Fast Radio Burst.

    - FRBs are millisec-duration, radio pulses that appear to be extragalactic. - The repeater demonstrates that its source survives the bursts and rules out a class of models requiring catastrophic explosions.

  • 2015

    NANOGrav (North American Nanohertz Obs for Gravitational Waves) uses an array of high-precision millisec pulsars to search for gravitational waves from supermassive black hole binaries.

    - Upper limits are already constraining models of space-time strain.

  • 2015

    Thermospheric atomic metal layers over Arecibo are not necessarily associated with the Ionospheric tidal ion layers (TILs).

    - This work represents the first simultaneous and (very nearly) common‐volume observations of Ne and two alkali metals (Na and K) in the thermospheric region over Arecibo (120 to 150 km high). The results imply that neutral layers cannot result only from direct neutralization of metal ions in the TILs. (Raizada, S., et al. 2015, Geophysics Research Letter, Volume 42, 10,106– 10,112)

  • 2015

    Fast electrons detection in the night sky are recurrent.

    - TDaylight plasma lines in the ionosphere, excited by photoelectrons, have long been known. However, the origin of the nighttime plasma lines, observed in the Arecibo sky from 1982, is not yet known. A long term study published in 2015 found that these lines are recurrent and variable. Although the source of the fast electrons that engender the lines remains unclear, one strong possibility is leakage from Earth's radiation belts. (Carlson, H. C. et al. 2015, Geophysics Research Letter, Volume 42, 7255–7263)

  • 2015

    First ground-based Detection of Arcing on GPS and Van Allen Probe Satellites from the Arecibo Observatory.

    - These arc rates may be used as an instrument to measure the environment (like local tropospheric weather stations) and also to monitor the degradation of the solar array power on board of the satellites expected at end-of-life (EOL) (Ferguson and Hoffman, 2019)

  • 2014

    Arecibo was essential for VLBI result that resolved the Pleiades distance controversy with measurements of ultra-faint radio stars.

    - This distance was just confirmed by the Gaia Collaboration.

  • 2014

    The GALFA (Galactic Arecibo L-Band Feed Array) survey revealed slender HI structures in interstellar space that are well aligned with dust polarization & the magnetic field.

    - We can now measure the foreground dust signal more precisely, improving our ability to uncover the B-mode signature of inflation.

  • 2014

    Distance to Plediades

    - Arecibo was essential for VLBI result that resolved the Pleiades distance controversy with measurements of ultra-faint radio stars. This distance was just confirmed by the Gaia Collaboration. (Melis et al. 2014 Science, Volume 345, Issue 6200, pp. 1029-1032)

  • 2013

    First detection of a thermospheric descending layer of neutral potassium over Arecibo.

    - On March 12, 2013 an observation was made with the K Doppler lidar at the Arecibo and an intriguing descending layer was detected in the end of that night with a speed phase matching that of the semidiurnal tide. The discovery of this thermospheric K layer, coupled with the McMurdo discovery of similar Fe layers, might lead to a new approach to study the thermosphere in the altitude range of 100 – 150km with resonance fluorescence lidars. (J. Friedman et al. 2013, Journal of Atmospheric and Solar-Terrestrial Physics, Volume 104, 2013, P. 253-259.)

  • 2013

    Arecibo begins timing a pulsar in a triple system.

    - This will provide the best test for the strong equivalence principle, an order on magnitude better than the Sun-Earth-Moon system.

  • 2012

    Searching for ice on the Moon poles.

    - First radar observations of the Moon at large bistatic angles using the Arecibo radar transmitter and the Miniature Radio Frequency (Mini-RF) instrument on NASA’s Lunar Reconnaissance Orbiter (LRO) suggest the presence of near-surface deposits of water ice deposits in permanently shadowed regions near the poles of the Moon (Patterson et al. 2017)

  • 2011 - 2012

    Long-term changes in the thermospheric altitudes over Arecibo

    - Three independent studies show that the thermospheric neutral winds (TNW) show strong variations in time and seasonal over a window of 30 years. These variations result in a change in the altitude of the ionospheric peak density of about 600m/year. The origin of these variations, anthropogenic or not, is not yet known. (Tepley et al. 2011 J. Geophysics. Res., 116,A00H06; Santos et al., 2011, J. Geophys. Res., 116, A00H13 0; Brum et al., 2012, J. Geophysics. Res., 117, A00H14)

  • 2008

    Arecibo telescope finds critical ingredients for the soup of life in a galaxy far, far away.

    - Arecibo have detected the molecules methanimine and hydrogen cyanide -- two ingredients that build life-forming amino acids -- in a galaxy some 250 light years away (Salter et al. 2008 The Astronomical Journal, Volume 136, Issue 1, 389-399)

  • 2008

    Arecibo radar discovers the first triple near-Earth asteroid system: 2001 SN263 orbited by two moons.

    (Nolan, M. C. et al., Central Bureau Electronic Telegrams, #1254, 2008)

  • 2007

    Arecibo radar images and optical lightcurves reveal the YORP effect, a thermal torque which changes how asteroids rotate, on near-Earth asteroid (54509) 2000 PH5.

    (Taylor, P. A. et al. Science, Volume 316, 5822, pp. 274, 2007)

  • 1996

    AO first simultaneous measurement of both proton O+ and H+ temperatures from ISR spectra obtained in the lower topside ionosphere.

    - This is the first time that two ion temperatures and three ion composition (O+, H+ and He+) have been measured from the same IS spectrum, and, as far as we know, one of the first measurements of both ion temperatures in the ionosphere (Sulzer, M. P. & Gonzalez, S. A., Geophysical Research Letters, 1996, 23, 3235-3238)

  • 1992

    Arecibo discovered the first ever exoplanet.

    In subsequent observations, an entire planetary system was found around the pulsar PSR 1257+12.

  • 1992

    Arecibo radar observations suggest ice deposits at the north and south poles of Mercury, later confirmed by NASA’s MESSENGER spacecraft.

    - The ice persists in shadowed craters despite the high temperatures, 800 F, at the surface. (Harmon, J. K., and M.A. Slade, Science Volume 258, 640-643, 1992)

  • 1988

    Arecibo radar provides the first high-resolution (1.5-2 km) geologic maps of the surface of Venus, focusing especially on the volcanic features of the planet.

    (Campbell, D. B. et al. Science, Volume 246, 373, 1989)

  • 1982

    Arecibo discovered the first OH Megamaser in Arp 220, the nearest ultraluminous IR galaxy.

    - Arp 220 is a merger undergoing a burst of star formation; the population inversion is produced by IR radiation from dust.

  • 1982

    Arecibo discovered the first ever exoplanet orbiting around a pulsar.

    - In subsequent observations, an entire planetary system was found around the pulsar PSR 1257+12. (Wolszczan, A. and Frail, D. A. 1992, Nature, Volume 355, p. 145-147)

  • 1981

    Arecibo produced the first radar maps of the surface of Venus.

    - Optical images show only the top of the thick cloud layer.

  • 1975-79

    Arecibo radar reveals the anomalous backscattering properties of the icy Galilean satellites of Jupiter.

    - Up to this day, no other radar target has shown as high backscattering enhancement as the Galilean Moons. (Campbell, D. B. et al. Science, Volume 196, 650-653, 1977)

  • 1976

    Arecibo conducts the first S-band radar observations after a major upgrade that allowed the Arecibo planetary radar system to observe planetary bodies not only at P band, but also at S band.

    - The target was the near-Earth asteroid 1580 Betulia. (Pettengill, G. H. et al. Icarus, Volume 40, pp. 350-354, 1979)

  • 1974

    September 2007 - May 2010

    Arecibo discovered the first ever binary pulsar. Changes in periastron confirmed the predictions of General Relativity.

    - The 1993 Nobel Prize in Physics was awarded to Hulse and Taylor for this discovery. (Hulse R A 1994 Review of Modern Physics, Volumen, 66, 699.)

  • 1974

    Arecibo transmitted the first intentional message to extraterrestrials.

    - The Arecibo message is an interstellar radio message carrying basic information about humanity and Earth that was sent to globular star cluster M13 in 1974. It was meant as a demonstration of human technological achievement. The content of the message was designed by Frank Drake in collaboration with Carl Sagan and others.

  • 1970

    Arecibo makes the first radar interferometric maps of the Moon (7.5-m wavelength) and Venus (70-cm wavelength).

    - (Thompson, T. W., Icarus, 13, 3, p. 363-370, 1970; Campbell, D. B., et al. Science, Volume, 170, Issue 3962, pp. 1090-1092, 1970)

  • 1968

    Arecibo measured the 33-ms period of the Crab pulsar.

    - Only sporadic radio pulses from the Crab nebula supernova remnant were know before Arecibo.

  • 1967

    Arecibo radar pierces through the clouds of Venus and finds it rotates retrograde about once every 245 days.

    - Radar ranging of Venus and Mercury precisely determines the value of the astronomical unit, setting the scale of the Universe. (Dyce, B. R. et al. Astronomical Journal, Volume 72, p. 351, 1967; Muhleman, D. O., et al., Astronomical Journal, Volume 67, 191, 1962)

  • 1965

    AArecibo radar shows that Mercury rotates once every 59 days rather than once per orbit (88 days) as previously thought.

    - The rotation is not tidally locked, but rather, the rate is an orbital resonance with 2 orbits for every 3 rotations. (Pettengill, G. H. and Dyce, R. B. Nature, Volume 206, 4990, pp. 1240, 1965).