1963 Arecibo Ionospheric Observatory Commissioned for service, 1 November, 1963. [$9.7M]
1965 One of its first accomplishments: Establishing the rotating rate of Mercury, which turned out to be 59 days rather than the previously estimated 88 days.
1968 Sporadic radio pulses from the direction of the Crab Nebula supernova remnant found at Green Bank were shown by Arecibo to come from a 33-ms period pulsar situated at the center of the nebula.
1969 The National Science Foundation assumes operations from DoD, 1 October, 1969. The National Astronomy and Ionosphere Center (NAIC) begins.
1974 New high precision surface reflector installed, planetary radar transmitter installed. [$8M - $5M from NSF, $3M from NASA]
1974 The first pulsar in a binary system was discovered, leading to important confirmation of Einstein's theory of general relativity and a Nobel Prize 1993 for astronomers Russell Hulse and Joseph Taylor.
1979 22 September, 1979, a large, anomalous traveling ionospheric disturbance (that is, an upper atmosphere wave) moving southeast to northwest was detected in the early morning hours - something researchers had never before witnessed. These data helped define the probable cause as an air nuclear blast over the Indian Ocean. Richard Behnke reported his information to the American public on the final CBS news broadcast of Walter Cronkite.
1980 First radar ranging to an Earth-crossing asteroid (1862 Apollo)
1981 First radar maps of the geologic surface of Venus are produced.
1982 The discovery of strong "megamaser" emission from the hydroxyl (OH) molecule in the starburst galaxy Arp 220 (IC 4553).
1982 The discovery of millisecond pulsars, which rotate several hundred times per second. This demonstrated the existence of two classes of pulsars: the millisecond pulsars and the slower-rotating pulsars, which rotate about once per second.
1984 2102 Adonis -- extremely unusual echo polarization establishes extreme diversity of NEA surface properties
Late 80's Detailed maps of the distribution of galaxies in the universe produces 3D description of the Universe.
Late 80's Provided much of our pre-Magellan mission knowledge of the surface of Venus via 1.5 km resolution imagery of the surface through the planet's cloud cover using the radar system.
1989 The first measurement of hydrogen escape flux from earth is presented, based on velocity distribution measurements of the hydrogen airglow emission in the upper atmosphere.
Early 90's The first planets outside the solar system were discovered around Pulsar B1257+12, a rapidly rotating pulsar with three Earth-like planets in orbit. (early 1990s)
Oct. 1992 Ice is discovered in shadowed craters at Mercury's north pole. Later observations show ice in south pole craters as well. (Harmon et al., 1992 Science vol. 258, no. 5082, p. 640-643.) More Information...
1996 Gregorian dome with sub reflectors enhances telescope capability, new planetary radar transmitter installed ground screen installed. [$25M - $14M from NSF, $11M from NASA]
1990-1997 Investigations of ice craters at the polar regions of the planet Mercury with the radar system (1990s) and similar investigation of the lunar poles for evidence of ice (1997).
1998 A layer of helium ions is shown to be a common, but previously unrecognized feature in the low-latitude ionosphere near 600 km.
May 2000 Radar Observations of Asteroid 216 Kleopatra reveal a "dog bone" shaped metal-rich object, 220km x 90km x 80km. (Ostro et al., 2000 Science Volume 288, Issue 5467, pp. 836-839).
Sep. 2000 Discovery that 2000 DP107 was the first asteroid identified by radar as a binary system. The primary is roughly spherical with a diameter of 800 meters (a half mile) and the smaller secondary, which orbits it in 1.8 days, is about 300 meters (1000 feet). More Information...
2001 Arecibo Observatory is simultaneously recognized as an IEEE "Electrical Engineering Milestone" and a ISME "Mechanical Engineering Landmark".
Jan. 2001 Ice is discovered in shadowed craters at Mercury's north pole. Later observations show ice in south pole craters as well. More Information...
1998-2003 Solar and Heliospheric Observatory (SOHO) spacecraft, suffered communication loss with European Space Agency and NASA ground station on 24 June, 1998. Arecibo Observatory "found" the satellite 28 July, 1998, by bouncing a radar signal off the satellite, the mission was resumed thereafter, producing solar data through 2003.
2003 Evidence for hydrocarbon lakes on the Saturn satellite "Titan" is established using the Observatory planetary radar.
May 2004 Images and movies of the binary asteroid 2003 YT1 can be seen by clicking here.
- Arecibo Observatory is uniquely suited to search for signals from extraterrestrial life, by focusing on thousands of star systems in the 1,000MHz to 3,000 MHz range. No such signals have been found.
2005 Radar refinement of the 2029 miss distance of Apophis.
2006 Search for water ice in the permanent shadow of the lunar Shackleton Crater disputes evidence for water ice on the lunar surface.
2006 First ground-based images of a cometary nucleus are recovered.
Oct. 2006 Radar images of the south pole of the moon reveal no evidence for thick deposits of ice (Campbell et al. 2006 Nature Volume 443,Issue 7113, pp. 835-837)
Nov. 2006 Radar images of binary asteroid (66391) 1999 KW4 in May 2001 and again in June 2002 reveal exotic physical and dynamical properties which may be common among near-Earth binaries. (Ostro et al., 2006, Science Volume 314, Issue 5803, pp. 1276-1280, and Scheeres et al. Science Volume 314, Issue 5803, pp. 1280-1281)
Jan. 2007 Asteriod 2006 AM4 rotates in less than 5 minutes and is 150m in diameter, so there can be no loose material on the surface. A movie of the asteroid rotating can be found here...
Mar. 2007 Radar images of Mercury reveal features to be studied further by the Messenger spacecraft over the next several years. (Harmon et al, 2007 Icarus v187 pp374-405.)
2007 Core or planet Mercury is determined to be liquid using a coordinated experiment between Arecibo Observatory, the Goldstone Radar in California, and Green Bank telescope in West Virginia.
2007 The largest single-source collection of galaxy rotation velocity data coupled with galaxy intrinsic luminosity data in existence, including photometric data and spectroscopic data from Arecibo Observatory is made public. The data are used to accurately map distances to galaxies.
2007 54509 YORP (2000 PH5): Spin rate increasing due to YORP effect
2007 Previously undetected radio lines of the molecule hydrogen cyanide (HCN), and the presence of the molecule methanimine (CH2NH), were recently discovered in the distant galaxy Arp 220. Arp 220, a "starburst galaxy" in which massive stars are being born at a rate one hundred times that within our own Milky Way, lies 250 million light years away! Both methanimine and hydrogen cyanide are considered to be "pre-biotic", which is to say that they are among the fundamental building blocks for the production of more complex molecules with relevance to the origins of life in the Universe. Specifically, they are likely to participate in the interstellar formation of the simplest amino acid, glycine. The occurrence of these pre-biotic molecules in such a distant galaxy is surely telling us much of importance concerning how the ingredients for life may have arisen in the first place, and of how widespread they are likely to be in the cosmos.
May 2007 Radar observations of Mercury point to a molten core. The measurements show that the planet exhibits librations in longitude as predicted by theory. (Margot et al., 2007 Science Volume 316, Issue 5825, pp. 710).
Dec. 2007 Images of the nucleus of 8P/Tuttle were obtained at Arecibo. The nucleus has an unusual double-lobed appearance, which has not been seen before in a comet nucleus. Read more...
Jan. 2008 Asteroid 2007 TU24 passed only 1.4 times the distance to the moon. This rounded object is about 200m in diameter, and currently poses no threat of impact to the Earth. We transmitted from Arecibo and received the signal at the GBT to generate images of this object click here.
Feb. 2008 Discovery of the first triple asteroid system among the near-Earth asteroids. The asteroid, 2001 SN263, is about 2km (1.5 mi) in diameter, with two moons, 1km and 600m in diameter orbiting it. More information about this object click here.
2008-2012 Observations with Arecibo discover a radio outburst in the nearby galaxy NGC 660, ten times brighter than a radio supernova.

Observations of the effects of polar sudden stratospheric warmings on the tropical ionosphere provide further confirmation of their global effects.

Chau, J. L., N. A. Aponte, E. Cabassa, E., M. P. Sulzer, L. P. Goncharenko, and S. A. González, 2010.Quiet time ionospheric variability over Arecibo during sudden stratospheric warming events, J. Geophys. Res., 115, A00G06, DOI: 10.1029/2010JA015378. Published: SEP 11 2010.

Oct. 2010 Radar images of the nucleus of the comet 103P/Hartley 2 allowed the EPOXI spacecraft to image the nucleus during its flyby on November 5, 2010. The two-lobed appearance in the radar images was confirmed in the spacecraft images, and the complex rotation state was revealed.
2010-2012 Bistatic imaging of the moon using the orbiting Lunar Reconnaissance Orbiter (LRO) with the Arecibo Observatory acting as the transmitter and the spacecraft as receiver. This allows the measurement of radar reflectance at a range of phase angles, and can distinguish between rough rocky surfaces and the presence of ice.
2011 Observations of brown dwarfs find the coldest star to show radio emission.
2011 A shape model of the two-lobed near-Earth asteroid 1996 HW1 is derived from radar images taken in September, 2008. Light curves help to constrain this detailed model of the asteroid, 3.8x1.8 km in size. This object does not pose an impact hazard to the Earth in the foreseeable future.
Nov. 2011 Radar imaging of near-Earth asteroid 2005 YU55, which made a very close flyby. This dark, spheroidal asteroid was found to be about 350m in diameter and have a rough surface covered with 10-m scale boulders. This object does not pose an Earth impact hazard in the foreseeable future.
2012 Arecibo joins with telescopes in Europe and with the Russian Radio Astron satellite to form the largest telescope ever – 20 times the size of the Earth.
2012 Arecibo continues to obtain orbit refinement of potentially hazardous near-Earth asteroids (PHAs) to identify possible future Earth impactors. Rapid response to targets of opportunity and an extremely sensitive detection system allowed measurements of 65 near-Earth asteroids in 2012, of which 33 were PHAs.
2012 An ion-neutral chemistry model is developed to successfully describe thin layers of neutral metal atoms at above 100 km (62 mile) altitudes.