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Here are 10 of Arecibo’s most interesting achievements

The sun set on the iconic Arecibo telescope.

Since 1963, this colossal radio telescope in Puerto Rico has observed from space rocks passing through Earth to mysterious explosions of radio waves from distant galaxies. But on December 1, the 900-metric-ton platform of scientific instruments on top of the plate fell down, demolishing the telescope and spelling out the end of Arecibo's days of observation.

Arecibo made too many discoveries to include in a list of the top 10, so some of his big hits didn’t make the cut, like a strange class of stars that seem to turn on and off (SN: 1/6/17), and ingredients for life in a distant galaxy. But in honor of Arecibo’s 57 years of permanence as one of the world’s leading observatories, here are 10 of the telescope’s most interesting achievements, presented in reverse order of coolness.

10. Clock the crab nebula pulsar

Astronomers originally thought that seemingly flickering stars called pulsars, discovered in 1967, could be pulsating white dwarf stars (SN: 27/04/68). But in 1968, Arecibo saw the pulsar in the center of the crab nebula blink every 33 milliseconds, faster than white dwarfs can pulsate. (SN: 07/12/68). That discovery strengthened the idea that pulsars are actually rapidly rotating neutron stars, stellar corpses that sweep beams of radio waves around in space like celestial headlights (SN: 1/3/20).

Arecibo's observations on the frequency of radio flashes from the pulsar in the center of the Crab Nebula (red star in the middle) supported the idea that pulsars are rapidly rotating neutron stars.Optics: NASA, HST, ASU, J. Hester et al .; X-rays: NASA, CXC, ASU, J. Hester et al.

9. Reborn bracelets

In 1982, Arecibo made a pulsar, called PSR 1937 + 21, which flashed every 1.6 milliseconds, leaving the neutron star in the Crab Nebula as the fastest known pulsar (SN: 12/4/82). That discovery was puzzling at first because PSR 1937 + 21 is older than the crab nebula pulsar and the pulsars were thought to rotate more slowly with age.

Then astronomers realized that old pulsars can “spin” by sifting the mass of a companion star and flashing each one for up to 10 milliseconds. The NANOGrav project now uses fast-fire radio beacons as extremely accurate cosmic clocks to search for waves in space-time known as gravitational waves (SN: 2/11/16).

Push button illustrationPulsars usually rotate more slowly as they age. But Arecibo’s data showed that pulsars can “rotate” to rotate hundreds of times per second by sifting through the material of a neighboring star (as seen in this artist’s print; the pulsar in blue).ESA, Francesco Ferraro / Bologna Astronomical Observatory

8. Ice on mercury

Mercury looks like it would be an unlikely place to find water ice because the planet is so close to the sun. But Arecibo's observations in the early 1990s suggested that ice was lurking in craters with permanent shadow at Mercury's poles (SN: 11/9/91). NASA's MESSENGER spacecraft later confirmed those observations (SN: 11/30/12). The discovery of ice on Mercury has raised doubts about whether ice could exist in the craters of its moon, and the latest observations from spacecraft indicate that it does (SN: 5/9/16).

By the north of MercuryImages of Mercury taken by NASA's MESSENGER spacecraft in 2011 and 2012 confirmed that signs of water ice (yellow) seen on the planet by Arecibo reside in shady regions at Mercury's poles (north, shown; two craters labeled).NASA, JHUAPL, Carnegie Institution of Washington, Arecibo Observatory

7. Unveiling Venus

Venus is shrouded in a thick layer of clouds, but Arecibo's radar beams could cut through that haze and bounce off the surface of the rocky planet, allowing researchers to map the terrain. In the 1970s, Arecibo's radar vision obtained the first large-scale views of the surface of Venus (SN: 03/11/79). Their radar images revealed evidence of past tectonic and volcanic activity on the planet, such as ridges and valleys (SN: 22/04/89) and ancient lava flows (SN: 18/9/76).

radar image of the surface of VenusArecibo provided this initial view of the surface of Venus using radar in 1971.D.B. Campbell / Cornell University

Image of Venus 2015Technological advances have allowed Arecibo to get sharper views of Venus. This 2015 image shows the northern hemisphere of the planet.Smithsonian Institution, NASA GFSC, Arecibo Observatory, NAIC

6. The Mercury Revolution

In 1965, Arecibo's radar measurements revealed that Mercury rotates on its axis once every 59 days, instead of every 88 days (SN: 5/1/65). That observation clarified a long-standing mystery about the temperature of the planet. If Mercury rotated on its axis once every 88 days, as previously thought, then the same side of the planet would always be facing the sun. This is because it also takes the planet 88 days to complete an orbit around the sun.

As a result, that side would be much hotter than the dark side of the planet. The 59-day rotation coincided best with the observation that Mercury's temperature is fairly uniform over its entire surface.

MercuryArecibo's first radar observations measured Mercury's 59-day rotational speed (shown in this false-color image of data from the MESSENGER spacecraft, which highlights the chemical and mineralogical characteristics on the planet's surface).NASA, JHUAPL, Carnegie Institution of Washington

5. Asteroid mapping

Arecibo cataloged the characteristics of many asteroids close to Earth (SN: 5/7/10). In 1989, the observatory created a radar image of the asteroid 4769 Castalia, revealing the first known double-wolf rock in the solar system (SN: 25/11/89). Arecibo found space rocks orbiting each other in pairs (SN: 29/10/03) and trios (SN: 17/07/08).

Other strange finds included a space rock whose shadows made Arecibo look like a skull and an asteroid in the unlikely shape of a dog bone (SN: 24/07/01). Understanding the characteristics and motion of asteroids close to Earth helps determine which ones may pose a danger to Earth and how they can be safely deflected.

216 images of asteroids from CleopatraArecibo's radar images in 2000 revealed the strange shape of the dog's bone from an asteroid called 216 Kleopatra (shown from multiple angles).WSU, NAIC, JPL / NASA

4. Telephones E.T.

The Arecibo Observatory broadcast the first radio message aimed at a foreign audience in November 1974 (SN: 23/11/74). That famous message was the most powerful signal ever sent from Earth, intended in part to demonstrate the capabilities of the observatory’s new high-power radio transmitter.

The message, aimed at a cluster of about 300,000 stars about 25,000 light-years away, consisted of 1,679 bits of information. That binary code string detailed the chemical formulas of the DNA components, a sketch of a man’s figure, a schematic of the solar system, and other scientific data.

3. Repetition of radio explosions

Fast radio bursts or FRBs are short, bright bursts of radio waves of unknown origin. The first FRB known to emit multiple bursts was FRB 121102, which Arecibo first detected in 2012 and again in 2015 (SN: 3/2/16). Finding a repeated FRB ruled out the possibility that these explosions were generated by occasional cataclysmic events, such as stellar collisions. And because FRB 121102 kept repeating itself, astronomers were able to track it down to their home: a dwarf galaxy about 2.5 billion light-years away (SN: 1/4/17). This confirmed a decade-long suspicion that FRBs come from beyond the Milky Way.

rapid burst radio sourceA repeated source of radio waves discovered by Arecibo (radio image, left) was the first rapid radio explosion to go back to his home galaxy. The explosion originated in a dwarf galaxy about 2.5 billion light-years away (visible light image, right).H. Falcke / Nature 2017

2. Make waves

Gravitational waves were first detected directly in 2015 (SN: 2/11/16), but astronomers saw the first indirect evidence of ripples in space-time decades ago. That evidence came from the first pulsar found orbiting another star, the PSR 1913 + 16, first seen by Arecibo in 1974 (SN: 19/10/74).

By tracking the arrival time of radio explosions from that pulsar for several years, astronomers were able to trace their orbit and discovered that the 1913 + 16 PSR was spiraling toward its companion. As the orbits of the two stars contract, the binary system loses energy at the rate that would be expected if gravitational waves were being whipped (SN: 24/02/79). This indirect observation of gravitational waves won the 1993 Nobel Prize in Physics (SN: 23/10/93).

illustration of the star orbiting the pulsarThe first pulsar found orbiting another star, sighted by Arecibo in 1974, provided indirect evidence of the existence of ripples in space-time called gravitational waves (illustrated).ESO, L. Road

1. Pulsar planets

The first planets discovered around another star were three small rocky worlds orbiting the pulsar PSR B1257 + 12 (SN: 1/11/92). The finding was somewhat serendipitous. In 1990, Arecibo was being repaired, so he was trapped looking at a point in the sky. During his observations, the Earth's rotation swept PSR B1257 + 12 through the telescope's field of view. Small fluctuations in the time of arrival of radio bursts from the pulsar indicated that the star wavered as a result of the gravitational pull of unseen planets (SN: 05/05/94).

Thousands of exoplanets have been discovered orbiting other stars, including sun-like stars (SN: 10/8/19). However, recent surveys of exoplanets suggest that planets orbiting the pulsar are rare (SN: 9/3/15).

rocky planets orbiting the pulsar PSR B1257 + 12The first worlds to be seen beyond the solar system were three rocky planets (seen in this artist’s illustration) orbiting the PSR B1257 + 12 pulsar.NASA, JPL-Caltech, R. Hurt / SSC

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