Here are 10 of Arecibo’s coolest achievements

The sun has set on the legendary Arecibo telescope.

Since 1963, this gigantic radio telescope in Puerto Rico has watched everything from space rocks whizzing past Earth to mysterious explosions of radio waves from distant galaxies. But on December 1st, the 900-tonne platform of scientific instruments crashed over the bowl, destroying the telescope and marking the end of Arecibo’s observation days.

Arecibo has made too many discoveries to include in a top 10 list that some of his biggest hits didn’t make the cut – like an odd class of stars seemingly turning on and off (SN: 01/06/17) and ingredients for life in a distant galaxy. In honor of Arecibo’s 57-year tenure as one of the world’s premier observatories, here are 10 of the telescope’s coolest achievements, presented in roughly the reverse order of coolness.

10. Clocking of the Crab Nebula pulsar

Originally, astronomers thought that apparently blinking stars called pulsars, discovered in 1967, could be pulsating white dwarf stars (SN: 04/27/68). 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). This discovery confirmed the idea that pulsars are actually rapidly rotating neutron stars, stellar corpses that sweep beams of radio waves around like celestial lighthouses in space (SN: 1/3/20).

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

9. Pulsars Reborn

In 1982 Arecibo clocked a pulsar called PSR 1937 + 21, which flashed every 1.6 milliseconds and replaced the neutron star of the Crab Nebula as the fastest known pulsar (SN: 04.12.82). This find was initially puzzling, since PSR 1937 + 21 is older than the pulsar of the Crab Nebula and it was assumed that pulsars rotate more slowly with increasing age.

Then the astronomers realized that old pulsars can “turn up” by sucking off the mass of a companion star and flash every 1 to 10 milliseconds. The NANOGrav project is now using rapid-fire radio beacons such as extremely precise cosmic clocks to look for waves in spacetime known as gravitational waves (SN: 2/11/16).

Pulsar illustration — Pulsars usually spin more slowly as they get older. However, data from Arecibo indicated that pulsars can spin to spin hundreds of times per second by sucking material from a neighboring star (as seen in this artist’s impression; pulsar in blue).ESA, Francesco Ferraro / Bologna Astronomical Observatory

8. Ice on Mercury

Mercury seems an unlikely place to find water ice because the planet is so close to the sun. However, Arecibo observations in the early 1990s indicated that ice lurked in permanently shadowed craters on Mercury’s poles (SN: 11/09/1991). NASA’s MESSENGER spacecraft later confirmed these observations (SN: 11/30/12). Finding ice on Mercury raised the question of whether ice could also be present in shady craters on the moon – and recent observations from spacecraft show that it is (SN: 05/09/16).

Mercury's North Pole — Images of mercury captured by NASA’s MESSENGER spacecraft in 2011 and 2012 confirmed that evidence of water ice (yellow) that Arecibo has seen on the planet is in shady regions at Mercury’s poles (North Pole , shown; two craters labeled).NASA, JHUAPL, Carnegie Institution of Washington, Arecibo Observatory

7. Unveiling of Venus

Venus is shrouded in a thick layer of cloud, but Arecibo’s radar beams could cut through this haze and ricochet off the surface of the rocky planet, allowing researchers to map the terrain. In the 1970s, Arecibo’s radar sight received the first large-scale views of the surface of Venus (SN: 11/03/79). His radar images showed clues to past tectonic and volcanic activities on the planet, such as B. ridges and valleys (SN: 04/22/89) and ancient lava flows (SN: 9/18/76).

Radar image of the surface of Venus — Arecibo provided this early radar view of Venus’ surface in 1971.D.B. Campbell / Cornell University

2015 image of Venus — Technological advances have enabled Arecibo to take a closer look at Venus. This 2015 image shows the planet’s northern hemisphere.Smithsonian Institution, NASA GFSC, Arecibo Observatory, NAIC

6. Mercury’s Revolution

In 1965 Arecibo radar measurements showed that mercury rotates around its axis every 59 days, not every 88 days (SN: 05/01/65). This observation solved a long-standing mystery about the temperature of the planet. If Mercury had rotated around its axis once every 88 days, as previously assumed, the same side of the planet would always face the sun. That’s because it takes the planet 88 days to create an orbit around the sun.

As a result, this side would be much hotter than the dark side of the planet. The 59-day rotation was in better agreement with the observation that the temperature of mercury is fairly uniform on its surface.

5. Mapping of asteroids

Arecibo has cataloged the features of many near-Earth asteroids (SN: 05/07/10). In 1989 the observatory made a radar image of asteroid 4769 Castalia and revealed the first bilobed rock known in the solar system (SN: 11/25/89). Arecibo has since found space rocks that orbit each other in pairs (SN: 10/29/03) and trios (SN: 07/17/08).

Other strange finds were a space rock whose shadows made it look like a skull to Arecibo, and an asteroid with the improbable shape of a dog bone (SN: 07/24/01). Understanding the properties and movements of near-Earth asteroids can help you determine which asteroids pose a threat to Earth and how to safely deflect them.

216 Cleopatra asteroid images — Arecibo radar images in 2000 showed the strange dog-bone shape of an asteroid called 216 Cleopatra (shown from multiple angles).WSU, NAIC, JPL / NASA

4. Telephoning with E.T.

The Arecibo Observatory broadcast the first radio message to an extraterrestrial audience in November 1974 (SN: 11/23/74). This famous message was the strongest signal ever sent from Earth, in part to demonstrate the capabilities of the observatory’s new high-powered radio transmitter.

The message, which was aimed at a group of about 300,000 stars about 25,000 light years away, consisted of 1,679 pieces of information. This binary code string contained the chemical formulas for DNA components, a stick figure sketch of a human, a schematic representation of the solar system, and other scientific data.

3. Repeat radio bursts

Fast radio bursts, or FRBs, are short, brilliant explosions of radio waves of unknown origin. The first FRB known to deliver multiple bursts was the FRB 121102, which Arecibo first discovered in 2012 and 2015 (SN: 02.03.16). Finding a repeating FRB eliminated the possibility that these bursts were generated by one-time catastrophic events such as star collisions. And because FRB 121102 kept appearing, astronomers were able to trace it back to its home: a dwarf galaxy about 2.5 billion light years away (SN: 01/04/17). This confirmed decades of suspicion that FRBs came from beyond the Milky Way.

Source of the rapid radio burst — A repeating source of radio waves discovered by Arecibo (radio image, left) was the first rapid burst of radio waves traced back to its home galaxy. The eruption originated in a dwarf galaxy about 2.5 billion light years away (image with visible light, right).H. Falcke /*Nature 2017*

2. Make waves

Gravitational waves were recorded directly for the first time in 2015 (SN: 2/11/16), but astronomers saw the first indirect evidence of waves in spacetime decades ago. This evidence comes from the first pulsar to orbit another star, PSR 1913 + 16, first sighted by Arecibo in 1974 (SN: 10/19/74).

By tracking the arrival time of radio bursts from this pulsar over several years, astronomers were able to map its orbit and determine that PSR 1913 + 16 was heading for its companion. As the orbits of the two stars contract, the binary system loses energy at the rate that would be expected if they were to stir up gravitational waves (SN: 2/24/79). This indirect observation of gravitational waves was awarded the Nobel Prize in Physics in 1993 (SN: 10/23/93).

Illustration of the pulsar orbiting star — The first pulsar, which orbited another star in 1974 and was sighted by Arecibo, provided indirect evidence for the existence of waves in space-time known as gravitational waves (Figure).ESO, L. Calçada

1. Pulsar planets

The first planets to be discovered around another star were three small rocky worlds orbiting the pulsar PSR B1257 + 12 (SN: 01/11/92). The find was a bit accidental. In 1990 Arecibo was repaired, and so it got stuck and stared at a spot in the sky. During his observations, the Earth’s rotation moved PSR B1257 + 12 across the telescope’s field of view. Small fluctuations in the arrival time of radio bursts from the pulsar indicated that the star wobbled as a result of the gravitational drag of invisible planets (SN: 03/05/94).

Since then, thousands of exoplanets have been discovered orbiting other stars, including stars similar to the Sun (SN: 10/08/19). However, recent exoplanet research suggests that pulsar orbiting planets are rare (SN: 03.09.15).