Edgard Rivera-Valentín attended the Arecibo Observatory as a young child.
“I definitely remember that feeling of just being awesome,” says Rivera-Valentín. “When you look at this gigantic telescope … hearing about all of this neat work that has just been done … it definitely makes an impression.” Important science was happening right in the back yard of Rivera-Valentin’s hometown of Arecibo, Puerto Rico – and one day Rivera-Valentín wanted to be a part of it.
As an adult, Rivera-Valentín returned to the observatory to work as a planetary scientist. With Arecibo, he mapped the shapes and movements of potentially dangerous near-Earth asteroids. At the Lunar and Planetary Institute in Houston, Rivera-Valentín continues to use Arecibo data to study planetary surfaces. The latest news that the Arecibo Observatory would be closed was “heartbreaking”.
In August and November, two cables supporting a 900-tonne platform of scientific instruments above Arecibo’s shell unexpectedly broke. After assessing the damage, the National Science Foundation, which is funding Arecibo, announced that the telescope could not be safely repaired and would be demolished (SN: 11/19/20). But before the telescope could be dismantled, the entire instrument platform fell into the bowl on December 1st.
For Puerto Rico, the loss of Arecibo is like the loss of the Empire State Building in New York or the loss of the Golden Gate Bridge in San Francisco, says Rivera-Valentín – but with the added tragedy that Arecibo was not just a cultural and historical icon but a productive research facility.
“The loss of Arecibo is a great loss to the community,” said Tony Beasley, director of the National Radio Astronomy Observatory in Charlottesville, Virginia. “The Arecibo life cycle has been truly remarkable and has produced amazing scientific evidence.”
For example, the radar maps from the Moon and Mars Observatory helped NASA to select landing sites for the Apollo (SN: 05/01/65) and Viking Missions (SN: 07/17/76). And observations of the asteroid Bennu helped NASA plan their OSIRIS-REx mission to take a sample from the space rock (SN: 10/21/20). Arecibo views of Saturn’s moon Titan have discovered hydrocarbon lakes on its surface (SN: 10/01/03).
Beyond the solar system, Arecibo has observed mysterious flashes of radio waves from space known as rapid radio bursts (SN: 02/07/20) and the distribution of galaxies in the universe. Arecibo has also been used in the search for extraterrestrial intelligence for decades (SN: 11/07/92), and in 1974 the first radio message to aliens was broadcast into space (SN: 11/23/74).
After the Arecibo collapse, the radio astronomy community “needs to look at what’s going on at Arecibo and figure out how we can replace some of those skills with other instruments as best we can,” says Beasley.
However, many of Arecibo’s features cannot simply be replaced.
“Arecibo was unique in several ways,” says Donald Campbell, an astronomer at Cornell University and former director of the observatory. For starters, Arecibo was enormous. With a diameter of 305 meters and an area of around 20 acres, Arecibo has been the largest radio dish in the world since it was built in 1963 (SN: 11/23/63) until 2016, when China completed its five hundred meter aperture (FAST) spherical telescope. With such a large dish for collecting radio waves, Arecibo could see very faint objects and phenomena.
This incredible sensitivity made Arecibo particularly good at detecting difficult-to-see objects such as rapidly rotating neutron stars called pulsars (SN: 1/3/20). When a pulsar spins, it sweeps a beam of radio waves around space like a lighthouse that appears to Earth as a beacon that switches on and off.
“Arecibo was the king” when he discovered the capricious light of the pulsars, says Beasley. “There won’t be an easy solution to recreate this quilt.” The next largest radio dish in the US is the 100-meter-wide Green Bank Telescope in West Virginia. With smaller telescopes, it can take several hours to observe a target to collect enough radio waves for analysis, whereas Arecibo only took a few minutes.
In addition to its mammoth size, Arecibo could also transmit radio waves. “Most radio astronomy telescopes have no transmitters,” says Campbell. “They only receive radio waves from space.” With radar transmitters Arecibo was able to bounce radio waves from gases in the atmosphere (SN: 01/31/70) or the surfaces of asteroids and planets. The reflected signals that came back contained information about the target such as size, shape, and movement.
“The high-powered transmitters enabled the telescope’s original primary purpose – to study the Earth’s ionosphere,” says Campbell. The US military that funded the construction of Arecibo wanted a better understanding of the earth’s atmosphere in order to help develop missile defense (SN: 2/10/68). But Arecibo’s radar transmitters “were also used to study bodies of the solar system – the planets, the moons, including our own moon,” says Campbell. “More recently, the focus has been on studying near-Earth asteroids,” which may be on a collision course with Earth.
Other major radio courts like China’s FAST or the Green Bank Telescope are not equipped with radar transmitters. NASA’s Goldstone Deep Space Communications Complex in the Mojave Desert has a 70-meter dish with radar capabilities. But Goldstone “is used both as a military installation and as part of the Deep Space Network that communicates with spacecraft, so it doesn’t have much time,” says Rivera-Valentín. “And it’s not as sensitive as Arecibo,” so it can’t see as many asteroids.
At the time of its demise, the Arecibo Observatory still had a “bright scientific future,” says Joan Schmelz, an astronomer with the Universities Space Research Association in Mountain View, California, and former deputy director of the observatory. “It wasn’t just resting on its laurels.” For example, Arecibo was a key facility for the ongoing NANOGrav project, which uses pulsar observations to search for waves in spacetime triggered by supermassive black holes (SN: 9/24/15).
The Arecibo observation days may be over, but that doesn’t mean that data from the telescope isn’t making any further contribution to science, says Schmelz. Some of the most exciting discoveries in radio astronomy have come from re-analyzing old telescope data (SN: 07/25/14). “People will be analyzing Arecibo data for a while,” she says, “and we will hopefully see new scientific results when that data is analyzed and published.”