Scientists Reflect on Arecibo’s Doomed Big Dish

Aerial view of the big dish and platform, showing the Arecibo Observatory prior to the recent damage.

Aerial view of the big dish and platform, showing the Arecibo Observatory prior to the recent damage.
Image: NIAC

The big dish at the Arecibo Observatory in Puerto Rico is on the verge of collapse, leaving officials with no choice but to retire the famous radio telescope. Astronomers around the world are now having to face a grim reality: that this dutiful dish—in service for the past 57 years—is no more.

I have to admit, the thought that the 1,000-foot (305-meter) dish at Arecibo would have to be torn down never occurred to me when I first started to cover this story during the summer. The first disturbing development came on August 10, when an auxiliary cable slipped out from its socket, crashing through the dish below. The falling cable created an unsightly 100-foot scar, but at the time, the incident seemed more of a nuisance than a catastrophic problem. And indeed, officials with the observatory soon made arrangements to repair the damage and replace the missing cable.

Things took a dramatic turn for the worse on November 6, when a main cable snapped and also fell onto the structure. This was the moment when I really started to worry. A missing auxiliary cable is one thing, but a missing auxiliary cable and a main cable? Not good. In my mind’s eye, I imagined the 900-ton platform, which is suspended 450 feet (137 meters) above the dish, being held together by string. A new image of a badly frayed cable didn’t ease my anxiety.

The platform above the dish.

The platform above the dish.
Image: NIAC

I reached out to the Arecibo Observatory, the National Science Foundation, and the University of Central Florida, which manages the facility on behalf of the NSF. On the morning of Thursday November 19, I woke up to an NSF email alerting me to a press conference that was to be held later the same morning. Finally, I thought, I would be able to report on pending repairs and a strategy for bringing the beleaguered facility back online. After registering for the press conference, however, the NSF sent me further details: The iconic dish was slated for demolition.

It felt like a punch to the stomach.

Engineering teams brought in to evaluate the situation said the platform could undergo a catastrophic collapse at any time, making it unsafe for workers. The dish, in operation since 1963, would have to undergo controlled disassembly in such a way to preserve other assets at Arecibo, including a LIDAR facility and visitor’s center.

Illustration for article titled A Magically Surreal Symbol of Human Ingenuity: Scientists Reflect on Arecibo’s Doomed Big Dish

While scientific work at the Arecibo Observatory will continue, the radio dish is done. And that’s a huge shame. In addition to its cultural importance, the dish fostered some excellent science, including the first detection of a binary pulsar (which earned the team a Nobel Prize in Physics), the first radar maps of Venus, the detection of potentially hazardous asteroids, the first exoplanets ever discovered, and insights into gravitational waves. The facility

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Doomed Philae lander accidentally did a science by denting the comet

The close-ups highlight the bright ice exposed in the boulders when Philae struck them during its second touchdown (green box above).
Enlarge / The close-ups highlight the bright ice exposed in the boulders when Philae struck them during its second touchdown (green box above).

The Rosetta mission’s attempt to drop the Philae lander on a comet in 2014 didn’t go according to plan. The harpoon mechanism meant to stick Philae to terra-not-quite-firma didn’t work, and poor Philae ended up bouncing around and landing under a dark cliff overhang, unable to deploy its solar panels and complete its tasks. But let it not be said that Philae failed to leave its mark. Because it did. Quite literally.

To extract value from Philae’s accidental adventure, researchers have worked hard to identify the spots where the craft impacted the surface of the comet. This required painstaking analysis of Philae’s motion sensors to reconstruct its trajectory, along with a terrifically complex game of “one of these things is not like the others” played with before-and-after images of the comet’s jumbled surface.

The site of the initial bounce was easy enough to find, but the path from there to its resting place was another story. A new study led by the European Space Agency’s Laurence O’Rourke reveals another spot where Philae dented comet 67P. And the size of that dent actually tells us something remarkable about what comets are like.

Dented

Researchers eventually found a spot they dubbed “skull-top ridge” where a pair of boulders separated by a crevice appeared to have met Philae. After the landing, a bright spot appeared in that crevice, as if surface dust had been removed to expose water ice in the boulder. And indeed, spectral data from imagery confirms that the bright spot is largely water ice. While water ice makes up a substantial portion of comets—which are often somewhat rudely referred to as “dirty snowballs”—a comet’s surface is composed of a layer of dust left behind as sunlight drives off the outermost ice, so actually seeing ice there is telling.

A simple animation to show how the team thinks Philae interacted with the pair of boulders.

Philae’s initial touchdown location was in a flat spot likely covered by a thick layer of that dust. Its encounter with this boulder represents an interaction with something more similar to the comet’s interior.

The team estimates the depth of the dent it left behind at about 25 centimeters. Using the recorded velocity of the 100-kilogram craft, this allowed them to calculate the boulder’s sturdiness—or lack thereof, as it turns out. They found that the boulder was actually about as soft as fluffy snow on Earth.

Mixed measurements

This illustrates something that Rosetta successfully measured: the comet is extremely porous. The high water ice and CO2 ice content might make you think the comet is a hard, frozen block, but around 75 percent of its volume is void space in between grains of ice and dust. Without strong gravity to pull things together, comets just aren’t that dense.

This isn’t the first estimate of the comet’s material strength to come out

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