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When it comes to telescopes, size matters. That’s because, ultimately,  telescopes are light collectors.

Whether it’s a backyard rig or a  multi-million-dollar space mission, all telescopes work by  gathering light from the stars. The more light you can gather,  the more detail you can see. There are lots of tricks you can play to  make small telescopes collect more light, like unfolding mirrors, looking at you James Webb.

But when you get right down to it, there’s no substitute for good  old-fashioned square footage. And Arecibo’s radio telescope held the crown for having the most square footage  to collect light for 53 years! But how did this giant telescope end up in the middle of the Puerto Rican rainforest?

In the late 1950s, the American  military wanted to understand more about the upper atmosphere, also  known as the ionosphere, to detect nuclear missiles  re-entering the atmosphere. So the Advanced Research Projects  Agency funded Cornell University to build a radio telescope that  could study the ionosphere. The university looked around  for places to build a giant dish and selected a big sinkhole in  Puerto Rico near the town of Arecibo.

They chose Puerto Rico  because it was easy to access, in a politically stable country,  and close to the equator. The latitude is important because if you  can only point your telescope straight up, you want to build near the  equator so you can observe as much of the sky as  possible as the earth rotates. A telescope pointed up at the  North pole could only ever see the same chunk of sky around the North star.

And they chose a sinkhole for  the simple reason that they wouldn’t have to dig a brand new hole  and could save on construction costs. The dish was a giant half-sphere, 305  meters across at its widest point. To put that into perspective, the  Chrysler Building in New York City is just under 319 meters tall.

Arecibo is designed to bounce radio waves from the sky into a receiver suspended above. This is how satellite dishes for satellite  TV work, but on a /much/ bigger scale. But Arecibo’s telescope is different because it’s more spherical compared to  the parabola of a satellite dish.

Most satellite dish-makers choose parabolas  because when parallel rays of light bounce off of them, they get  focused onto a single point. That makes the signal very clear. On the other hand, when parallel  radio waves hit Arecibo’s dish, they bounce off and focus  into a line, not a point, making them much harder to collect and analyze.

But if the researchers could  deal with that fuzziness, then they could take advantage  of something unique to spheres: the curve is exactly the same no  matter which part you look at. That means you could point the telescope  by moving the receiver, not the dish. It was a challenge designing the right  receiver, but in 1997 they installed an assembly made of two additional curved mirrors that could focus incoming light,  called the Gregorian Dome.

And with that upgrade complete, the  entire 305-meter dish in the bottom of a sinkhole could be effectively pointed 20 degrees in any direction just by moving  the Gregorian dome above. And Arecibo wasn’t only great  at receiving radio signals; it could also transmit them  with a huge radio transmitter! In 1974, right after the dish upgrade, it  famously used this transmitting ability to send a binary message to outer space,  saying who we are and where to find us.

Astronomers also used Arecibo to direct  radio waves out into space to bounce off of moons, planets, comets, asteroids, or  the atmosphere, and listen to the echoes. The echoes revealed mountains on Venus, the shape of nearby asteroids,  and much else besides. By looking very carefully at the moon,.

Arecibo also found secret Soviet radar stations. That’s right, Arecibo was so sensitive  that it essentially used the moon as a mirror to find out where on Earth  Soviet radar signals were coming from. Arecibo was also at the heart of lots of  other exciting discoveries, like the fact that some pulsars go quiet, and that there are  planets around stars other than our own.

It also helped researchers find that  there are building blocks of life in other galaxies, and that  Mercury’s day lasts 59 days compared to the 88 days we once thought. If that sounds like a varied list, that’s  because Arecibo was a powerhouse of scientific discovery for more than half a century. And we’ve just barely scratched the surface!

Arecibo was amazing in part because  its collecting area was huge, so it could pick up signals  that other telescopes couldn’t. Unfortunately, due to a combination of  age, climate, and difficulty in repairs, in December 2020 the cables supporting  the receiver assembly snapped, bringing the Gregorian dome crashing down. We talked about this shortly after it happened in an episode of SciShow Space News.

Engineers are still assessing  the reasons it happened and ground crews are still cleaning it up. This might not be the end for Arecibo, though. Astronomers have already proposed  a successor for the site, dubbed the Next Generation Arecibo Telescope.

The idea is to install a series of smaller dishes at Arecibo telescope’s previous  location, and then make it steerable. Its 1,000 dishes would let them peer  further into the cosmos to spot incoming asteroids and understand the effects  of climate change on the atmosphere. But it’s an expensive proposition,  costing $400 million compared to the over $9 million that the original  Arecibo telescope cost in 1960, about $90 million in 2021-dollars.

And we’ve already passed the  decadal survey that determines the astronomy community’s priorities and  it will be ten years until the next one. But Arecibo is a name that carries a lot  of weight in the scientific community, so it may get the funding it needs. Whether or not the Next  Generation Arecibo gets built, there will always be a place in our  hearts for the Arecibo Telescope.

Which is why we’re immortalizing  it as this month’s space pin! It’ll be available all month at DFTBA.com/SciShow. It’s only available in November,  so make sure to order yours soon.

Because in December, we’ll  have a whole new pin for you. [♪ OUTRO].