The Arecibo Observatory's radio telescope is the largest single dish in the world. Here, the platform above the dish is illuminated by the afternoon light. (Nadia Drake/WIRED) An early artist's conception of the telescope. The shape of the dish is the same, but the tower system changed. (Photo courtesy of Arecibo Observatory) An early photo of the observatory site. Two towers are built, and the sinkhole is being sculpted to hold the spherical dish. (Photo courtesy of Arecibo Observatory) Workers installing the original dish surface. The entire reflector is suspended from cables anchored to the sinkhole walls. The first surface was made from coarse mesh; in 1974, it was upgraded to a perforated aluminum surface. (Photo courtesy of Arecibo Observatory) A 1974 view of the original dish, before resurfacing began. You can see through the dish to the spiraling road beneath. (Photo courtesy of Arecibo Observatory) The same linoleum floor still runs through much of the control room. (Image courtesy of Arecibo Observatory) Bravery in the early days. Here, workers are installing a radio feed on one of the cables connecting the platform to the towers. It's being used to observe a pulsar. (Photo courtesy of Frank Drake) A 1974 photo of the dish resurfacing in progress. (Photo courtesy of Arecibo Observatory) An early photo from the telescope's platform. (Photo courtesy of Arecibo Observatory) In 1974, the observatory upgraded the dish surface from coarse mesh to perforated aluminum. Each panel (stacked) is the size of a large dinner table, and lets enough light through to allow vegetation to grow and prevent erosion. (Photo courtesy of Frank Drake) High aerial view of the Arecibo Observatory, showing the rough karst terrain in which it sits. (Photo courtesy of Arecibo Observatory) The control panels for the observatory's transmitter looks like something out of Star Trek. (Nadia Drake/WIRED) Looking down from the platform, which is 450 feet above the dish. Its exact altitude varies by about a foot as the temperature rises and falls. (Nadia Drake/WIRED) You can phone home from the top of the platform. (Nadia Drake/WIRED) Looking down at the platform and the rotating joint that lets the whole thing swivel. (Nadia Drake/WIRED) Teams are installing transmitters at the telescope's base. These dipole antennas, each about 70 feet tall, will be used to heat the ionosphere above the site and study how plasma particles behave. (Nadia Drake/WIRED) Only five people at a time are allowed on the catwalk. There's also a cable car that runs between the control room and the platform. Maximum capacity: Four people. (Nadia Drake/WIRED) Beneath the dish it's shaded and cool, and home to an army of tiny frogs. It's quiet except for the sounds of machete-wielding staff trimming the vegetation, and another crew installing a transmitter at the dish's base. (Nadia Drake/WIRED) A discarded pile of old line feeds rusts in the sinkhole beneath the dish. This is what it looks like inside one of them. (Nadia Drake/WIRED)

A 1960 photo of the sinkhole that would later hold the world's largest radio telescope. Already, some land is being moved to make way for the dish. (Photo courtesy of Arecibo Observatory)

ARECIBO, Puerto Rico – The Arecibo Observatory, home of the world’s largest single radio telescope, celebrates its 50th birthday today. In that half-century, the giant, iconic dish and its three towers have become a symbol of the quest to understand the cosmos and whether we are alone in the universe. Its endurance also points to the ingenuity of the scientists and engineers who dreamed up and then installed a massive, world-class observatory in a Caribbean sinkhole in the early 1960s.

“Arecibo is one of the most powerful and amazing instruments ever made – ever,” said physicist Richard Behnke of the National Science Foundation at a conference celebrating the observatory’s 50th trip around the sun.

I’m here with my dad, Frank Drake, who was once the director of the observatory; it’s not the first time he’s brought me to Arecibo, though I barely qualified as an intelligent life form on my last visit (I was four months old).

What began as a complex engineering challenge has since become a destination for both researchers and tourists. Now, more than 100,000 visitors come to see the telescope each year. And unless you’re a VIP with helicopter landing pad privileges, which we were not, the journey to the observatory involves traversing 10 miles of winding, narrow roads past colorful concrete houses and thick vegetation.

The road is carved into the area’s rugged karst terrain where rounded hills bubble up as though the earth were frozen in mid-boil. It’s a sticky and unforgiving landscape, and it’s more wildly beautiful than I could have imagined.

The most obvious thing to say about the telescope is that it’s enormous. Even when you’re standing right next to it, or on it, the size of the telescope is hard to comprehend.

The dish is 1,000 feet across and covers 20 acres. Above, a 900-ton platform is suspended from 3-inch-thick cables. A funny-looking dome hangs beneath the platform and holds two secondary reflectors – a relatively late addition to the site’s hardware that helps focus incoming radio waves. The cables attach to three towers that rise as much as 365 feet above the hills like a trio of sentinels keeping watch over a sensitive, cosmic ear.

A 1961 photo of telescope designer William Gordon, with some sketches in the background. (Photo courtesy of Arecibo Observatory)

The observatory grew out of a 1950s plan to study Earth’s ionosphere. Fifty years ago, radio astronomy was still in its infancy. Sputnik had just been launched, NASA had just been born, and the U.S. government was concerned about whether we could detect Soviet satellites. One of the telescope’s many objectives was to find out whether satellites zooming through the atmosphere left detectable trails of charged particles in their wake.

So, with funding and support from what was then called the Advanced Research Projects Agency (now DARPA), William E. Gordon, an astronomer and engineer, began designing a giant radio telescope that would also have a radar transmitter. He needed to build it somewhere in the tropics, where all the planets in the solar system pass nearly overhead – in case radio astronomers wanted to study them. Ideally there would be a natural sinkhole, so a giant hole wouldn’t have to be dug for the telescope’s enormous dish. Oh, and it should be located in a place with a friendly government.

That meant Cuba was out, and Puerto Rico was in.

Construction began in 1960. Three years and $9 million later, the telescope opened for business on November 1, 1963, managed by Cornell University.

At first it didn’t work very well – the spherical reflector, built so that the telescope could be aimed at stuff instead of staring at a fixed point, wasn’t very efficient (most radio telescopes use parabolic reflectors that concentrate incoming waves at a single point, and Arecibo’s spherical surface concentrated them over a 96-foot-long line).

After a few upgrades – including an early fix for that initial inefficiency, a new surface comprising 38,778 aluminum panels in 1974, and the 1997 installation of the dome, which houses secondary reflectors that focus incoming signals – radio astronomy had grown up into a mature discipline, and the telescope was among the most sensitive cosmic detectors on the entire planet.

Today it still listens for the tap-dancing transmissions from distant pulsars, for the humming of hydrogen atoms and the murmurings of interstellar and extragalactic molecules, for the whispers from faraway civilizations. But it’s not a one-way receiver – the observatory also sends signals into space.

In 1974, the original dish surface was replaced with aluminum tiles. Cornell University president Dale Corson slipped the last tile – #38,778 – into place. (Photo courtesy of Frank Drake)

One of the most famous was the 1974 interstellar postcard beamed toward the globular cluster M13. My dad and his colleagues designed the message, which used binary code to describe such things as Earth’s place in the solar system, the structure of DNA and the four base pairs, chemical elements, the Arecibo telescope, and a human. The connection between Arecibo and civilizations among the stars is strong: The observatory has been the site of numerous SETI searches and still compiles the data distributed by UC Berkeley’s SETI at Home project. It was also part of the set for the 1997 movie Contact.

The observatory can also turn the skies overhead into a laboratory. One early experiment, for example, involved launching rockets carrying barium payloads into the air above Arecibo. At a high altitude, the rockets would release the barium into the atmosphere so scientists could observe how the barium atoms behaved when ionized by the sun. Barium, one of the alkaline earth metals, is highly reactive and easily broken into charged particles by sunlight, forming a plasma. Scientists wanted to use that plasma as a proxy to study how the actual ionosphere behaves should a nuclear bomb explode above it.

But first, researchers needed a rocket launch pad – something capable of launching Aerobee rockets. “There was an abandoned military base near there,” said Frank Drake, who was director of the observatory at the time. “So we went in and made a concrete launch pad for the rockets.”

After two failed launches in 1968, the final available rocket set the pre-dawn sky above the observatory ablaze with the brilliance of a second sun as Earth’s home star illuminated the barium cloud. When the cloud ionized and broke apart – the charged particles being guided by magnetic fields – it changed color. “I went outside and looked up at the sky, and wow,” said space physicist Herb Carlson, now at Utah State University, who worked on that early experiment. “It looked like a bull’s eye – different colors, and with rings.”

Later, in 1979, the observatory played a role in the infamous Vela incident, in which Israel reportedly conducted a nuclear test off the coast of South Africa.

Turns out, if you explode a nuclear bomb high up in the atmosphere, the folks at Arecibo will know: Atmospheric waves produced by the blast travel through Earth’s ionosphere carrying a distinct, high-energy signature. In 1979, that’s just what Richard Behnke saw.

At the time, he was at the observatory studying the atmospheric ripples produced by an Atlas rocket launch from Cape Canaveral. Then he noticed a weird signal in the data he’d been processing. It looked as though something huge had exploded high above the Earth. The shock wave had smashed into the ionosphere and set it trembling, sending atmospheric gravity waves, which are kind of like ocean waves, around the planet. Behnke calculated the speed of the waves as they arrived over Arecibo (about 600 meters per second) and the direction they came from (the southeast), and reached the unmistakable conclusion that something had dumped enormous amounts of energy into the atmosphere off the South African coast.

Behnke didn’t know yet that the American Vela satellite had also detected something strange – a double pulse of light, the sign of a nuclear explosion – off South Africa. In the next few months, word began circulating that the Israelis, in cooperation with South Africa, had conducted a nuclear test over the Indian Ocean.

The U.S. government then convened a panel of scientists to review the observations. They reached a very different conclusion: The event was the result of a meteorite hitting the satellite. The story, and Behnke’s differing conclusions, aired during Walter Cronkite’s last two evening broadcasts on CBS.

“What are the odds that some young Ph.D. is going to end up talking about the ionosphere and gravity waves on the last two Walter Cronkite news shows?” Behnke said.

A 1974 photo of then-graduate student Russell Hulse working in the observatory's control room (check out the tick marks on the side of the box). That year, they discovered the first binary pulsar. (Photo courtesy of Joe Taylor)

Arecibo’s less controversial discoveries include showing that Mercury’s rotation rate was nowhere near what people thought it was (59 days instead of 88), the identification of the first millisecond pulsar, the discovery of the first pulsar planets, and the Nobel Prize-winning discovery of a binary pulsar.

In the 1970s, pulsars were trendy in radio astronomy. But no one had ever seen two of them circling one another – until 1974, when graduate student Russell Hulse and his advisor Joe Taylor found the first binary system. At the time, Hulse was in Arecibo and Taylor was back in Amherst, Mass., teaching classes – so they communicated by writing letters or by using a faster but fairly complicated relay of short-wave radio and phone calls.

By studying the binary pulsar system, Hulse and Taylor were able to test Einstein’s theory of general relativity. More specifically, they indirectly proved Einstein’s prediction of gravitational waves. The binary pulsar was “a very ideal relativity laboratory,” Taylor said, “with what amounts to a very accurate clock, moving in an orbit where only gravity is important.”

Not every experiment conducted at the observatory moved science forward in prize-winning leaps. Once, graduate students tried to predict how many times a basketball, hurled sideways at the dish’s rim, would circle the reflector before falling toward the center (answer: zero. It went straight down). Another experiment confirmed that, yes, one could ride a motorcycle up the catwalk to the suspended platform, according to a scientist with first-hand knowledge of the event, who insisted on anonymity.

For decades, these scientists and engineers have been coming back to Arecibo. The telescope still operates 24 hours a day, peering through dust and clouds and searching for answers.

It’s a hard place to say goodbye to, and I hope it won’t be three more decades before I can return. We left when darkness fell and the stars turned on, when the red lights on the towers became blazing beacons in the night and the coqui frogs began singing a sweet nighttime melody – a song interrupted only by the whirring of the telescope as it swiveled to listen to the murmurings of another faraway object.

Arecibo at sunset. (Nadia Drake/WIRED)

Special thank you to director Robert Kerr and the staff of the Arecibo Observatory for help in reporting this story.