Iconic Arecibo Observatory, too unsafe to repair, faces demolition

The Arecibo Observatory as seen in a file photo taken before the failure of two cables helping support the suspended receiver platform. Engineers say the structure is too unstable to safely repair. Image: University of Central Florida

The iconic Arecibo Observatory has made its final observations.

Following the failure of a second cable helping support the radio telescope’s 900-ton instrument platform, the U.S. National Science Foundation opted to carry out a controlled demolition after engineers concluded the structure was too unstable to attempt repairs.

“After reviewing the engineering assessment, we have found no path forward that would allow us to (carry out repairs) safely,” Sean Jones, assistant director for the Mathematical and Physical Sciences Directorate at the National Science Foundation, told reporters 19 November.

“We know that a delay in decision making leaves the entire facility at risk of an uncontrolled collapse, unnecessarily jeopardizing people and also the additional facilities.”

The observatory is operated for the NSF by the University of Central Florida. It is made up of a fixed 305-metre (1,000-foot) dish antenna built into a natural depression that reflects radio waves or radar beams to the receiver array suspended 137 metres (450 feet) above by cables stretching from three support towers.

While the dish itself only moves with Earth’s rotation, the instrument platform features a moveable receiver pallet that allows astronomers to observe targets up to 40 degrees away from the telescope’s vertical axis.

Since first light in 1963, the observatory has played a significant role observing deep space targets, bodies in the solar system and, at an adjacent facility using powerful lasers, the composition and behavior of Earth’s upper atmosphere.

But on 10 August, an auxiliary cable installed in the 1990s pulled free of its socket on one support tower and crashed onto the dish below, ripping a 30-metre-long (100-foot) gash.

Engineers were developing repair plans when one of the main 7.6-centimetre-wide (3-inch-wide) cables attached to the same tower unexpectedly snapped on 6 November, causing the instrument platform to tilt. That put additional stress on the remaining cables.

An analysis showed the cable failed at about 60 percent of of its minimum breaking strength. Inspections of other cables showed fresh wire breaks and slippage in several auxiliary cable sockets.

A view beneath the Arecibo radio telescope dish shows the damaged caused when a support cable failed last August. Image: University of Central Florida

An engineering firm assessing the structure concluded it would be unsafe to proceed with repairs. Even stress tests to determine the strength of the remaining cables could trigger a catastrophic collapse.

Instead, engineers recommended a controlled demolition, bringing down the suspended receiver platform in a way that would prevent damage to other structures around the dish.

“The telescope is at serious risk of an unexpected, uncontrolled collapse,” said Ralph Gaume, director of NSF’s Division of Astronomical Sciences. “According to engineering assessment, even attempted stabilization, or testing the table could result in accelerating the catastrophic failure.

“Engineers cannot tell us the safety margin of the structure, but they have advised NSF that the structure will collapse in the near future on its own.”

Plans for bringing down the instrument platform have not yet been finalized.