Arecibo Telescope

A dish in a sinkhole

William E. Gordon, a Cornell engineer, proposed a giant radar dish to study the ionosphere, and a natural limestone sinkhole near Arecibo in northern Puerto Rico offered a bowl for a fixed reflector 305 metres across. The Arecibo Ionospheric Observatory opened on 1 November 1963. A platform of about 900 tonnes hung on cables above the dish carried the receivers, which moved to point at different parts of the sky.

In 1964 radar echoes from Mercury showed that the planet rotates in about 59 days, not the 88 days of its orbit as had been believed.

Pulsars, planets and a message

In 1974 Russell Hulse and Joseph Taylor discovered the first binary pulsar, PSR B1913+16, with Arecibo; the slow shrinking of its orbit matched the energy loss predicted for gravitational waves, and they received the 1993 Nobel Prize in Physics. In November 1974 the telescope transmitted the Arecibo message, a 1,679-bit picture addressed to the globular cluster M13. In 1992 Aleksander Wolszczan and Dale Frail found the first confirmed planets outside the solar system, around the pulsar PSR B1257+12, in Arecibo observations.

The telescope also studied near-Earth asteroids by radar and the upper atmosphere by lidar.

Collapse

In 2018 a consortium led by the University of Central Florida took over operations. In August 2020 an auxiliary cable slipped from its socket, and in November a main cable broke; on 1 December the 900-tonne platform fell into the dish. The National Science Foundation had decided to decommission the telescope before the fall. No one was hurt. The site now hosts education and the observatory's remaining instruments.

The legacy after the collapse

The loss of the dish ended an era of high-sensitivity radar astronomy in the United States and removed the world's most sensitive radio telescope, but the observatory's data live on: decades of pulsar timing, radar images of asteroids and the Arecibo Legacy Fast ALFA survey of neutral hydrogen. Puerto Rico's community lost a local landmark, a major employer and an education centre. The site now supports a new science and education centre, funded by the National Science Foundation. A decommissioned observatory is not simply a loss, because its archive, its people and its place in the community can be carried into the next project.

Instruments and technologies

• 305 m spherical reflector A fixed dish of perforated aluminium panels, whose spherical shape needs a line feed to correct the aberration; later a Gregorian dome added secondary and tertiary mirrors.
• Planetary radar A transmitter of up to 1 megawatt that sent signals to planets and asteroids and recorded their echoes.
• Pulsar timing back end Digital receivers recording pulse profiles at microsecond resolution.

The people

• William E. Gordon Cornell engineer who conceived the observatory.
• Russell Hulse Discovered the binary pulsar as a graduate student.
• Joseph Taylor Hulse's supervisor; shared the 1993 Nobel Prize in Physics.

Timeline

• 1963 Opens on 1 November as the Arecibo Ionospheric Observatory.
• 1964 Radar shows Mercury rotates in about 59 days, not 88.
• 1974 Discovery of the first binary pulsar; the Arecibo message is transmitted.
• 2018 A consortium led by the University of Central Florida takes over operations.
• 2020 Cable failures in August and November; the platform collapses on 1 December.

Key lessons for observation today

• Monitor the structure as carefully as the signal Cable failures preceded the collapse by months. Structural health monitoring is part of any large observing system.
• Keep the data when the facility closes The telescope is gone but its archives remain in use. Open data and clear archives outlast hardware.
• A multi-purpose instrument has more than one constituency Radio astronomers, radar scientists and atmospheric researchers all used the dish. Governance should serve them all.

Sources

• Wikipedia, Arecibo Telescope
• University of Central Florida, Arecibo Observatory's telescope collapses
• Nobel Prize, Physics 1993 press release
• Wikipedia, William E. Gordon