The news this week that the Big Dish at Arecibo has reached the end of its life has hit the astronomy and SETI communities hard. Let’s look back over the working life of this amazing telescope.
An Ambitious Project
In the early 1960s, William E. Gordon from Cornell University designed a telescope to study the ionosphere.
The site on Puerto Rico was chosen for two reasons:
- It was close to the equator, which is always a good site for large telescopes.
- The karst geography was prone to large sinkholes, which allowed for the easy construction of the big dish.
The initial design was a fixed parabolic reflector with a 150 m tower, but designers soon realized that this would greatly limit its utility, and the tower was redesigned to have an azimuth arm, which would carry the actual antennae to receive the signal and allow them to be pointed at any part of the sky. Thus, the only limitation was the Earth’s rotation; sometimes astronomers would have to wait for their target to come into view.
The big dish was at the time the largest single aperture telescope constructed; and it held that title until the Five hundred meter Aperture Spherical Telescope was constructed in 2016.
But for the last sixty years, the words “Big Dish” have been synonymous with “Arecibo.”
What has it been Used For?
Arecibo is a radio telescope, which means that instead of observing in the (very limited) visible light spectrum, it observes radio waves.
Thus, the primary purpose of the Big Dish has been radio astronomy, but it’s also been used in atmospheric science (as Gordon initially envisioned) and radar astronomy.
Time on the Big Dish has also been used by SETI. Arecibo’s transmitter has been used to send messages to the stars, although none have yet obtained a response.
The dish has been upgraded twice. In 1973, the original mesh replaced by aluminum panels, allowing for a higher usable frequency. And in 1997, a more advanced reflector system was installed, which gave more flexibility, and a more powerful transmitter was added.
But Arecibo’s star was about to wane.
Lack of Funding and Disasters
In 2006, Arecibo’s funding was cut, mostly by NASA. The budget cuts included a recommendation for closure, but researchers came together to find the money.
NASA eventually restored funding, especially for radar astronomy, which plays a role in planetary defense.
Then Puerto Rico was struck by a series of natural disasters. The always storm-prone island was torn apart by Hurricane Maria in 2017. An earthquake swarm hit the island this last winter, lasting from December into January, and culminating at 6.4. Combined with other lesser storms, damage was done to the aging observatory.
On August 10, 2020, an auxiliary cable detached from the center tower and fell to the ground, cutting a 30 meter gash in the dish and damaging the Gregorian dome that holds the receivers and secondary reflectors.
Right as repair work was about to start, one of the main cables, perhaps over-strained by the loss of the auxiliary, simply snapped on November 6. Over the next few days, three separate engineering teams examined the dish.
All three agreed that the structure cannot be made safe. Repairing it would be too risky for the workers and repairs would likely not last long.
The decision to decommission and demolish the big dish was made on November 19, 2020. The visitor center and remaining facilities will be preserved for educational purposes.
There is no other telescope that can do Arecibo’s specific work. The FAST telescope is larger, but less versatile and sophisticated.
So, will Arecibo be replaced? At this point it seems unlikely (but not impossible) that a new, modern big dish will be built at the site. Certainly we need to think about replacing Arecibo’s planetary defense functions.
Radio and radar astronomy continues at other sites. Modern radio telescopes generally take the form of an array, that is to say, multiple small reflectors. This allows for a much larger “dish,” but many scientists still appreciate Arecibo’s advantages.
The biggest problem is that we only have two radar astronomy facilities. Well, now we have one (Goldstone).
Without Arecibo, we have a far higher chance of being hit by a big rock we didn’t see coming. For this reason alone, Arecibo should be replaced. We may have better facilities to study the stars.
We have none to help keep our planet safe. So, we need to find a way to replace Arecibo.
Or better yet, to duplicate its capabilities in an even better form.