5-08-2026 7:07pm

Sunspot Telescope leak was small, but dangerous

Only 1 percent of the mercury in the Dunn Solar Telescope leaked on Jan. 5, none of it outside the building to endanger the forest or nearby communities.

These and other details come from a National Science Foundation notice of a sole-source Federal contract awarded to a New York-based company, Thornton Tomasetti, for mercury removal and remediation services. With a motto of “When others say No, we say ‘Here’s How’,” it specializes in difficult engineering projects. Thornton Tomasetti was selected because of their forensic investigation into the 2021 collapse of the 1,000-foot-wide Arecibo Radio Telescope in Puerto Rico and other work at heights and on large structures.

The one-year, $320,000 contract was awarded on March 20, but the Federal System for Acquisition Management (SAM) did not publish the notice until April 30. The contract itself is not published as it contains “Controlled Unclassified Information,” a measure used to protect information proprietary to a contractor.

The NSF notice was a “Justification and Approval” explaining why Thornton Tomasetti received the contract without the usualrequest for proposals. NSF justified it because of “The immediate need to assess and stabilize the structure and prevent a catastrophic mercury release … .”

The concept of mercury bearings dates from 1825 when Augustin Fresnel, a French scientist, proposed it as a way to provide smooth, rapid rotation of the flat, lightweight lenses he invented for lighthouses. The first such use was in 1892. Several large astronomy telescopes have used mercury bearings since then.

The Dunn rides on two mercury bearings at 30 and 70 feet above the observing floor to ensure smooth rotation of the 200-ton telescope. Another bearing, at the bottom of the telescope barrel, about 200 feet underground, stabilizes the base of the telescope but is not involved in the leak. The Dunn bearings hold about160 gallons — 18,000 pounds — of mercury.

According to the notice, a weld between a valve and the bottom of a bearing cracked. The exact cause remains unknown. Mercury corrodes many metals, but iron and steel are resistant, and the telescope is 57 years old.

A hazmat team removed and disposed about 186 lbs., equivalent to 1.6 gallons, of mercury by Jan. 15 and identified the source the next day. The leak stopped although the crack has not been sealed, and a container was placed under it, so NSF considers the situation to be active. NSF mentions mercury only reaching the main floor.

“[T]he concern is that the crack may widen suddenly,” NSF wrote, “or that additional undetected structural failures may occur, causing a catastrophic leak in which a portion or all the mercury in the bearing could be released in a very short time [~5 minutes] from a great height.”

Given various risks, NSF stated, all the mercury will be removedand render the Dunn inoperable in its original form.

The author is a veteran science writer and former education officer at Sunspot.