Earthquake-Proofing the World's Largest Telescopes

In a country as volatile as Chile, engineering for earthquakes is essential. That's one reason the seventh strongest earthquake ever recorded didn't damage the 23-ton mirror of Chile's Very Large Telescope (VLT), one of the largest in the world.

In the aftermath of the massive 8.8-magnitude earthquake that left crumbled buildings, power outages, and hundreds dead, Chile's president declared a state of catastrophe.The pre-dawn quake was the most powerful to hit the country in a half century, and also unleashed a tsunami across the ocean, putting much of the Pacific Rim on alert for potentially devastating waves.

But the VLT escaped unscathed.

Built in 1988, the European Southern Observatory's aptly named Very Large Telescope isn't actually a single unit: The  VLT -- which the ESO calls the world's most advanced optical instrument -- actually consists of four individual main mirrors 27 feet in diameter and four auxiliary mirrors 6 feet in diameter. The telescopes can work together, in groups of two or three.

The telescope is situated in La Silla, while the epicenter of the earthquake was 71 miles north of Concepción. about 680 miles away. But it's not just the distance that saved the ultraprecise mirror at the heart of the telescope from damage.

The Edge of physics explains just how such a massive hunk of glass can survive an earthquake of magnitude 8.8:

Because of its weight, the mirror's precise shape can warp when it is tilted, so 150 actuators, upon which the mirror rests, continually push and pull at least once a minute to ensure that the optimal curvature is maintained.

More impressive than the actuators are the clamps around the edges of the mirror, which can, at a moment's notice, lift the entire mirror, all 23 tons of it, off the actuators and secure it to the telescope's support structure in case of an earthquake. The entire telescope is designed to swing during an earthquake, and securing the primary mirror prevents it from rattling against the metal tubes that surround it.

Despite the remarkable escape from danger, ESO officials urged astronomers slated to visit Chile to use its three telescope facilities to put their plans on hold until further notice.

"ESO expresses its deepest condolences to the families of the victims, and its sympathy and support to all those affected by the earthquake," ESO officials said in a statement. contributed to this report.