Few things in science appear to be as delicate or precarious as the giant mirrors at the hearts of modern telescopes. These mirrors — doughnuts of glass meters in diameter, weighing tons and costing millions of dollars — are polished within a fraction of a wavelength of visible light into the precise concavity required to gather and focus starlight from the other end of the universe.

When not at work, they are sheltered in lofty domes that protect them from the distortions of humidity, wind and changes in temperature. But this cannot shield them from all the vicissitudes of nature and humanity, as I was reminded on a recent visit to the Las Campanas Observatory in Chile.

As my hosts showed off one of their prized telescope mirrors — 20 feet of shiny, immaculately curved aluminum-coated glass — I couldn’t help noticing a small, suspicious smudge. It looked like the kind of smear you might find on your windshield in the morning, especially if you had parked under a tree.

“Birds,” one astronomer grumbled when asked what it was.

It happens all the time, other astronomers say. Michael Bolte, now an emeritus professor at the University of California, Santa Cruz, recalled giving the governor of Wyoming a tour of the Wyoming Infrared Observatory, outside Laramie, in 1981. “We went up on the service platform and looked down, and there were bird droppings all over the mirror,” he said. “It looked awful.”

It’s not only birds that can deface a mirror. Mike Brotherton, the current director of the Wyoming observatory, posted a picture on Facebook of frost that had accumulated on his mirror while the dome was open for observation. “It’s hard to keep a mirror pristine,” he said. “It’s a balance between opening to take data and protecting the mirror.”

Bird residue has a special place in astrophysical lore. In the early 1960s, the radio astronomers Arno Penzias and Robert Wilson, both then at Bell Labs, were trying to calibrate an old horn antenna to study galaxies. In an effort to get rid of a persistent background hum, they shoveled vast amounts of pigeon guano out of their telescope, only to eventually learn that the hum was cosmic: It was the hissing remains of radiation from the Big Bang, and it firmly settled the question of whether the universe had a distinct beginning.

Luckily, such biodegradable insults to the mirrors are temporary and don’t block much light. Observatories periodically wash their mirrors, strip off the old aluminum coatings and apply a fresh layer, which involves removing the mirror from the telescope.

That can be a ticklish operation. Last fall, the 8-meter-diameter primary mirror of the Gemini North telescope, on Mauna Kea in Hawaii, was nicked on its outer edge while being moved for cleaning and recoating. The damage was not to the part of the mirror that collects light, but the telescope’s managers opted to repair it anyway. On March 31, Jen Lotz, the observatory director, reported that the repairs were complete and that the telescope, she hoped, would be back in operation sometime in May.

Some things are less easy to fix. On Feb. 5, 1970, a new employee at the McDonald Observatory in West Texas took a gun to work and opened fire, first at his boss and then several times point-blank at the primary mirror of the observatory’s new 2.7-meter reflecting telescope. Then he went at it with a hammer.

Preliminary reports indicated that the mirror had been destroyed; when the sheriff had arrived, he had noted that it had a big hole in it. In fact the mirror, of a common type called Cassegrain, was designed and built with central holes to permit light to pass through to instruments behind it.

Nobody was hurt during the assault. And aside from seven small bullet holes, which affected only about 1 percent of the mirror’s surface area, the telescope was virtually unscathed.

“The telescope resumed its observing program the following night,” the observatory’s director, Harlan Smith of the University of Texas, reported to the International Astronomical Union soon after, “producing some of the best photographs (of quasar fields) so far obtained with this instrument in its first year of use.”

Which is to say, telescope glass is tougher than you think. When I first visited the 200-inch Hale Telescope on Palomar Mountain in California — a rite of passage for a young science writer — I was startled to discover, looking down the barrel of what was then the world’s largest and most famous telescope, a dinner-plate-size gash left by a tool that a worker had dropped years earlier.

Dr. Bolte described a close call at the Canada-France-Hawaii Telescope on Mauna Kea. He and a colleague were up in the dome, working on a camera in the telescope, when they noticed that the covers that normally protected the mirror were open. They managed to radio down to the floor and get the covers closed.

“We did whatever we were going to do, and were getting ready to come down,” Dr. Bolte wrote in a Facebook conversation. “You counted all the tools you took to the prime focus cage and made sure the count on the way up matched the count on the way down. Just as I was saying to Bob, ‘I think we are one tool short,’ a big crescent wrench fell out of the cage and made an incredible racket, smacking the mirror cover.”

The most famous example of what can go wrong with a mirror occurred in 1990, when the Hubble Space Telescope was launched with a misshapen mirror that could not focus.

Astronauts were able to fix it, and Hubble is still going strong. But the episode led NASA to be extra cautious with Hubble’s successor, the James Webb Space Telescope, scheduling extensive tests that vastly increased the telescope’s cost and construction time.

The Webb was launched spectacularly and successfully on Dec. 25, 2021, but space is a shooting gallery, too. The telescope had barely set up shop when it was pelted by a larger-than-expected micrometeorite, which left a tiny crater in one of the telescope’s mirror segments. NASA has since modified its protocols to minimize the amount of time that the telescope is aimed into meteor streams.

And so it goes. The cosmos has a way of guarding its secrets.