The history of science is filled with lucky breaks, cases where researchers have stumbled across something unexpected that ended up leading to a major breakthrough—it was a scientist, after all, who said "chance favors the prepared mind." But a perspective published today in Science brings the year of astronomy to a nice end by discussing just how often luck has played a role in the progress of that field.

One of the stories, of course, is Galileo—it's the anniversary of his first use of a telescope. But it's easy to forget that the first instrument wasn't actually intended to be a telescope at all; instead, it was a spyglass that was expected to find use as an instrument of war. Nor was Galileo especially intent on completely upsetting Europe's view of its place in the Universe when he pointed the spyglass at the skies. That just happened to be a side effect of seeing comets and the moons of other worlds for the first time.

The perspective does tread some familiar ground, discussing the accidental discovery of the cosmic microwave background by Bell Labs researchers who initially considered it annoying background noise. In that case, the prepared minds were at a completely different institution, and they correctly interpreted the noise in a paper that was published separately from its description.

But what's stunning is a catalog of just how common this sort of event has been. Herschell was looking for faint stars when he happened across the planet Uranus, while Piazi was simply creating a star catalog when he observed the object that turned out to be the first asteroid to ever be described, Ceres I.

Someone I'd never heard of, Vesto Slipher, obtained critical data while looking at something that the entire field had completely misidentified. Other galaxies had once been thought to be what we'd now call a proto-planetary disk, with the central bulge being the star and the spiral arms a collection of planet-forming dust. Slipher was attempting to get some measurements of the arms when he discovered that they were moving faster than anything ever previously identified, far faster than dust could orbit a star. It was an essential step on our road to recognizing an expanded and expanding Universe.

Technology has also played a key role. Radio astronomy might have taken decades longer to catch on if it hadn't been for the rapid development of radar technology during the second world war. X-rays don't penetrate the atmosphere, so the cold-war era space race was critical in enabling us to start high-energy astronomy.

Even then, luck played a role. Researchers were trying to look for X-rays originating on the Moon (they were expected to be induced by solar radiation) when their instruments picked up the first sources of X-rays outside our solar system. Pulsars were found by researchers looking for fluctuations driven by the solar wind, while the first cosmic gamma-ray sources were picked up by sensors designed to spot nuclear tests. All of these sources have also turned out to be a playground for theoretical physicists, given that they appear to involve exotic objects like black holes, neutron stars, and gravity waves.

A few other examples don't even make the cut for the article, like the discovery of the first exoplanets. My understanding (which a reader now tells me is in error) is that astronomers found them while looking for binary star systems by scanning stars for periodic orbital wobbles. They just happened to do tests that were sensitive enough to pick up smaller objects. I'm also not sure if anyone was even talking about dark energy when the WMAP probe was designed, but its detailed look at the cosmic microwave background apparently contains evidence that about three-quarters of the stuff in the Universe appears to be dark energy. Nobody expected the Voyagers to spot volcanoes on Io, either.

(This sort of thing is hardly unique to astronomy, either. An expedition to the deep sea rift zones was supposedly so completely unprepared to find thriving ecosystems that they had to raid the ship's vodka supply to preserve the samples.)

In any case, the article has helped me end the Year of Astronomy with a profound sense of excitement. We've currently got some absolutely superb single-purpose instruments in orbit, like the planet hunters CoRoT and Kepler, that appear to be performing their expected functions extremely well. But the Universe has been full of surprises, and there seems to be a better-than-ever chance that one of those observatories will stumble onto something they weren't designed to see. I can't wait to find out what it is.

Science, 2009. DOI: 10.1126/science.1183653