It was one of the most important astronomical discoveries of the twentieth century, and it became one of the more controversial when only one of the discoverers received a Nobel prize. Now a fascinating new footnote has been added to the story of how pulsars were discovered with the revelation that some had previously been observed by a US Air Force staff sergeant at a remote Alaskan outpost.

Earlier this month, 81-year-old Charles Schisler came forward to tell the story of how he used a military radar to identify around a dozen radio sources, some of which were pulsars. Astronomers who have seen Schisler's meticulous logs believe that he spotted a bright pulsar in the nearby Crab Nebula months before the first scientific observation of a pulsar was published in Nature (A. Hewish et al. Nature 217, 709–713; 1968). Although Schisler never knew exactly what he was seeing, the story should be counted as an early pulsar spotting, says Jocelyn Bell Burnell, an astronomer at the University of Oxford, UK, and one of the authors on the original paper. “He happened to be a very observant person,” Bell Burnell says.

Schisler's story began in the summer of 1967 during a mind-numbing four-hour shift on an early-warning radar at Clear Air Force Station in Alaska. He was using the Ballistic Missile Early Warning System, a massive set of radars that looked some 4,800 km across Siberia for incoming warheads. As he sat at his station, the bored, 41-year-old staff sergeant noticed a faint signal on his scope. During the course of the summer, the source continued to show up. “I kept seeing it week after week,” he says.

An early-warning radar in Alaska picked up pulses from the Crab Nebula (left) before astronomers did. Credit: BETTMANN/CORBIS

Then one day Schisler noticed something — the mysterious blip appeared 4 minutes earlier than the day before. Four minutes meant a lot to the airman: before being stationed at Clear, he had been a navigator on a B-47 bomber, and he knew that stars rise 4 minutes earlier each night as a result of Earth's motion around the Sun.

Schisler calculated the source's approximate position in the sky and wrote it down on a scrap of paper. Over the weekend, he drove 125 km to the University of Alaska at Fairbanks, where he met an astronomy professor who showed him the location of his source — the Crab Nebula, some 6,300 light years from Earth. At the centre of the Crab, a supernova remnant, there sits a bright pulsar.

Schisler returned to Clear with the coordinates of other radio sources that he thought he might be able to detect with the radar. Throughout the late summer and early autumn, he began a meticulous log of anything he could spot on the scope. Because the radar was designed to pick up man-made pulses bouncing off incoming missiles rather than steady signals, Schisler believes most of the things he saw were pulsars. By his own count, he spotted about a dozen sources. “My commanders didn't know what the hell I was doing,” he recalls.

The work preceded by several months the observations made by Bell Burnell, then at the University of Cambridge, UK, which led to the first paper on the subject. A Nobel prize for the discovery was subsequently awarded to her supervisor Antony Hewish, but, controversially, not to her. Schisler was not the only one to “pre-discover” a pulsar, though, according to Bell Burnell. “There are actually a lot of stories,” she says. In the 1950s, a woman visiting the observatory at the University of Chicago, Illinois, pointed out that there was a regularly pulsating source of visible light in the Crab Nebula. Elliot Moore, an astronomer at the university, dismissed the woman's claim, telling her that all stars seem to flicker. Another radio astronomer she knows of will, after a drink or two, confess to having dismissed observations of a pulsating source as the result of faulty equipment. “He's a bit embarrassed now,” says Bell Burnell.

For his part, Schisler says he never quite understood what he was looking at until he heard of Hewish and Bell Burnell's discovery on a short-wave radio. When he learned that they had discovered a pulsating radio star, Schisler says, the significance of his own work became clear. But he says he didn't dare speak about the log until nearly half a century later, when the old early-warning system at Clear was finally decommissioned. He says that he feels he deserves no credit for his work, but he still regrets that he was unable to share what he had seen. “I wish we had had a way to communicate with the scientific community,” he says.