Borisov's discovery invited an irresistible comparison - to Clyde Tombaugh, the amateur astronomer and self-taught telescope builder who found Pluto in 1930. Tombaugh, though, made his find on the professional astrograph of Lowell Observatory. Borisov did something arguably harder: he used instruments he designed and constructed himself. Over the years he built a small fleet of telescopes with names like GENON and GENON Max, and it was his 0.65-meter HGB-650 that caught the interstellar comet. Born in 1962 in Kramatorsk, Borisov works as an engineer at the Crimean Astronomical Station of Moscow State University's Sternberg Astronomical Institute, and pursues his own hunt for comets in his spare time, from a personal observatory he calls MARGO.

What made 2I/Borisov extraordinary was not that it was a comet - Borisov had found others - but where it came from. Its path through space was too steep, too fast, and too open to be bound by the Sun's gravity. It was passing through, not orbiting: a chunk of ice and dust flung from some other planetary system, drifting for who knows how many millions of years before threading briefly through ours. Only one interstellar object had ever been identified before it, the strange, cigar-shaped 'Oumuamua in 2017. But 'Oumuamua looked like a bare rock. Borisov's find behaved like a proper comet, growing a glowing coma and a tail, giving astronomers their first real chance to study material from beyond the solar system as it warmed in our Sun's light.

The interstellar comet was the crown, but Borisov's watchfulness has caught more than one object streaking near home. On 21 January 2023, he spotted a small asteroid, 2023 BU, just five days before it swept past Earth - closer than the ring of geostationary satellites, though still above the low-orbit spacecraft. It was one of the nearest approaches by a known asteroid ever recorded, the kind of fleeting, fast-moving target that rewards a patient observer with a homemade telescope and a dark sky. Between 2013 and 2019 alone, Borisov turned up nine comets, along with several near-Earth objects - a tally that would be the envy of many a professional facility.

Borisov is clear-eyed about the future of his craft, and a little wistful. "In 2016, only I discovered a comet," he has said. "In 2013, there were seven of us. Every year there are less and less. There are more and more huge telescopes. Amateurs will soon have nothing left." Vast automated surveys now sweep the sky faster than any lone observer can, hoovering up new objects before a human eye ever settles on them. That an amateur, at his own instrument, could still beat those machines to the single most surprising comet of the century is precisely what makes the story of 2I/Borisov linger. It may be among the last discoveries of its kind.