Saturn's 100th moon was spotted in 2004 — but it took astronomers 19 years to prove the 2-km speck was really orbiting the planet.
Saturn's 100th announced moon, a 2-kilometer speck first glimpsed by Japan's Subaru Telescope in 2004, wasn't confirmed as a real moon until 2023 — a 19-year gap between the first image and the announcement.
The object now catalogued as S/2004 S43 showed up on the Subaru Telescope's detector in 2004. It took until May 2023 for anyone to say, with confidence, that it was a moon of Saturn. Nineteen years passed between the first pinprick of light and the announcement — not because astronomers lost the file, but because proving that a faint dot belongs to a specific planet is genuinely hard.
Why it took 19 years to confirm one moon
The problem is scale. According to the Subaru Telescope's own release from the National Astronomical Observatory of Japan, S/2004 S43 is only about 2 kilometers across and glows at roughly 26th magnitude — millions of times fainter than the dimmest star your eye can catch on a dark night. At that brightness, a single image tells you almost nothing. You have a smudge. Is it a background galaxy? An asteroid drifting through the frame? A cosmic-ray hit on the sensor? Or a genuine moon looping around Saturn?
The only way to know is to catch it again and again, tracking how it moves relative to the planet over years. A real satellite traces a closed, repeating orbit; an interloper does not. Scott Sheppard's team, who first observed the object with Subaru's Suprime-Cam (the Subaru Prime Focus Camera), needed additional years of follow-up observations to pin down that orbit. Only once the path resolved into a stable loop around Saturn could the object be certified as a moon rather than a passing rock.
That certification came in a burst. The University of British Columbia announced 62 new Saturnian moons on May 16, 2023, in work led by Edward Ashton and Brett Gladman. As Jeff Hecht reported for *Sky & Telescope*, the Minor Planet Center's announcements from that batch pushed Saturn's tally to 145 known moons — more than doubling its count of small, distant "irregular" satellites and vaulting the planet past Jupiter in the moon race. S/2004 S43 was one of the objects that carried Saturn across the 100-moon line.
How you find something this faint
The technique behind these discoveries is called "shift and stack." Because a 2-kilometer moon is far too dim to register in any single exposure, astronomers take a long series of images, then digitally shift each one to follow the predicted motion of an object at Saturn's distance and stack them together. The moon's light, spread thinly across dozens of frames, piles up into a detectable point, while the smeared trails of stars and galaxies wash out. It's the difference between hearing a whisper once and hearing it repeated a hundred times until you're sure of the word.
The vivid detail worth holding onto: the 2004 data already contained this moon. The photons had been recorded, sitting in an archive, for nearly two decades before the follow-up work confirmed what they were. The discovery didn't require new light — it required patience and enough return trips to the same patch of sky.
That's also why the number keeps climbing. Saturn's officially confirmed count reached 146 moons per the International Astronomical Union's Minor Planet Center, and there's every reason to expect it will keep rising. These small irregular moons are thought to be fragments of larger bodies shattered by ancient collisions, which means the debris field around Saturn may hold many more objects still below the threshold of confirmation.
And here's the seam. When a "moon" is a 2-kilometer chip of rock on a distant, wobbling orbit, the word starts to strain. There is no agreed lower size limit for what counts as a moon versus a mere ring particle or captured boulder. If Saturn has hundreds of kilometer-scale fragments waiting in its archives, the real open question isn't how many moons Saturn has — it's what we should still be willing to call one.