The Sharpest Images Ever Taken of Asteroid 44 Nysa Show Three Lobes Split by Valleys — and a Hidden Moon

Asteroid 44 Nysa has been known since 1857 and observed for well over a century. It took until now for anyone to see that it appears to be made of three pieces — and that it is not traveling alone.

A team led by Kate Minker of Lowell Observatory pointed adaptive optics instruments on the Large Binocular Telescope in Arizona and the Very Large Telescope in Chile at Nysa and produced the sharpest visible-light images of it ever taken. Adaptive optics measures how the atmosphere is smearing incoming light hundreds of times a second and deforms a mirror to cancel that distortion, which is what makes it possible to resolve surface features on a body in the main asteroid belt at all. The results were posted to the arXiv preprint server.

What came out was a body with "a three-lobed structure even stranger than astronomers anticipated," segmented by distinct valleys that cut the asteroid into separate sections. No other object in the asteroid belt has been documented with this shape.

The images also turned up something else: a faint, small companion in orbit around Nysa, making it a binary system. Small satellites around asteroids are no longer surprising — they are common enough that missions like NASA's DART and Japan's Hayabusa2 have been sent to systems like them — but a moon around a body with three lobes complicates the reconstruction of how the whole system came together.

The team offers two readings, and both are uncomfortable for existing models. The first is that Nysa is a contact trinary: three separate rocks orbiting each other so tightly that they came to rest touching. Contact binaries are well documented — the Kuiper Belt object Arrokoth, visited by New Horizons, is the famous example — but a stable three-body version would mean that multi-body coalescence in the belt is more common, and more durable, than anyone has assumed.

The second reading is that Nysa is one solid object with an extraordinarily irregular shape that only looks segmented. That would be no easier to explain. Asteroids of Nysa's size are generally expected to relax toward smoother, more regular forms under their own gravity and the accumulated battering of impacts over billions of years. A single body holding a deeply lobed geometry would mean asteroid interiors can support far stranger configurations, for far longer, than the models allow.

Distinguishing between the two possibilities will require better measurements of Nysa's mass and density, which is exactly what the newly found companion makes possible. Tracking the moon's orbit yields the mass of the primary directly; combined with a shape model from the adaptive-optics images, that gives density — and density is what tells you whether you are looking at a solid rock or a loosely bound pile of rubble that happens to be stuck together in three clumps.

Either answer changes something. The asteroid belt is the leftover construction material of the solar system, and how those leftovers assemble, stick and survive is the same physics that governed how planets were built in the first place. An object that does not fit the models is, for now, the most useful kind of object there is.