Adaptive optics images taken with two of the largest ground-based telescopes show that (44) Nysa, a bright asteroid in the main belt between Mars and Jupiter, has a three-lobed structure and carries a previously unseen small moon. The findings appear in a paper led by Kate Minker of Lowell Observatory and colleagues, submitted to Astronomy & Astrophysics and posted on arXiv on July 28.
Nysa, discovered in 1857, is one of the largest known E-type asteroids, a rare class whose surfaces match the light-reflecting signature of the iron-free mineral enstatite. More than a century of observations, including Hubble Space Telescope measurements, radar from Arecibo and stellar occultation campaigns, hinted that the object was elongated or possibly bilobate, but its true shape stayed unresolved.
The team imaged Nysa with the SHARK-VIS instrument on the Large Binocular Telescope in Arizona on February 15 and March 21, 2026, and with SPHERE/ZIMPOL on the European Southern Observatory’s Very Large Telescope in Chile across ten epochs between March 9 and April 5. The images show two valleys that appear to wrap around the asteroid along its narrowest axis. The researchers interpret these as colli, the neck-like connections that join separate lobes in contact-binary systems, and identify three lobes in total, along with several craters and other indentations.
From the images and photometric light curves gathered at observatories including TRAPPIST-North and South, the team built a three-dimensional shape model with the ADAM algorithm. The model yields a rotation period of 6.4214175 hours, a spin pole near ecliptic coordinates of about 102.7 degrees longitude and 55.3 degrees latitude, and a volume-equivalent diameter of 75 km (47 miles), with an asymmetric uncertainty of plus 3 and minus 1 km (1.9 and 0.6 miles).
The team’s conclusion is that Nysa is most likely a binary system consisting of a primary that is either a contact trinary, three components connected at their necks, or a highly irregular coherent body, together with a small satellite. They note that Nysa is larger than any previously imaged contact binary and that its central indentation is more extreme than anything seen before, so the interpretation remains open.
The moon, temporarily designated S/2026 (44) 1, was detected independently in both observing campaigns, appearing in the residuals of blind deconvolution processing that subtracted the asteroid’s bright halo, and in a separate principal component analysis built from the exoplanet-imaging technique of angular differential imaging. It was observed at projected separations of 180 km (112 miles) in February and 170 km (106 miles) in March, with a brightness difference of about 9 magnitudes from the primary. Assuming the two objects have similar albedos, the satellite is about 1 km (0.6 miles) across, with an uncertainty of 0.5 km (0.3 miles).
The paper does not settle how the structure formed. The favored scenarios are a low-velocity reaccumulation of fragments from an ancient collision, possibly before Nysa reached its current position in the main belt, or the debris of a hit-and-run collision between larger bodies. The team considers heavy cratering reshaping less likely, because it does not readily explain the two valleys. They also note that no large enstatite-rich family surrounds Nysa, which argues against a recent in-situ shattering of a parent body.
The satellite offers a way to discriminate between these models. Tracking its orbit will let researchers measure Nysa’s mass and density, which differ under the competing scenarios: a battered core would imply a dense interior, while a coherent shard of enstatite-rich rock would be less dense. The paper states that work on the satellite’s dynamics and on the long-term stability of the system is ongoing.
Because E-type asteroids share chemical traits with the material that built the inner planets, their shapes and histories bear on how the early solar system assembled. If the three-lobed structure is confirmed, Nysa would join a small group of objects whose forms preserve evidence of ancient collisions and mergers.
Sources
- Minker et al., “Unmasking (44) Nysa: Evidence for a trilobate structure”, arXiv:2607.25786, https://arxiv.org/abs/2607.25786
- Lowell Observatory, “PRESS RELEASE: ASTEROID (44) NYSA MAY BE THE FIRST-KNOWN THREE-LOBED WORLD”, https://lowell.edu/press-release-asteroid-44-nysa-may-be-the-first-known-three-lobed-world/
- University of Arizona News, “Three-lobed asteroid is a world unlike any other”, https://news.arizona.edu/news/three-lobed-asteroid-world-unlike-any-other
- Space.com, “1st three-headed asteroid found in our solar system – and it has a little moon”, https://www.space.com/astronomy/asteroids/1st-three-headed-asteroid-found-in-our-solar-system-and-it-has-a-little-moon
- New Scientist, “Strange three-lobed asteroid seems to have its own tiny moon”, https://www.newscientist.com/article/2582275-strange-three-lobed-asteroid-seems-to-have-its-own-tiny-moon/