A large main-belt asteroid long suspected of having an unusual shape may be considerably more complex than earlier observations indicated. High-resolution images of asteroid (44) Nysa reveal two deep valleys that researchers interpret as necks connecting three separate lobes. The observations also uncovered a small satellite orbiting the asteroid.
The researchers describe Nysa as a probable binary system consisting of a highly irregular or contact-trinary primary and one separate satellite. Although the available evidence supports this interpretation, the three-lobed structure and its formation history have not yet been conclusively established. The study is currently available as an arXiv preprint.
Adaptive Optics Reveals Nysa's Shape
Nysa was observed with SHARK-VIS on the Large Binocular Telescope on February 15 and March 21, 2026. Researchers also used the SPHERE/ZIMPOL instrument on the Very Large Telescope during ten observing sessions between March 9 and April 5.
Both instruments use adaptive optics to compensate for atmospheric distortion, allowing astronomers to obtain sharper visible-light images from the ground. The observations were processed using lucky-imaging and blind-deconvolution techniques to improve the visibility of Nysa's surface features.
The team combined these resolved images with photometric light curves from several smaller observatories. The data were then processed using the All-Data Asteroid Modeling algorithm, known as ADAM, to reconstruct a three-dimensional model of the asteroid.
The resulting model gives Nysa a volume-equivalent diameter of approximately 75 kilometres, with an uncertainty ranging from 1 kilometre smaller to 3 kilometres larger. The asteroid completes one rotation in about 6.42 hours.
Evidence for Three Connected Lobes
The images show two persistent shadowed valleys positioned near Nysa's narrowest regions. These features appeared in multiple viewing geometries and seemed to connect indentations on opposite edges of the asteroid.
The researchers interpret the valleys as colli, or necks connecting separate pieces of material. Under this interpretation, Nysa consists of three lobes joined together in a contact-trinary configuration. Several other surface depressions are interpreted as impact craters.
A contact-trinary asteroid is similar in principle to a contact binary, but it contains three connected components instead of two. This proposed arrangement applies only to Nysa's primary body. The wider system is described as binary because it contains the primary asteroid and one physically separate satellite.
No colour difference greater than approximately 5% was detected between the apparent lobes in the observed filters. This surface similarity is consistent with the components having originated from the same parent body, although it does not by itself prove that they formed together.
A Satellite Roughly One Kilometre Wide
The team detected a faint point source near Nysa in Large Binocular Telescope observations from both February and March 2026. The object was recovered through two independent image-processing methods designed to separate faint companions from the bright halo surrounding the asteroid.
The source remained fixed relative to Nysa for more than three hours during each observing session while moving across the background sky with the asteroid. This behaviour allowed the researchers to rule out an unrelated stationary background object.
The satellite, designated S/2026 (44) 1, appeared at projected distances of approximately 180 kilometres and 170 kilometres from Nysa during the two observing periods. It was about nine magnitudes fainter than the primary.
Assuming that the primary and satellite reflect the same proportion of incoming sunlight, the researchers estimate the satellite's diameter at approximately 1 kilometre, with an uncertainty of about 0.5 kilometre. Its true orbit and physical properties will require further observations.
How Could Nysa Have Formed?
Nysa is one of the largest known E-type asteroids. These relatively bright asteroids are associated with enstatite-rich material, which is also found in certain meteorites and is connected to some of the building materials of the terrestrial planets.
One possible explanation is that Nysa began as a single coherent object and was heavily reshaped by impacts. However, the researchers note that repeated cratering does not easily explain the two valleys that appear to extend around the asteroid's narrow sections.
The team's preferred explanation is that Nysa consists of several fragments that merged at low velocity and remained connected. Such fragments might have been produced during a hit-and-run collision involving a larger differentiated parent body.
Another possibility is that material reassembled after a giant impact. However, a major collision at Nysa's current location would be expected to leave a detectable family of related, enstatite-rich fragments. No such extensive asteroid family has been identified, suggesting that Nysa's unusual structure may have formed before the object entered its present region of the main belt.
Future Observations Could Reveal Nysa's Interior
Tracking the satellite's orbit should allow researchers to estimate the mass and density of the Nysa system. These measurements could indicate whether the primary has enough internal strength or cohesion to preserve its unusual shape over long periods.
Further modelling will also test whether a three-lobed configuration can remain dynamically stable and whether the proposed collision and reassembly scenarios can reproduce the observed structure. For now, Nysa stands out as a rare asteroid whose external shape, possible internal construction and tiny companion may preserve evidence of an ancient and complex formation history.