The telescope of the University of Siena Observatory reveals that the asteroid (7132) Casulli is a binary system

Wednesday, March 25, 2020
1,9 km satellite discovered orbiting around asteroid Casulli. It's a new minor body of the solar system
Rappresentazione artistica di un asteroide binario (fonte NASA)

(7132) Casulli is a main belt asteroid discovered in 1993 Sept. 17 by A. Vagnozzi at Stroncone (Italy) and is named in honor of Silvano Casulli (1944 - 2018), who was the first amateur astronomer in the world to obtain precise astrometric positions of minor planets using a CCD camera (Minor Planet Circular 30800).  It orbits with a semi-major axis of about 2.309 AU, eccentricity 0.209, and a period of 3.51 years; it has an absolute magnitude H = 13.6 , a diameter D = 9.015 ± 0.064 km and a geometric albedo of  0.089 ± 0.003 (JPL Small-Body Database Browser -, 2020; Masiero et al., Main-belt Asteroids with WISE/NEOWISE: Near-infrared Albedos, Astrophys. J., 791, 2014).


Collaborative photometric observations carried out from 2020 February 20 to 2020 March 20 inside the Asteroids Research Section of the UAI (Italian Amateur Astronomers Union) reveal that the minor planet (7132) Casulli is a binary system with an orbital period of 36.54 +/- 0.02 h. 
The rotational light curve of the primary has a period of 3.5238 +/- 0.0002 h with an amplitude of 0.14 mag, suggesting a nearly spheroidal shape. Mutual eclipse/occultation events that are 0.11 to 0.16 mag deep indicate a secondary-to-primary mean-diameter ratio of 0.21 +/- 0.04.



We forgot to mention in the ATel #13590 that Raoul Behrend (Observatoire de Genève) and collaborators already guessed the binary nature of (7132) Casulli on the basis of their observations in 2013 (as showed in this web page) although they could not conclusively solve the system.

We also acknowledge that the binary nature of (7132) Casulli was almost contemporarily guessed within the BinAst collaboration via independent observations and analysis (Petr Pravec, private communication).


The phase plots for the primary (rotational) and secondary (orbital) periods of the system are available below.