Physical characteristics
2000 SY178's effective radius, when calculated from its absolute magnitude and geometric albedo, is 5.187 ± 0.171 km (3.223 ± 0.106 mi). [3] : 3 Analysis of its lightcurve, or variations in observed brightness over time, shows that it has a rapid rotation of 3.33±0.01 hours. The amplitude of 2000 SY178's lightcurve suggests that it is elongated, with an a/b ≥ 1.51. Based on its rotation period and elongated shape, 2000 SY178's inferred density is about 1.5 g/cm3 [a] —typical for C-type asteroids but denser than comets. 2000 SY178 is small enough that its current rotation period is unlikely to be primordial, suggesting that some mechanism has modified its spin. However, it is large and distant enough that it lies near the threshold where the YORP effect can significantly modify its spin. If YORP spinup is not responsible for its current rotation period, then an impact event or the sublimation of ice on its surface could explain its rapid rotation. [3] : 5–6
2000 SY178 is a dark asteroid, with a geometric albedo of about 0.0653. Spectroscopic observations reveal that it has a flat spectral slope that, in combination with its low albedo, is consistent with a C-type classification. Its color closely correlates with those of other Hygiea family members, which is primarily composed of C-type asteroids. [3] : 3–4
Activity
On 28 March 2014, 2000 SY178 was coincidentally imaged by the Cerro Tololo Inter-American Observatory's Dark Energy Camera (DECam) during a survey searching for extreme trans-Neptunian objects. Three DECam images discovered a faint tail extending about one arcminute away from the asteroid, making 2000 SY178 the thirteenth identified active asteroid in the main asteroid belt. Followup observations did not detect a coma, and by 29 August the tail had disappeared. [3] : 2, 8 2000 SY178 passed perihelion on 21 March 2013, about one year before its tail was first detected. The delay is consistent with ice sublimation with thermal lag, but the asteroid's low eccentricity and high perihelion suggest that near-perihelion activity may be coincidental. [3] : 3 2000 SY178's fast rotation, close to the point of breakup, likely plays a role in its activity. The alignment of its tail suggests that it is composed of relatively large particles, which could be the remnants of a prior outburst or material ejected by rapid rotation. The rapid rotation also likely shifts material on its surface, which could expose fresh ice that could then sublimate away. [3] : 6–7
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