Astronomers Discovered A New Class Of Super-Slow Rotating Asteroids (Planetary Science)

A team of international astronomers reported the discovery of a new class of super-slow rotating asteroids, having rotational period more than 1000 hours, in data extracted from the Asteroid Terrestrial-impact Last Alert System (ATLAS) and Zwicky Transient Facility (ZTF) all-sky surveys. Their study recently appeared in Arxiv.

Many asteroid rotation periods have already been extracted from the ATLAS photometry data set. In, previous study Erasmus and colleagues reported the shape-models and rotation periods of ∼2750 asteroids of which a few had periods around 800-900 hours and the longest period found was 1236 hours. Later, they published two more papers in which they reported the colours and rotation periods of >1000 main-belt family members and ∼ 40 Jupiter Trojans, respectively, of which the longest period found out of those two studies was 165 hours.

Now, Erasmus and colleagues presented photometry for a smaller number of asteroids (39), some observed for over 5 years in the ATLAS and ZTF surveys. However, each of these 39 asteroids has a rotation period greater than 1000 hours i.e. 42 days (min: 1013 hours; median: 2390 hours; max: 4812 hours), meaning that they have discovered a class of “super slow” rotating asteroids that has never been seen before.

Of the 39 rotation periods they reported, 32 of these objects have periods longer than any previously reported unambiguous rotation periods currently in the Asteroid Light Curve Database. In their sample, 7 objects have a rotation period > 4000 hours and the longest period they reported is 4812 hours (∼ 200 days).

“We do not observe any correlation between taxonomy, albedo, or orbital properties with super-slow rotating status.”

The most plausible mechanism for the creation of these very slow rotators is if their rotations were slowed by YORP spin-down. Super-slow rotating asteroids may be common, with at least 0.4% of the main-belt asteroid population with a size range between 2 and 20 km in diameter rotating with periods longer than 1000 hours.

Figure 1. The collisional (solid) and YORP (dashed) timescales for an object of a given size at a range of rotation periods. The red, blue and green lines indicate the timescales for an object with diameter 5, 15 and 30 km respectively © Erasmus et al.

“The forthcoming LSST will increase the number of known asteroids by a factor of five in a uniform and sparse survey that will be ideal for extending this work to more and much smaller asteroids.”

— concluded authors of the study.


1) YORP effect: In the YORP effect the body’s shape has a more effective role than albedo in altering the spin rate. For small asteroids (< 10 km), YORP can cause measurable changes in rotation rate. The effect can even speed up the rotation leading to disintegration.

Reference: N. Erasmus, D. Kramer, A. McNeill, D. E. Trilling, P. Janse van Rensburg, G. T. van Belle, J. L. Tonry, L. Denneau, A. Heinze, H. J. Weiland, “Discovery of Super-Slow Rotating Asteroids with ATLAS and ZTF photometry”, Arxiv, pp. 1-12, 2021.

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