Superfast rotator 2001 OE84 - a large monolithic body


Amor, q = 1.201 AU, a = 2.279 AU, i = 9.4 deg (MPEC 2001-U02)


This Amor asteroid has been observed by P. Kusnirak and P. Pravec on two nights of 2001 Oct. 15.1 and 16.0. The observations revealed a synodic rotation period of (29.190 +/- 0.002) min. The rotation is so fast that the object cannot be held together by self-gravitation and it is a body with a non-zero tensile strength (see Pravec et al. 2000, Pravec and Harris 2000). The composite lightcurve ( Fig. 1) has an amplitude of 0.60 mag (at phase angle 25 deg) that indicates an elongated figure with equatorial axis ratio > = 1.4.

Figure 2 shows the observed lightcurve of the second observational night. A sampling was one point per 140 seconds during most of the time (130-sec integration time plus a 10-sec readout), therefore consecutive points fall on fairly different rotation phases.

Figure 3 shows spin rate vs diameter plot for 987 asteroids of all sizes (updated from Pravec et al. 2001). With its estimated size of about 0.9 km (using H=17.7 from MPEC 2001-U02 and assuming a geometric albedo of 0.18 typical for near-Earth asteroids), 2001 OE84 is an exceptional object - all other known superfast monolithic rotators are objects less than about 0.15 km diameter. All other asteroids greater than ~0.15 km with known periods are below the critical rotation "barrier" of 12 rev/day (period > 2 h) that is an evidence that most of them are strengthless objects, so-called "rubble piles" (e.g., Pravec and Harris 2000). 2001 OE84 is the largest known asteroid with significant tensile strength, the first known in the km size range. It is an evidence that coherent bodies exist also among asteroids as large as ~1 km.

Fig. 1:

Composite lightcurve of 2001 OE84. The curve is the best-fit 7th-order Fourier series. A smoothing of the lightcurve due to the finite integration time (130 sec) is small; a true amplitude was greater by ~0.05 mag than the observed one.
2001 OE84 - composite lightcurve

Fig. 2:

Lightcurve of 2001 OE84 taken around October 15.96. The filled circles are the observed data, the curve is the 7th-order Fourier series expansion.
2001 OE84 - lightcurve of Oct. 15.96

Fig. 3:

Asteroids' spin rate vs diameter plot. 2001 OE84 is an exceptional object, a superfast rotating monolithic body in the size range dominated by strenghtless "rubble piles" that all lie below the critical spin "barrier" of 12 rev/day (period 2 h) plotted as the horizontal dashed line.
asteroids spin rate vs diameter


Pravec, P., and A. W. Harris 2000. Fast and slow rotation of asteroids. Icarus 148, 12-20.
Pravec, P., C. Hergenrother, R. Whiteley, L. Sarounova, P. Kusnirak, and M. Wolf 2000. Fast rotating asteroids 1999 TY2, 1999 SF10, and 1998 WB2. Icarus 147, 477-486.
Pravec, P., A. W. Harris, and T. Michalowski 2001. Asteroid rotations. Chapter submitted to the Asteroids III book.

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