2002 TD60 - a tumbling monolith

Updated 2002 December 18.

Orbit and absolute magnitude:

Amor, q = 1.103 AU, a = 1.202 AU, i = 7.4 deg, H = 19.5 (MPEC 2002-V32)

Observations and interpretation:

Lightcurve observations made:
Lenka Sarounova, Petr Pravec and Peter Kusnirak, Ondrejov, 2002 Nov. 7.1, 13.1, 14.1, 24.0, Dec. 1.9, and 11.9
Carl Hergenrother, Catalina, 2002 Nov. 7.5 and 8.5
Peter Brown and Gil Esquerdo, University of Western Ontario, 2002 Nov. 8.3, Dec. 3.2, and 7.3
Gianluca Masi, Claudia Belmonte, and Franco Mallia, Campo Catino, 2002 Nov. 11.0
Adrian Galad, Leos Kornos, Modra, 2002 Dec. 8.0 and 8.9

The observations revealed a complex lightcurve with two periods, P1 = 2.00746 +/- 0.00005 h and P2 = 2.8507 +/- 0.0002 h (fit to the 2002 Nov. 7.1-14.1 data; see Pravec et al. 2002c). The lightcurve shape is fitted with the two-dimensional Fourier series with most signal in the terms with 2*f1, 2*f2, and (2*f1-2*f2), where f1=2*pi/P1 and f2=2*pi/P2. The lightcurve is shown in Fig. 1. Such lightcurve behaviour indicates that the asteroid is in a non-principal axis rotation state (see, e.g., Kaasalainen 2001, and references therein). The large amplitude of the lightcurve (1.4 mag at the phase angle 7 deg --- a phase-corrected amplitude is 1.2 mag at 0 deg according to the empirical formula by Zappala et al., 1990) indicates that the body is quite elongated with a/c about 3. The fast tumbling elongated body cannot be held together by self-gravitation only and must be a coherent body with non-zero tensile strength for a plausible asteroidal bulk density <8 g/cm^3. The damping timescale for a rigid rock of the size and spin rate of 2002 TD60 is estimated to be on the order of 10^8 yr (A. W. Harris, personal communication), i.e., comparable to the collisional lifetime of asteroids of that size. While the size is not well known yet, its absolute magnitude (H = 19.5) suggests that it is about 0.4 km for p=0.18 (typical for S-type asteroids). Coherent bodies are rare among objects with D > 0.2 km; most of the bodies in that size range have negligible tensile strength as derived from the statistics of their spin rate (Pravec et al. 2002a, and references therein). In fact, 2002 TD60 is apparently the second largest coherent asteroid known, the largest one being 2001 OE84 (Pravec et al. 2002b).

References:

Kaasalainen, M. 2001. Interpretation of lightcurves of precessing asteroids. Astron. Astrophys. 376, 302-309.
Pravec, P., A. W. Harris, and T. Michalowski 2002a. Asteroid Rotations. In Asteroids III (Eds. W. F. Bottke, A. Cellino, P. Paolicchi, and R. P. Binzel), Univ. of Arizona Press, pp. 113-122. Reprint available here.
Pravec, P., P. Kusnirak, L. Sarounova, A. W. Harris, R. P. Binzel, and A. S. Rivkin 2002b. Large coherent asteroid 2001 OE84. In Proceedings of the Asteroids, Comets, Meteors 2002 Conference. Preprint available here.
Pravec, P., L. Sarounova, C. Hergenrother, P. Brown, G. Esquerdo, G. Masi, C. Belmonte, F. Mallia, and A. W. Harris 2002c. 2002 TD_60. IAU Circ. 8017
Zappala, V., A. Cellino, A. M. Barucci, M. Fulchignoni, and D. F. Lupishko 1990. An analysis of the amplitude-phase relationship among asteroids. Astron. Astrophys. 231, 548-560.

Fig. 1: Lightcurve data. The curves are the best-fit two-dimensional Fourier series to the 2002 Nov. 7.1-14.1, Nov. 24.0-Dec. 3.2, and Dec. 7.3-11.9 data, respectively.

Fig. 2: Data folded with the period of 2.008 h. The interplay with the second period causes that the data cannot fit together well. This figure is just to illustrate the behaviour of the lightcurve in the form most observers are used to.

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