Binary NEA (66063) 1998 RO1

Updated 2003 October 6.


Aten, q = 0.277 AU, a = 0.991 AU, i = 22.7 deg (MPO 48545)

Lightcurve observations made from Ondrejov Observatory during 2003 Sept. 16.9 to 27.9, from Carbuncle Hill Observatory during Sept. 21.1 to 22.2, and from Elginfield Observatory of University of Western Ontario on Sept. 21.2 revealed that 1998 RO1 is an asynchronous binary asteroid as we suspected from observations made from Ondrejov and Elginfield in September 2002 (see here for the early announcement.).

Estimated parameters of the binary system:

Primary's rotation period (synodic): 2.493 h (error <0.001 h)

Amplitude of the primary's rotation lightcurve: 0.12 mag. It indicates that the primary's shape is slightly elongated with equatorial axes ratio about 1.1 (assuming we had an equator-on aspect).

Orbital period (synodic): 14.53 +/- 0.01 h

The secondary-to-primary mean diameter ratio lower limit is 0.5. The small variation seen at the bottom "plate" of the long-period component's secondary attenuation event (between orbital phases 0.2 and 0.3 in the figure below) in the best nightly lightcurves indicates that the occultation/eclipse events were not quite total, so a true size of the secondary may be greater than that derived from the depth of the secondary attenuation event.

Amplitude of the long-period lightcurve component outside the eclipse minima: 0.11 mag. It is interpreted as being caused by a synchronous rotation of the secondary that is somewhat elongated (axes ratio estimated ~1.5). The secondary showed a maximum brightness at times about in the middle between the occultation/eclipse events, at the mean anomaly about 90/270 deg. (I define the mean anomaly to be 0/180 deg at the times of the occultation/eclipse events.) So, the secondary's long axis is approximately aligned with the primary-secondary vector.

Semi-major axis of the mutual orbit - 1.7 primary diameters, assuming primary's bulk density of 2 g/cm3.

Eccentricity - small; to be constrained later on.

Absolute R magnitude (H_R): 17.6 +/- 0.1

The estimated parameters of the binary system are strikingly similar to parameters of other known binary NEAs.

Participating people :

Petr Pravec, Peter Kusnirak, Lenka Sarounova, Ondrejov Observatory
Peter Brown, Gilbert Esquerdo, Nicole Kaiser, University of Western Ontario
Don Pray, Carbuncle Hill Observatory

Fig. 1: Lightcurve data from 2003 Sept. 16.9 to 21.9 folded with the orbital period. The data include rotational variations of the two bodies as well as their mutual eclipse/occultation attenuation events.

Fig. 2: The same data as in Fig. 1 but with the primary's rotational component removed. They show a rotational variation of the elongated secondary (fitted with the curve) and mutual eclipse/occultation attenuation events between the two components.

Check of the 2002 data with the periods derived above shows a consistency despite the poor orbital phase coverage of the 2002 data, see here.

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