Extreme Kuiper Belt Object 2001 QG298 and the Fraction of Contact Binaries

Extensive time-resolved observations of Kuiper belt object 2001 QG₂₉₈ show a light curve with a peak-to-peak variation of 1.14+/-0.04 mag and single-peaked period of 6.8872+/-0.0002 hr. The mean absolute magnitude is 6.85 mag, which corresponds to a mean effective radius of 122 (77) km if an albedo of 0.04 (0.10) is assumed. This is the first known Kuiper belt object and only the third minor planet with a radius greater than 25 km to display a light curve with a range in excess of 1 mag. We find the colors to be typical for a Kuiper belt object (B-V=1.00+/-0.04, V-R=0.60+/-0.02), with no variation in color between minimum and maximum light. The large light variation, relatively long double-peaked period, and absence of rotational color change argue against explanations due to albedo markings or elongation due to high angular momentum. Instead, we suggest that 2001 QG₂₉₈ may be a very close or contact binary, similar in structure to what has been independently proposed for the Trojan asteroid 624 Hektor. If so, its rotational period would be twice the light-curve period, or 13.7744+/-0.0004 hr. By correcting for the effects of projection, we estimate that the fraction of similar objects in the Kuiper belt is at least ~10% to 20%, with the true fraction probably much higher. A high abundance of close and contact binaries is expected in some scenarios for the evolution of binary Kuiper belt objects.