Extreme Kuiper Belt Object 2001 QG 298 and the Fraction of Contact Binaries

Abstract

Extensive time-resolved observations of Kuiper belt object 2001 QG 298 show a light curve with a peak-to-peak variation of 1.14 AE 0.04 mag and single-peaked period of 6.8872 AE 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 (BÀV = 1.00 AE 0.04, VÀR = 0.60 AE 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 QG 298 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 AE 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.