2001 QW322: Antipholus and Antipholus

Discovered by the CFEPS team on 24 August, 2001, Kuiper Belt binary 2001 QW322was immediately recognized as an exceptional object because the measured separation of 125,000 km (about one third of the distance from Earth to the Moon), far larger than any other small body binary.

two images of 2001 QW322

Two images of the binary system 2001 QW322.

The two components have nearly identical brightness, implying essentially equal size/mass.

The large separation implies a mutual-orbit period of more than ten years.  Most small body binaries have orbits of a few months to years in length. Six years of intense tracking by the CFEPS team and our collaborators, using 4-to-8 m class telescopes, revealed that 2001 QW322 has a nearly circular (e < 0.4) mutual-orbit with a separation of 105,000–135,000 km – greater than any other known minor-planet binary – and a period in the range 25–30 years.

Observations from VLT and Gemini observatories confirmed that the two components have essentially the same brightness and both components are unusually blue in colour (suggesting an exposed ice surface).  The colour similarity implies a surface similarity, meaning equal surface reflectivity (albedo) for the two components. The most numerous observations in the red filter give a magnitude m_{r’} = 23.7, with an estimated difference of  1 -to- 5%. This is currently the smallest measured brightness difference between components of a binary system in the Solar System. Using our measurement of the brightness and the known distance to the binary, 43.4 AU, we derive a radius of 54 km, based on an assuming a reflectivity of 16% (typical of small objects in the outer solar system).

The heliocentric orbit of 2001 QW322 makes it part of the cold main classical Kuiper Belt, a population that has been shown to contain a large fraction of binaries. However, its color is on the blue extreme of the color distribution of that belt’s dynamical sub-class. It could then be a low-inclination end-member of the inclination distribution of the hot Kuiper Belt. This latter possibility raises questions on the possibility to retain such a wide binary during the excitation of the hot Kuiper Belt.

In the current collisional environment of the Kuiper Belt, the lifetime of 2001 QW322due to interaction with interlopers is 0.3-1 Gyr. Its formation requires a much denser Kuiper Belt than the present one, thus this binary is certainly primordial. This implies one of two things: (1) Either 2001 QW322 was created with its current mutual-orbit early in the history of the Solar System, in which case it is one of the few survivors of a population at least 50-100 times larger; (2) Or this is a transitory object, evolving, due to perturbation from interactions with smaller KBOs, from a population of more tightly bound binaries. Asserting this latter hypothesis would require better orbital statistics for moderately large KB binaries (separation of 1-2″).