Figure 1: Artist’s impression of how a Kuiper Belt Object might look like.
2001 QW322 is a binary Kuiper Belt Object (KBO). It lies beyond the orbit of Neptune, roughly 6 billion kilometres from the Sun. What makes 2001 QW322 extreme is the remarkably huge separation between its two roughly equal-sized components - nicknamed Antipholus and Antipholus. Both objects are estimated to have diameters of roughly 100 kilometres, and are separated by a whopping ~125,000 kilometres (about one-third the Earth-Moon separation distance), roughly 10 times the separation of other near-equal-mass binaries. Antipholus and Antipholus are so far apart that both objects take ~25 to 30 years to complete one orbit around each other.
This system is the most tenuously bound pair of objects known in the Solar System. Such a tenuously held binary is difficult to create and maintain. Since this system is prone to disruption by massive interlopers, it is estimated to remain bound for no more than ~1 billion years. Antipholus and Antipholus circle around each other with an average orbital speed of just ~0.85 m/s or ~3 km/h, comparable to a slow human walking pace. On any one of the two Antipholuses, the surface gravity is predicted to be ~600 times weaker than on Earth. From the surface of one component, the existence of the other component would be visually perceptible since the other component would appear as large as a pinhead held at arm’s length and can appear as bright as Saturn seen from Earth.
Figure 2: Secondary-to-primary mass ratio versus average separation (in units of the primary’s radius). The dashed box represents the known binary asteroids; all on the left side of the plot (the largest separation barely exceeds 100). Also shown are the most extreme, outer-planet irregular satellites and several other binary KBOs. 2001 QW322 clearly stands out in the top-right corner of the diagram as the widest near-equal-mass binary.
Petit et al. (2008), “The Extreme Kuiper Belt Binary 2001 QW322”, Science 322 (5900): 432-434