Spectroscopic observations of unbound asteroid pairs using the WHT

Recently over 62 pairs of asteroids have been shown to have very similar orbital elements. Backward integration of their orbits indicates that the asteroids in each pair likely had very close encounters at low relative velocities, consistent with models of the spin-up and rotational fission of asteroids. Although linked dynamically, the observation of highly similar spectra would suggest that the asteroids share a common composition, which we would expect if they formed from a common parent body. We have begun an observational campaign whose aim is to obtain optical and/or NIR spectra of a large sample of these unbound asteroid pairs to determine whether the asteroids in each pair exhibit similar spectra. We present optical spectroscopic observations of four complete pairs obtained using the William Herschel Telescope. We find that the components of pairs 1979-13732 and 19289-278067 share very similar spectra and likely have a common origin. Our current spectra of 17198-229056 are sufficiently different to suggest that they do not have a common origin, although this is contrary to the strong dynamical linkup of these asteroids demonstrated in the current paper and previous studies. Further observations of this pair are encouraged to examine why the spectra are so different. It is unclear whether the spectra of the final pair, 11842-228747, are a match due to the low S/N of the secondary's spectrum. We discuss the process of space weathering and present new dynamical analyses which confirm the previously estimated ages of the observed pairs. The time-scale for space weathering appears to be longer than 1 Myr for at least some pairs. We also present an efficient method which can be used to determine the positional convergence of unbound asteroid pairs.