  Following its flyby and first imaging the Pluto-Charon binary, the New
Horizons spacecraft visited the Kuiper-Belt-Object (KBO) (486958) 2014 MU69
(Arrokoth). Imaging showed MU69 to be a contact-binary, made of two individual
lobes connected by a narrow neck, rotating at low spin period (15.92 h), and
having high obliquity (~98 deg), similar to other KBO contact-binaries inferred
through photometric observations. The origin of such peculiar configurations is
puzzling, and all scenarios suggested for the origins of contact-binaries fail
to reproduce such properties and their likely high frequency. Here we show that
semi-secular perturbations operating only on ultra-wide (~0.1-0.4 Hill-radius)
KBO-binaries can robustly lead to gentle, slow-speed binary mergers at
arbitrarily high obliquities, but low rotational velocities, that can reproduce
MU69's (and similar oblique contact binaries) characteristics. Using N-body
simulations, we find that ~15% of all ultra-wide binaries with cosine-uniform
inclination distribution are likely to merge through this process. Moreover, we
find that such mergers are sufficiently gentle as to only slightly deform the
KBO shape, and can produce the measured rotation speed of MU69. The
semi-secular contact-binary formation channel not only explains the observed
properties of MU69, but could also apply for other Kuiper/asteroid belt
binaries, and for Solar/extra-solar moon systems.
