Following its flyby and first imaging the Pluto-Charon binary , the New Horizons spacecraft visited the Kuiper-Belt-Object ( KBO ) ( 486958 ) 2014 MU _ { 69 } ( Arrokoth ) .
Imaging showed MU _ { 69 } 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 ( \sim 98 ^ { \circ } ) [ 1 ] , similar to other KBO contact-binaries inferred through photometric observations [ 2 ] .
The origin of such peculiar configurations is puzzling , and all scenarios suggested for the origins of contact-binaries [ 3 , 4 , 5 ] fail to reproduce such properties and their likely high frequency .
Here we show that semi-secular perturbations [ 6 , 7 ] operating only on ultra-wide ( \sim 0.1 - 0.4 Hill-radius [ 8 ] ) KBO-binaries can robustly lead to gentle , slow-speed binary mergers at arbitrarily high obliquities , but low rotational velocities , that can reproduce MU _ { 69 } ’ s ( and similar oblique contact binaries ) characteristics .
Using N-body simulations , we find that \sim 15 \% of all ultra-wide binaries with cosine-uniform inclination distribution [ 5 , 9 ] 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 MU _ { 69 } .
The semi-secular contact-binary formation channel not only explains the observed properties of MU _ { 69 } , but could also apply for other Kuiper/asteroid belt binaries , and for Solar/extra-solar moon systems .