We assess the existence of the “reddening curve” seen in optical/NIR measurements of trans-Neptunian surfaces into the near-ultraviolet. We measured the colour of 52 TNOs with simultaneous CFHT -band and Gemini photometry, acquired as part of the Colours of the Outer Solar System Origins Survey (Col-OSSOS). We find that the majority of our sample fall along the reddening curve. A few of our TNOs are 2- or 3- outliers in UV-red and UV-blue. UV-red objects may be asteroid interlopers emplaced during planetary migration: a drop-off in the UV is a key feature of C-type asteroids, due to the presence of ferric oxide. We discuss our identification of one potential asteroid-like TNO in this significant near-UV survey. Using the quantified biases in our targets' discovery survey (the Outer Solar System Origins Survey: OSSOS), we estimate the population fraction of asteroid interlopers in the Kuiper Belt.
We also report upper limits on the u-band magnitudes of 112 additional OSSOS discoveries, forming the largest -band TNO sample to date. These very low SNR or non-detection measurements are of objects with precision orbits, which were serendipitously within the CFHT MegaCam field of view during the simultaneous observations with Gemini. We discuss the implications of these results and place them in context with our current understanding of the UV-optical colour-space.