New Horizons wasn’t always guaranteed a visit to an object like Ultima Thule. The vehicle’s primary mission was to fly by Pluto, with the possibility of passing by a second Kuiper Belt object later. Astronomers didn’t even know if there was an object close enough to Pluto that New Horizons could target.

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https://www.theverge.com/2018/12/27/18156181/nasa-new-horizons-pluto-ultima-thule-kuiper-belt-flyby