Ultima Thule was first discovered in 2014 by astronomers using the Hubble Space Telescope.
At about 19 miles in diameter, Ultima Thule is a bit larger than Manhattan. The rock is located in the Kuiper Belt, a massive, doughnut-shaped region of icy bodies beyond the orbit of Neptune.
The New Horizons spacecraft was launched from Earth 13 years ago and flew by Pluto in 2015. The craft will come three times closer to Ultima Thule than it did Pluto, according to NASA.