While the scientists will celebrate the moment of flyby as if it were New Year’s, they will have no idea how the mission is actually going at that point. The spacecraft, busy making its science observations, will not turn to send a message back to Earth until a few hours later. Then it will take six hours for that radio signal, traveling at the speed of light, to reach Earth.

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Tell me about this small frozen world

Based on suggestions from the public, the New Horizons team chose a nickname for the world: Ultima Thule, which means “distant places beyond the known world.” Officially, it is 2014 MU69, a catalog designation assigned by the International Astronomical Union’s Minor Planet Center. The “2014” refers to the year it was discovered, the result of a careful scan of the night sky by the Hubble Space Telescope for targets that New Horizons might be able to fly by after its Pluto encounter.

No telescope on Earth has been able to clearly spot MU69. Even sharp-eyed Hubble can make out only a dot of light. Scientists estimate that it is 12 to 22 miles wide, and that it is dark, reflecting about 10 percent of the light that hits it.

Four billion miles from the sun, MU69 is a billion miles farther out than Pluto, part of the ring of icy worlds beyond Neptune known as the Kuiper belt. Its orbit, nearly circular, suggests that it has been undisturbed since the birth of the solar system 4.5 billion years ago.