"Go New Horizons!" said lead scientist Alan Stern as a crowd cheered at the Johns Hopkins Applied Physics Laboratory in Maryland to mark the moment at 12:33am (1.33pm Hong Kong time) when the New Horizons spacecraft aimed its cameras at the space rock four. Instead, team members and their guests gathered nearby for back-to-back countdowns at midnight and again 33 minutes later.
The twin planetary feats coincided with the 50th anniversary of the first time humans ever explored another world, when USA astronauts orbited the Moon aboard Apollo 8 in December, 1968.
FILE - This composite image made available by NASA shows the Kuiper Belt object nicknamed "Ultima Thule," indicated by the crosshairs at center, with stars surrounding it on August 16, 2018, made by the New Horizons spacecraft.
As for the "cold, classical" part, Ultima Thule's orbit has a very low "inclination", meaning that it travels around the Sun in roughly the same plane as all the planets (except Pluto), and its orbit is almost circular (unlike Pluto).
At 11:30 a.m., scientists involved with the New Horizons mission will describe the images and data returned by the spacecraft.
New Horizons, which is the size of a baby grand piano and part of an $800 million U.S. mission, was expected to hurtle to within 3,500 km of Ultima Thule, considerably closer than the Pluto encounter of 2015.
U.S. space probe New Horizons flew past 2014 MU69 Ultima Thule asteroid at the very edge of the solar system, making it the farthest from the Earth ever visited by a probe from our planet, NASA said on Tuesday.
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"Beyond the terrestrial planets lie the giant planets, and beyond the giant planets lies the Kuiper Belt", Stern says. It's a tougher encounter than at Pluto because of the distance and the considerable unknowns, and because the spacecraft is older now.
'Ultima was formed in the middle of the Kuiper Belt, where temperatures are close to absolute zero, ' Stern wrote.
Despite the partial US government shutdown, sparked by a feud over funding for a border wall with Mexico between President Donald Trump and opposition Democrats, NASA administrator Jim Bridenstine vowed that the USA space agency would broadcast the flyby.
Asteroid 2014 MU69 was discovered via the Hubble orbital telescope back in 2014. Ultima Thule orbits the sun from the Kuiper Belt - a massive region of frozen rocky bodies beyond Neptune's orbit. It is thought to be potato-shaped and dark-colored with a touch of red, possibly from being zapped by cosmic rays for eons.
"We are ready to science the heck out of Ultima Thule", Stern said. "We will find out".
"We're going to learn more very soon", John Spencer, New Horizons deputy project scientist, said Monday.