It's the final destination on NASA's planetary tour of the solar system, which began more than a half-century ago. Pluto was still a full-fledged planet when New Horizons rocketed away in 2006, only to become demoted to dwarf status later that year.
The 3-billion-mile (5-billion-kilometer) journey from Cape Canaveral, Florida, culminates Tuesday at 1149 GMT. That's when the spacecraft is due to fly past Pluto at 31,000 mph (50,000 kph).
The New Horizons team gathered at Johns Hopkins University's Applied Physics Laboratory in Laurel, Maryland, won't know for many hours if everything went well. The spacecraft will be too busy taking photographs and collecting information to "phone home." A confirmation signal is expected at around 0100 GMT Wednesday.
New Horizons has already beamed back the best-ever images of Pluto and big moon Charon. Pluto also has four little moons.
"The Pluto system is enchanting in its strangeness, its alien beauty," principal scientist Alan Stern told reporters Monday.
Discovered in 1930, Pluto is the largest object in the so-called Kuiper Belt, considered the third zone of the solar system after the inner rocky planets and outer gaseous ones. This unknown territory is a shooting gallery of comets and other small bodies.
An extension of the $720 million mission, not yet approved, could have New Horizons flying past another much smaller Kuiper Belt object, before departing the solar system.
Related story
Scientists just figured out how Pluto got its heart
