The first Nasa explorer of its kind has taken off on a seven-year quest, chasing after a big, black, unexplored asteroid to gather a few handfuls of gravel and bring them back to Earth.
It is heading to collect dust and gravel from the surface of an asteroid, bringing samples back in a couple of years.
Scientists say there is a slim chance the asteroid could crash into Earth in about 150 years time.
Professor Dante Lauretta said everything had gone to plan.
"We got everything just exactly perfect. This represents the hopes and dreams, blood, sweat and tears of thousands of people," he said.
The bite-size bits of ancient space rock from asteroid Bennu could hold clues to the origin of life, not just on our planet but potentially elsewhere in the solar system.
Thousands gathered to witness the evening launch of Osiris-Rex from Florida's Cape Canaveral. They robotic hunter looks something like a bird with its solar wings. The spacecraft took flight atop an Atlas V rocket.
Victory was declared an hour later as launch controllers shook hands and embraced after the spacecraft shot out of Earth's orbit, bound for Bennu.
"Tonight is a night for celebration. We are on our way to an asteroid," said Nasa's chief scientist, Ellen Stofan. "We've just done something amazing."
It will take two years for Osiris-Rex to reach Bennu, which is circling the sun in a slightly wider orbit than Earth's. The boxy spacecraft will go into orbit around the asteroid, seeking out the best spot before going in for a quick bite.
The SUV-sized spacecraft will have travelled more than four billion miles by the mission's end in 2023.
Nasa has gone after comet dust and solar wind particles before, but never anything from an asteroid. It promises to be the biggest cosmic haul since the Apollo moon rocks.
The roundish rock - about 1,600ft across and taller than the Empire State Building - is believed to harbour carbon dating back 4.5 billion years, to the beginning of our solar system. That makes it a time capsule and a scientific prize.
"We will make discoveries on this mission that we have not anticipated. It's exciting," said Bill Nye, chief executive officer of the Planetary Society.
The launch came 50 years to the day after the first Star Trek episode aired on TV. Nasa launch commentator Mike Curie referenced the anniversary, urging the spacecraft at lift-off "to boldly go" to Bennu and back.
Osiris-Rex may lead to asteroid-mining missions, according to scientists, and could help protect the planet from menacing space rocks.
Japan has already visited an asteroid and returned some specks, and is chasing another space rock for more samples. Osiris-Rex's bounty should surpass that. Mr Lauretta and his team want at least 60 grammes of dust and gravel when the big day comes in 2020. Ground tests have yielded eight times that in a single scoop, so hopes are high for four to five handfuls.
Osiris-Rex will hover like a hummingbird over Bennu, according to Mr Lauretta, as the spacecraft's 10ft mechanical arm touches down like a pogo stick on the surface for three to five seconds.
Thrusters will shoot out nitrogen gas to stir up the surface, and the loose particles will be sucked up into the device. Spacecraft managers call it "the gentle high five". They get just three shots at it before the nitrogen gas runs out and the effort is abandoned.
The team opted for this touch-and-go procedure instead of landing to increase the odds of success. Despite extensive observations of Bennu from ground and space telescopes, no-one knows exactly what to expect there, and it could be difficult if not impossible to anchor a spacecraft on the surface, Mr Lauretta said.
Osiris-Rex's freed sample container - the same kind used for the comet-dust retrieval - will parachute down with the pristine asteroid treasure in Utah, and the mother spacecraft will continue its orbit of the sun.
Among the 8,000 Nasa launch guests was the schoolboy who came up with the asteroid's name for a contest.
Twelve-year-old Mike Puzio of Greensboro, North Carolina, cheered as he watched his first up-close rocket launch: "It was awesome!"
The name Bennu comes from the heron of Egyptian mythology. Mike thought Osiris-Rex looked like a bird, with its twin solar wings and long arm outstretched for a sample grab, and with the spacecraft named after the Egyptian god Osiris, Bennu was an obvious choice.
Osiris-Rex is also a Nasa acronym for origins, spectral interpretation, resource identification, security-regolith explorer. The estimated cost of the mission is more than 800 million dollars (£600 million).
"Space exploration brings out the best in us," Mr Nye said shortly before Osiris-Rex began its journey. "It is an extraordinary use of our intellect and treasure to elevate humankind, to help us know our place in the cosmos."
It may also one day save the home planet.
Bennu swings by Earth every six years, and 150 years from now, could hit us. The odds are less than a tenth of 1%, according to Mr Lauretta. While this particular rock would not destroy Earth - just carve out a huge crater - other asteroids could cause more trouble.
Osiris-Rex will help scientists better understand the ever-changing paths of asteroids, and that could prove its biggest payoff.