The two-year Solar Terrestrial Relations Observatory (STEREO) mission will be the first to view the sun from two separate vantage points outside Earth's orbit.
The nearly identical satellites will act like a pair of human eyes, each picking up data that will be correlated, with data from observatories on the ground and in low-Earth orbit, into a third-dimension vision of the sun's eruptions and their impact on Earth.
A three-stage Boeing Delta II rocket carrying the observatories lifted off from Cape Canaveral Air Force Station in Florida, at 8:52 pm local time (0052 GMT).
The twin spacecraft, each about the size of a golf cart and weighing about
620 kilograms, separated without problem from the rocket after launch, NASA said.
Once in orbit, the observatories, each equipped with 16 instruments, will provide the first images in three dimensions of the sun and its corona.
The National Aeronautics and Space Administration said it expected to have the first 3-D images of the sun by mid-December.
"We are at the dawning of a new age of solar observations," Russ Howard of the Naval Research Laboratory said at a NASA news conference.
"We're going to be viewing things in a new dimension for us," Mr Howard said.
He said STEREO would have an unprecedented "broadside" view of the entire relationship between the sun and Earth, about 150 million kilometres apart.
And, he said, "for the first time we will be able to measure optically what the other instruments on STEREO ... are seeing in their instruments."
Scientists hope the mission will glean insight into solar activities such as coronal mass ejections (CMEs), the most violent explosions in the solar system.
When aimed at Earth, these billion-tonne eruptions of the sun's corona spew intense radiation, severely disrupting the Earth's environment and endangering astronauts and scientific spacecraft.
Information gleaned on the origins and behaviours of CMEs would allow more accurate forecasting, which would improve planning for space missions as well as over-the-pole flights, satellite communications and video transmissions.
At first the twin observatories will be placed in a highly eccentric Earth orbit, NASA said.
After a few weeks, the two spacecraft -- named "A", for "Advance", and "B", for "Behind" -- are expected to drift slowly apart but stay relatively close to each other as they line up for their close encounter with the moon about one month into the mission.
"This is when the relatively small distance between the two spacecraft makes all the difference," the space agency said on its website.
NASA will use the moon's gravity like a pinball flipper to send them into opposite and slightly distinct orbits around the sun.
"Behind" will be flung completely away from Earth, and will become a satellite of the sun, while the "Ahead" spacecraft will curve back to fly past the moon a second time six weeks later, and be flung out in the opposite direction.
Ahead will orbit ahead of the Earth in its rotation around the sun and the
Behind will follow Earth's orbit.
The total cost of the mission is US$550 million. Scientists from European countries are participating in the mission: Belgium, Britain, France and Germany.