NASA's unmanned Juno spacecraft on Monday began orbiting Jupiter, a key triumph for the $1.1 billion mission that aims to uncover the origins of the biggest planet in the solar system.
"Welcome to Jupiter," said a commentator at mission control at NASA`s Jet Propulsion Laboratory in Pasadena, California.
The room erupted in cheers as the solar observatory successfully entered its aimed-for orbit around Jupiter at 11:53 pm (0353 GMT Tuesday).
The spacecraft has traveled 1.7 billion miles (2.7 billion kilometers) since it launched five years ago from Cape Canaveral, Florida.
"We are in it," hollered Scott Bolton, NASA`s principal investigator.
Launched from Florida nearly five years ago, Juno had to be precisely positioned, ignite its main engine at exactly the right time and keep it burning for 35 minutes to shed enough speed so it can be captured by Jupiter's gravity.
Scientists are particularly interested in learning how much water Jupiter contains, which is key to determining where in the solar system it formed. Jupiter's origins, in turn, affected the development and position of the rest of the planets, including Earth and its fortuitous location conducive to the evolution of life.
The immense gravity exerted by Jupiter's sheer size - packing 2-1/2 times the mass of all the other planets combined - is thought to have helped shield Earth from bombardment by comets and asteroids.
"We are learning about nature, how Jupiter formed and what that tells us about our history and where we came from," Bolton said.
The Juno probe is named for the ancient Roman goddess, who was the wife and sister of Jupiter, the mythological king of gods, and had the power to see through clouds.
Only one other spacecraft, Galileo, has ever circled Jupiter, which is five times farther away from the sun than Earth and is itself orbited by 67 known moons. Bolton said Juno is likely to discover even more.
Seven other US space probes have sailed past the gas giant on brief reconnaissance missions before heading elsewhere in the solar system.
The risks to the spacecraft Juno will not end once it arrives in orbit. The probe must quickly turn around and face the sun so its 18,698 solar cells can begin recharging the battery.
"I won't exhale until we are back sun-pointing again," Bolton said.
Juno will fly in highly elliptical, egg-shaped orbits that pass within 3,000 miles (4,800 km) of the tops of Jupiter's clouds and inside the planet's powerful radiation belts.
Juno's computers and sensitive science instruments are housed in a 400-pound (180-kg) titanium vault for protection. But during its 37 orbits around Jupiter, Juno will be exposed to the equivalent of 100 million dental X-rays, said Bill McAlpine, radiation control manager for the mission.
The spacecraft, built by Lockheed Martin, is expected to last for 20 months. On its final orbit, Juno will dive into Jupiter's atmosphere, where it will be crushed and vaporized.
Like Galileo, which circled Jupiter for eight years before crashing into the planet in 2003, Juno's demise is designed to prevent any hitchhiking microbes from Earth from inadvertently contaminating Jupiter's ocean-bearing moon Europa, a target of future study for extraterrestrial life.