* Juno to spend a year in polar orbit around Jupiter
* Journey to Jupiter will take 5 years
* Planet believed to hold clues to solar system formation
By Irene Klotz
CAPE CANAVERAL, Fla., Aug 5 (Reuters) - An unmanned Atlas 5 rocket blasted off from Cape Canaveral Air Force Station on Friday, sending a robotic scout on its way to Jupiter to sniff out details about how the solar system formed.
The rocket carrying NASA's Juno spacecraft lifted off at 12:25 p.m. (1625 GMT), the first step in a five-year, 445-million mile (716-million km) journey to the largest planet in the solar system.
Launch was delayed almost an hour while United Launch Alliance, a Boeing (BA.N)-Lockheed Martin (LMT.N) joint venture that builds and flies Atlas and Delta rockets for NASA, the military and commercial customers, fixed a technical problem with ground support equipment that supplies a helium purge to the rocket.
Upon arrival in July 2016, Juno is to spend a year in an unprecedented polar orbit around the giant planet, measuring its water content, mapping its magnetic fields and searching for signs of a solid core.
With more than twice the mass than all its sibling planets combined, Jupiter is believed to hold a key piece to the puzzle of how the planets formed some 4.65 billion years ago from the gas and dust left over after the birth of the sun.
"We're really looking for the recipe for planet formation," said Juno lead scientist Scott Bolton, with the Southwest Research Institute in San Antonio, Texas.
"We're going after the ingredients of Jupiter by getting the water abundance as well as very precise measurements of the gravity field that will help us understand whether there's a core of heavy elements or a core of rocks in the middle of Jupiter," he said.
The measurements will help scientists discriminate among theories about what the early solar system looked like and how Jupiter, believed to be the first planet to form, was created.
To make its observations, Juno will soar as close as 3,100 miles (5,000 km) above Jupiter's cloud tops, the first spacecraft to fly inside the planet's radiation belts.
With its sensitive electronics housed in a vault of titanium, Juno should last through 33 orbits around Jupiter, which is about a year on Earth. Its last maneuver will be a suicidal plunge into the planet's thick atmosphere, which will incinerate the probe to avoid possible contamination of Jupiter's water-bearing moons.
Now that NASA has retired its shuttle fleet, the U.S. space spotlight could shift toward the robotic probes and observatories have brought the biggest leaps toward understanding the cosmos. [ID:nN1E7730GW]
The Juno mission is the second in NASA's lower-cost, scientist-led New Frontiers program, and it was accomplished on schedule and within its $1.1 billion budget.
The spacecraft was built by Lockheed Martin Astronautics of Denver, Colorado.
In addition to launching science probes and other satellites, United Launch Alliance is in the process of certifying its Atlas 5 rockets to fly astronauts to the International Space Station, one of several possible commercial rockets contending to replace NASA's now-retired space shuttle fleet.
Editing by Jane Sutton and Doina Chiacu