NASA's Juno spacecraft, an energy-efficient probe now being built in Colorado, is less than two years from beginning its journey to map Jupiter in hopes of unlocking secrets about the enormous planet's ancient formation.

Artist's concept of the Juno spacecraft at Jupiter. Credit: NASA/JPL

Early construction work on the spacecraft's primary structure is already underway after the $700 million mission passed its critical design review in April, according to Jan Chodas, Juno's project manager at the Jet Propulsion Laboratory in Pasadena, Calif.

The next step will be building flight hardware for electronic components and Juno's eight science instruments.

"Our instruments are well along," Chodas said. "They've all got pretty mature engineering models and most, if not all, have started their flight builds."

Scheduled to launch on an Atlas 5 rocket in August 2011, Juno will coast through the solar system for five years before reaching Jupiter in mid-2016. The probe will swing by Earth in 2013, using the planet's gravity to sling-shot Juno into the outer solar system.

Juno is constrained to a launch period between Aug. 5 and Aug. 26 in 2011 because Earth and Jupiter must be in the right positions to allow the spacecraft to reach its destination.

NASA's Curiosity rover, also called the Mars Science Laboratory, is slated to launch on another Atlas 5 booster in October or November of 2011. Atlas rockets launching from Florida must use the same launch pad, leaving NASA leadership concerned about delays that could affect both missions.

Senior officials requested extra manpower from United Launch Alliance to cut the processing time between missions from 60 days to about 45 days, but there are no guarantees, said William Wrobel, NASA's assistant associate administrator for launch services.

NASA Headquarters also asked the Juno project to open their launch period a few days earlier, giving the probe more time to get off the ground in early August.

"The earlier that we launch in our launch period, the sooner it frees up that one Atlas pad to start MSL preparations," Chodas said.

To accommodate the change, Juno will now follow a trajectory that arrives at Jupiter a month earlier than originally planned.

This year has marked a crucial milestone in the project since it was selected by NASA in 2005.

"I've always felt that when you reach the two-years-from-launch point, you get into a different mindset," Chodas said. "Now it's time to buckle down and get it built, tested, integrated and launched."

"You really can go through a tour of Juno hardware already," said Scott Bolton, Juno's principal investigator at the Southwest Research Institute in San Antonio.

Lockheed Martin Corp. in Denver is managing the spacecraft's construction. Eight instruments and a public outreach visible camera are being developed by science teams around the world.

"It's very exciting to see this mission actually become real, to go from a study to something that's tangible," Bolton said.

Bolton said two of Juno's biggest technological hurdles were Italian star trackers that will have to work in the harsh radiation environment at Jupiter and a collection of ultra-sensitive solar cells to produce electricity.

Engineers believe they have a grasp of both challenges. The star trackers have passed early testing and Juno's spacecraft has three large solar panels to collect as much sunlight as possible, according to Bolton.

Juno will use special solar cells designed to operate in the low light and low temperature conditions at Jupiter. No spacecraft to visit the outer planets has ever been powered by solar electricity.

"We're pushing the envelope on how solar cells can perform, and the things that you learn are applicable not only for space, but even here on the Earth," Bolton said.

Juno designers have screened solar cells from suppliers to pick the most efficient units for the mission. Engineers are also trying to make the energy-saving spacecraft as lean as possible, in an effort to save precious power.

"We've got both sides of it, a very efficient spacecraft and a very good set of solar cells that are designed to work in the low temperature and the low light conditions," Bolton said.

Workers will begin integrating major subsystems and payloads to the spacecraft next year.

Bolton said Juno is currently on schedule and under its development cost cap.

"We're doing a lot with nine instruments for this kind of a budget. We're really doing flagship-class science with far less resources. That's a new way of doing business at the giant outer planets," Bolton said.

Juno will map Jupiter in a looping 11-day-long elliptical orbit over the planet's enigmatic poles. At its closest approach to Jupiter during arrival, Juno will pass just 3,000 miles above the planet's swirling cloud tops.

Spinning at a few revolutions per minute for stability, Juno's instrument package will study Jupiter for at least 30 science orbits lasting almost one Earth year.

"We dive in close to the cloud tops and we have about a 6-hour intense data-gathering period, and then we swing back out," Chodas said.

"We're investigating the interior structure, so we're studying both the gravity field and the magnetic field," Bolton said.

Other sensors will focus on Jupiter's thick and stormy atmosphere. A microwave radiometer with six frequencies will peer through Jupiter's clouds to probe the deep atmosphere.

Because Juno will be slowly spinning, the craft will not be able to collect the volume of spectacular visible imagery gathered by Galileo, a NASA mission that studied Jupiter from 1995 to 2003.

"It's hard to take those beautiful Galileo photos of the moons and Jupiter," Chodas said. "We're concentrating on fields and particles measurements."

But Juno is carrying a small visible camera specifically for public engagement. That imager will take the first good pictures of Jupiter's poles.

Juno will be flying much closer to Jupiter than Galileo, which gave scientists their first comprehensive view of the planet and its moons.

"We have a much more comprehensive mapping mission of the planet," Chodas said.

The lower orbit will expose Juno to harsh radiation that will degrade electronics needed to control the spacecraft, limiting its lifetime.

Juno's mission at Jupiter will not last much more than one year because officials want to crash the spacecraft into Jupiter to protect the planet's moons. The probe must still be operating to ensure that maneuver is successful.