Solar-powered probe set to skimcloud tops to learn what’s below
is preparing to launch another spacecraft to Jupiter that will try to uncover some of the mysteries the planet kept hidden the last time a human-built robot ventured into its cold, radioactive and relatively dark neighborhood.
Named Juno, after the Roman god Jupiter’s wife, the unique spacecraft will set a record by running on solar power rather than the nuclear radioisotope thermoelectric generators (RTG) previously used to operate spacecraft that far from the Sun. It will pick up where the Galileo mission left off in September 2003, when that probe was sent crashing into the gas giant’s clouds at the end of its service life to prevent accidental contamination of one of Jupiter’s moons.
Juno is essentially a flying windmill rotating around a titanium box. Most of the spacecraft’s 66-ft. diameter is taken up with huge arrays designed to capture as much solar energy as possible in the dim sunlight at Jupiter. And to shield delicate hardware from the high-energy radiation at the massive planet, most of the instrument electronics is crammed into a box with titanium walls as much as half an inch thick, tucked right in the middle of the spacecraft for maximum protection.
In orbit at Jupiter, Juno will turn at 2 rpm as its nine instruments collect data on the gas giant’s structure and composition. Unlike Galileo, which orbited at the planet’s equator where it could also visit the Jovian moons, Juno will take up an orbit over Jupiter’s poles to avoid the worst of the radiation belts.
“We have this elliptical orbit which dives down between the innermost radiation belt and the cloud tops at about 4,000 km [2,500 mi.],” says Jan Chodas, Juno project manager at the(JPL). “Our prime data-taking pass is about 6 hr. around perijove, close to the planet, and then we swing back out in this 11-day orbit and relay the data back and set up for the next orbit. So it’s that knowledge of where the radiation belts are, and the precise navigation to pinpoint the spacecraft on that trajectory, that is mission-enabling.”
The space around Jupiter is so hot that mission designers expect Juno to survive there for only 30 orbits—less than a year. Planning to handle the radiation started early, Chodas says, and included developing the cubic-meter titanium box known on the project as “the vault.”
Aside from the solar arrays, the spacecraft is “short and squatty,” says Tim Gasparrini, Juno program manager atin Denver. The body measures 3.5 meters (11.5 ft.) tall and 3.5 meters in diameter. Right in the middleis the titanium vault. Measuring 0.8 X 0.8 X 0.6 meters, it weighs 150 kg. (330 lb.) and is “not your typical aerospace construction,” he says.
The mass of the titanium walls provides a radiation shelter where all of the spacecraft avionics are housed. “All the sensors are outside,” Gasparrini says. “But as much of the critical avionics [as possible] is sandwiched in the vault. [Exterior equipment] is as hard as it could reasonably be within cost constraints. It’s expected that after 30 orbits you’ve pretty much taken your radiation beating, and you’re not going to survive much longer.”
The approach, like the decision to use solar power, was taken to curtail the cost of the mission, which was developed under the cost-capped New Frontiers program. One of the biggest challenges was making all of the pieces fit inside the vault.
“It’s like a jigsaw puzzle,” says Chodas. “You’ve got a cube, and you’ve got things mounted on a central panel and on all the panels that fold up to make the cube. That layout is ‘easy to do,’ but the cabling that runs from box to box and in and out of the vault is a work of art. The first time I saw the flight cable I said, ‘Wow, you guys are going to stuff all that in there?’”
When it reaches Jupiter in July 2016 and begins taking science data, Juno will set the record for distance from the Sun for a spacecraft operating on solar power. The Stardust comet sample return mission used solar power at 2 astronomical units (AU), twice the distance of the Earth from the Sun. But Juno will operate at 5 AU. To obtain the power the mission needs at that distance, the probe’s solar arrays are very large—two wings of four solar panels each, and one with three, plus the boom that holds Juno’s magnetometer.
The “ultra triple junction” gallium arsenide solar cells, built for Juno by’s Spectrolab Inc. subsidiary, will be able to convert more than 28% of the solar energy hitting them into electricity at the beginning of the mission. They have thicker cover glass than Earth-orbiting arrays for extra radiation protection, and still will darken over time under the radiation effects, reducing their efficiency at the planet. Even so, they should generate plenty of power for the job they have to do.
“At Earth the solar arrays generate around 18 kw,” says Gasparrini. “At Jupiter they generate roughly 400 watts. So in general, if you can have enough energy to light a 100-watt bulb at Earth, you’ll only have enough energy to light a nightlight at Jupiter.”
Of the 400 watts the huge arrays will generate at Jupiter, half will go to the heaters and other thermal subsystems that keep the electronics warm. That is another factor in operating so far from the Sun. At Mars, for example, only a third of a probe’s power is needed for warmth. But even the 200-250 watts remaining for Juno’s science package will be enough to take data, record it and send it back to Earth through the high-gain antenna.
“For talking purposes, half of it goes to thermal subsystems; half of it goes to everything else,” Gasparrini says of Juno’s power distribution. “Certainly we don’t have as much power as some of the other missions that we do, but it isn’t four or five times different. It’s in the family.”
Juno’s solar arrays actually consume less spacecraft mass than an RTG, but the question was moot for the mission designers because there wasn’t an RTG available to them. The last one in the U.S. inventory is being used on the Mars Science Laboratory to be launched this fall, and developing a new set of RTGs for Juno “would have broken the bank,” Gasparrini says. Like the precise navigation and the titanium vault, the solar arrays were mission-enabling, he says.
As with any spacecraft, mass is crucial, and Juno is no exception because of all of the maneuvering involved in getting to Jupiter and collecting data. The spacecraft itself is baselined at 1,500 kg, with a 2,000-kg fuel load. Gasparrini says it will actually come in with about 100 kg more fuel than planned as final hardware weights are booked against the 3,625 kg allowed the spacecraft on its Atlas V 551 launch vehicle.
The Atlas variant is the same vehicle that lofted the New Horizons mission on Jan. 19, 2006, toward a Pluto flyby in July 2015. That launch set a record for the fastest escape from Earth’s gravity that stands today, and New Horizons reached Jupiter in only 13 months (AW&ST Jan. 9, 2006, p. 46). Juno is heavier and will proceed at a more stately pace to its orbital insertion in 2016.
The launch window for Jupiter will open on Aug. 11, with flight opportunities through Aug. 31. The Atlas V will propel the spacecraft toward Jupiter, Gasparrini says, and its Centaur upper stage will send it in the right direction. After that it will be a relatively straight shot to Jupiter, requiring only one Earth flyby—in September 2013—before moving on to its polar orbit at Jupiter on July 5, 2016, or a little later.
The arrays will start to deploy 70 min. after launch, unfolding at their spring-loaded hinge lines at roughly 30-min. increments while the spacecraft spins at 0.5 rpm. The launch vehicle will spin Juno up, and it will continue to spin at different rates throughout the mission.
During cruise to Jupiter, the normal spin rate will be 1 rpm, increasing to 5 rpm at orbit insertion. It will spin at 2 rpm while collecting science data. “You basically can stabilize yourself for free, with no power, by the fact that you’re spin stabilized,” Gasparrini says. “That was a consideration for the design of the payloads; they all had to be designed to do their science while we were spinning.”
Missions funded under’s New Frontiers program are led by the principal investigator who suggested it. Scott Bolton of Southwest Research Institute in San Antonio, Texas, proposed Juno, and he says the idea arose when he was using the radar on the Cassini Saturn probe to study Jupiter’s radiation belts.
The “glow” in radio wavelengths from Jupiter’s relatively warm atmosphere was noise that obscured his listen-only observations, which in effect made the Cassini radar into a long-range microwave radiometer.
In discussions with colleagues pursuing the question—left over from the Galileo mission—of how much water is in Jupiter, Bolton realized that it might be possible to make microwave measurements of the atmospheric glow from a polar orbit below the radiation belts, which otherwise obscure precise temperature measurements.
“If I get this measurement in the atmosphere at a whole bunch of different frequencies, I can compare them with my model of how warm Jupiter is, and the only thing that’s the variable is how much water’s in there,” Bolton says.
There were other questions that could be answered with a polar-orbiting probe flying under the radiation belts, including those involving Jupiter’s magnetosphere and gravity fields. That led to the Juno proposal, which was the second New Frontiers project selected by NASA.
The science payload that emerged carries nine instruments and 25 sensors. At its core is the microwave radiometer (MWR) that grew out of Bolton’s work with the Cassini radar. The MWR will be able to sound much deeper into the planet’s atmosphere than the physical probe that Galileo dropped, using six wavelengths to measure composition over a range of altitudes.
Other instruments in the suite are:
•Jovian Auroral Distributions Experiment (JADE), which will study the plasma structure of the planet’s aurora, measuring particle distributions in the polar magnetosphere.
•Jupiter Energetic-particle Detector Instrument (JEDI), for measuring hydrogen, helium, oxygen, sulfur and other ions in the polar magnetosphere.
•Ultraviolet Spectrograph (UVS), an imaging spectrograph that works while the spacecraft is spinning by recording data on ultraviolet photos detected during each rotation.
•Jovian Infrared Auroral Mapper (Jiram), which will study the linkages between the auroras and the planet’s magnetosphere.
•Plasma Wave Instrument (Waves), which will measure radio and plasma spectra in the aurora.
•Fluxgate Magnetometer (FGM), two sensors that will measure the strength and direction of the planet’s magnetic field.
•Advanced Stellar Compass (ASC), a star tracker designed to give extremely accurate data on the magnetometers’ pointing.
•Gravity Science (GS), which will use Doppler effects on the spacecraft’s communications with Earth to study the planet’s mass properties. (For more on the science Juno will deliver, see p. 54.)
In addition, the spacecraft will carry a three-color camera called JunoCam to deliver imagery for public engagement and outreach, as well as for scientific context.
Unlike the Mars Science Lab and the James Webb Space Telescope—both suffering cost overruns—Juno is one of the first major space science missions to go through a NASA process called the Joint Confidence Level (JCL), which uses outside reviews to estimate cost at a 70% level of certainty. In budget hearings before Congress this year, Administrator Charles Bolden has boasted that Juno and the lunar Gravity Recovery and Interior Laboratory (Grail) missions have benefitted from the process.
“We have independent assessments on our cost and schedule,” he told the House Appropriations subcommittee that sets NASA spending. “Grail and Juno are coming in on cost and on schedule because they were subjected to the JCL process, where we had independent assessments as to what our real cost is going to be.”
In general, New Frontiers missions like Juno are said to be capped at $700 million. But the accounting for Juno is a little more complex because of early changes in the budget profile for the project.
According to Jim Adams, deputy director of planetary science in the Science Mission Directorate at NASA headquarters, the original cap on the second New Frontiers mission—which became Juno—was $650 million, including the launch, as measured in fiscal 2003 dollars.
When Juno was selected two years later, the cap quoted was $842 million in then-year dollars, including the launch. The difference was attributed to inflation and changes in NASA accounting schemes, and could be traced back credibly to the $650 million figure, Adams says.
That figure called for a launch date in 2009; but NASA managers almost immediately stretched out the program twice to meet other agency needs—first to a launch in 2010 and then to the current planetary window in August. With the first change, the cost estimate went up to $931 million to account for the additional time, inflation and rising launch costs. The second stretchout took it to a $1.07 billion estimate.
After that the project was subjected to a JCL review to gain the 70% confidence level required under the process. That boosted the estimated total cost to $1.107 billion, where it stands today. Of that amount, an internal figure called the “principal investigator cost cap” is $1.09 billion, which leaves the project “a little bit of headroom” for rising costs during the operations phase coming up.
Those most likely would arise if Juno encounters an unexpected technical problem during cruise to Jupiter or while it is gathering science; but from a technical standpoint, it has indeed remained on track once the schedule was set for good at NASA headquarters. As of March 9, the program had 21 days of schedule margin remaining before the launch window opens, and the spacecraft had finished thermal vacuum testing at Lockheed Martin’s Denver facility.
Chodas, the JPL project manager who runs the program for Principal Investigator Bolton, and Gasparrini, who developed the flight system at Lockheed Martin, both attribute their success in keeping on track to several engineering factors.
Those factors hinge in large part on the experience their respective engineering organizations have gained in previous planetary science missions. That may have implications for future deep-space exploration if NASA’s current budget problems force the agency to curtail large missions in the years ahead.
“The people who are in the jobs, across the board, have the right set of experiences, and the right collaboration,” Chodas says. “People move around on different jobs and so they’ll say, ‘Oh, watch out for this, or have you thought about that,’ because they did that job on a previous development.”
That collaboration was particularly useful on Juno because it relied heavily on heritage hardware and software from previous missions—another factor its top managers say contributed to Juno’s relatively smooth development. The spacecraft avionics suite was derived from equipment originally developed for the Mars Reconnaissance Orbiter and other spacecraft.
Beyond that, the two delays early in the program gave its developers more time to prepare for the difficult environment Juno will encounter, and think through potential pitfalls carefully.
“The environmental challenges that we knew we would face were tackled early on in the design, so it wasn’t like getting halfway through building and testing it and suddenly realizing, ‘Oh, you mean we have to be magnetically clear; we didn’t use the right materials, we didn’t use the right techniques?’ A lot of foresight and planning was put in. We had a lot of the typical issues that crop up when you build hardware and software, but not that sort of thing.”