A composite image of Jupiter’s cloud formations as seen through the eyes of Juno’s Microwave Radiometer, which can see up to 250 miles into the planet's atmosphere with its largest antenna. The belts and bands visible on the surface are also visible in modified form in each layer below. Image credit: NASA/JPL-Caltech/SwRI/GSFC
Last week, NASA's Juno spacecraft reached perijove, the closest point of its 53.5-day orbit around Jupiter, when it passed less than 3000 miles from the gas giant's clouds. But during its approach, the onboard computer suddenly detected an unexpected condition and turned off unnecessary subsystems, entering “safe mode.” The solar-powered spacecraft then went "power positive," shutting down the cameras and reorienting itself toward the Sun, where it linked up with the Deep Space Network back on Earth. Then it waited for humans to evaluate the situation and provide guidance.
It was a disappointing outcome for the Southwest Research Institute scientists leading the mission, including principal investigator Scott Bolton. Because the science instruments were shut down during the flyby, no data were collected. But this outcome was also a necessary one. In space, power is king. Engineers can often fix—or find inventive workarounds to—problems of enormous complexity, even from hundreds of millions of miles away. The one thing that is non-negotiable, however, is power. The spacecraft must be alive to receive commands. So in this case, "safe mode" is a good thing—the robot did exactly what it was supposed to do in this situation.
According to the original plan, the October 19 maneuver was not meant to be a science orbit, but rather, a "period reduction maneuver." The Juno team initially intended to fire the same rocket motor that performed the daring insertion maneuver on July 4, when it purposefully slowed its engines enough to be caught by Juno’s gravity and orbit the poles. If successful, last week's rocket firing would have slowed the spacecraft and changed its orbit from 53.5 days to two weeks.
While preparing for the maneuver, however, the team noticed that the valves in the spacecraft's propulsion system were behaving sluggishly, as though the valves were "sticky." Rather than take any chances with the spacecraft's delicate orbit, they decided to postpone the maneuver and switch on the science instruments instead, making this a science pass.
The scientific investigation of Jupiter is tied to a two-hour window every orbit when the spacecraft reaches perijove. During that time, the spacecraft travels from Jupiter's north pole to its south. Whether it makes this traversal following a 14-day orbit or roughly 7.5-week orbit makes no difference at all; the current longer orbit simply means it will take longer to reach the completion of the mission.
Then the plan for a science pass fell through too when the spacecraft switched into safe mode.
Although these are two disappointing events in a row, everything will be okay, Bolton said at a press event during the 2016 meeting of the American Astronomical Society's Division for Planetary Sciences. The team can still fire the rocket in the future. Until then, they will work to determine what caused the safe mode and why the valves were behaving oddly. Bolton explained that the team is in no rush. "Fortunately, the way we designed Juno, and the orbit we went into, is very flexible," he said. "It allows very flexible science."
Though this flyby was a wash, a previous, successful flyby on August 27 has yielded extraordinary science. Then, an instrument called a microwave radiometer peered into Jupiter's atmosphere, giving scientists the first-ever look beneath the planet's clouds. Peeling away layers of the atmosphere as though it were an onion and looking as deeply within as 250 miles, scientists discovered that the atmosphere retains the famous structure of the zones and belts of clouds visible from telescopes.
"Whatever is making those colors—whatever is making those stripes—is still existing pretty far down into Jupiter," Bolton said. "That came as a surprise to many of the scientists. We didn't know if [Jupiter's appearance] was skin deep—just a very thin layer—or whether it goes down." Another surprise was that the colorful zones and belts also appear to evolve and change at various depths. This hints at the deep dynamics and chemistry of Jupiter's atmosphere, though the details still require much analysis.
During that same pass, Juno's camera captured images as the spacecraft crossed the "terminator" of Jupiter—that is, the line between the sunlit side of the planet and the side in darkness. Think of a half-moon: The terminator is the line where the bright half meets the dark half.
The above image of the sunlit half was created by citizen scientist Alex Mai using data from the spacecraft's JunoCam instrument. (Raw images from the mission are available at JunoCam for both public and professional use.) Meanwhile, the shadows revealed the topology of Jupiter's atmosphere—another first. A particularly pronounced feature was a cyclone raging even above Jupiter's base atmosphere. It's 53 miles tall and 4350 miles wide—half the size of the Earth.
"Imagine the kind of atmosphere you're dealing with," marveled Bolton.
For now, scientists will need to imagine a little longer. Juno's next flyby of Jupiter will be on December 11.