Researchers eager for Juno’s first chance for Jupiter science

Animation of Juno 14-day Orbits Starting in late 2016. Juno is currently in an even larger 53-day orbit. Credit: NASA/JPL-Caltech Credit: NASA
Animation of Juno 14-day Orbits Starting in late 2016. Juno is currently in an even larger 53-day orbit. Credit: NASA/JPL-Caltech

NASA’s Juno spacecraft fine-tuned its orbit around Jupiter this week, helping steer the spinning space probe on a course to dip beneath the giant planet’s radiation belts in late August for the first time with all its science instruments activated.

The orbit correction maneuver Wednesday began around 2:45 p.m. EDT (1845 GMT; 11:45 a.m. PDT) and lasted about one hour, using Juno’s 12 reaction control system thrusters to change the orbiter’s speed by about 11 mph, or 4.9 meters per second, according to Rick Nybakken, Juno’s project manager at NASA’s Jet Propulsion Laboratory in Pasadena, California.

The rocket burn Wednesday did not use Juno’s main engine, a more powerful thruster which fired when the spacecraft slipped into orbit around Jupiter on July 4.

Up to two more orbit trim maneuvers are planned before Juno completes its first elongated, egg-shaped orbit around the solar system’s biggest planet Aug. 27, when the craft will dive within 3,000 miles (about 5,000 kilometers) of Jupiter’s swirling, banded cloud tops.

The Aug. 27 flyby — called a “perijove” by orbit dynamicists — will be the second time Juno approaches so close to Jupiter. But the July 4 encounter occurred during Juno’s make-or-break orbit insertion burn, and mission managers elected to turn off the spacecraft’s science instruments to focus all of the probe’s energy and computing power on executing the engine firing.

Next time, Juno will have its eyes, ears and nose ready.

Scott Bolton, Juno’s principal investigator from the Southwest Research Institute in San Antonio, said scientists plan to release the initial findings from the late August perijove around Sept. 1.

According to Nybakken, Juno will turn itself into an orientation to allow its microwave radiometer to make measurements of Jupiter’s deep atmosphere when it swings back close to the planet next month. That scenario also allows Juno’s visible camera, called JunoCam, to take the best pictures of the mission, officials said.

Juno’s flight plan calls for the solar-powered spacecraft to complete 37 orbits of Jupiter, and other close-up encounters will have the probe in a different pointing position favoring measurements of the gas giant’s strong gravity field, an area of study that will help researchers learn whether the planet has a solid core at its center.

Juno’s mission is aimed at peering inside Jupiter with a suite of cameras, spectrometers and particle detectors to track the distribution of high-energy electrons around the planet, measure the water inside Jupiter’s atmosphere, search for the trigger of the gas giant’s brilliant auroras, and probe the source of its expansive, teardrop-shaped magnetosphere, a bubble that blocks the solar wind from reaching Jupiter.

Data from Juno will help scientists learn about the origin of Jupiter, which experts believe was the first planet to form from a ring of dust and gas surrounding the adolescent sun. Understanding how and where Jupiter formed could help explain how the rest of the solar system evolved, including Earth.

In an interview with Spaceflight Now this week, Nybakken said the Lockheed Martin-built Juno spacecraft is healthy after completing its 1.7 billion-mile (2.8 billion-kilometer) journey to Jupiter.

Ground controllers have turned on most of Juno’s nine science instruments, comprising 29 total sensors, since July 4, Nybakken said.

“Most of them are on and working just fine,” he said.

Two more science instruments are due to start returning data in the coming days. Juno’s atmosphere and aurora-mapping ultraviolet spectrograph and a special Ka-band radio system to track the spacecraft’s position, and help derive the strength of Jupiter’s gravity field, will be turned on this weekend.

Engineers finished testing of Juno’s scientific payload before it reached Jupiter, but Nybakken said a handful of calibrations are planned later this month.

“Everything is working well,” Nybakken said. “The instruments that are on are taking data and performing nominally, and they’re really getting excited about their first perijove pass on Aug. 27.”

This color view from NASA's Juno spacecraft is made from some of the first images taken by JunoCam after the spacecraft entered orbit around Jupiter. Credit: NASA/JPL-Caltech/SwRI/MSSS
This color view from NASA’s Juno spacecraft is made from some of the first images taken by JunoCam after the spacecraft entered orbit around Jupiter. Credit: NASA/JPL-Caltech/SwRI/MSSS

JunoCam is taking pictures of Jupiter and its moons every day through its 53-day circuit around the planet. Juno is the first mission to orbit over Jupiter’s poles, giving the imager a unique perspective.

“We’re working on something we call a marvel movie, because when we start the movie now Jupiter is pretty small, but we’ll go all the way around and create a full orbit movie, which will be pretty cool when we go over the poles for the first time and zip by the planet,” Nybakken said.

Earlier this week, NASA released one of the first images from JunoCam since the mission reached Jupiter.

Taken July 10, the picture shows Jupiter and three of its large Galilean moons — Io, Europa and Ganymede — from a distance of 2.7 billion miles (4.3 billion kilometers). The wide-angle camera, based on technology originally developed for NASA’s Curiosity Mars rover, resolved the planet’s colorful cloud bands and Jupiter’s iconic Great Red Spot.

The views will only get better as Jupiter pulls Juno toward its Aug. 27 close-up.

“This scene from JunoCam indicates it survived its first pass through Jupiter’s extreme radiation environment without any degradation and is ready to take on Jupiter,” Bolton said in a statement. “We can’t wait to see the first view of Jupiter’s poles.”

Officials are pleased with the outcome of Juno’s July 4 arrival burn, the third of four firings by the craft’s British-made main engine planned of the course of the nearly seven-year mission.

“We were aiming for a 53.4-day orbit, and we ended up in a 53.4-day orbit,” Nybakken said. “I think it was well within a 1-sigma performance. That’s the third time we’ve fired the main engine. We’re getting consistent, predictable performance out of it, which helps because we don’t have auto restart on the fourth one.”

Engineers programmed Juno’s flight computer to immediately resume the July 4 engine burn if it ran into trouble or suffered a computer reset triggered by Jupiter’s intense radiation belts. That feature, called auto restart, will not be installed on Juno for the next main engine maneuver set for Oct. 19 to steer the probe into a tighter, 14-day orbit for regular science observations.

The down side of employing the automatic engine restart software logic is that engineers on Earth would not have a chance to correct any underlying problem that triggered a computer fault in the first place.

Engineers were willing to take the risk July 4 because Juno had only one shot to enter orbit around Jupiter and complete its $1.1 billion mission.

One key unanswered question before Juno’s arrival at Jupiter was the exact location and intensity of the inner limit of the planet’s radiation belts, doughnut-shaped clouds of high-energy electrons surrounding the equator.

The radiation is a concern for Juno’s health because particles can penetrate the spacecraft, jeopardizing computing functions and degrading electronic components. Planners designed Juno’s orbit to stay away from the worst of the radiation, but doses will increase before the end of the mission in February 2018, and will likely determine when Juno’s campaign of exploration must conclude.

Nybakken said Juno did not gather any detailed information on the radiation environment July 4, but telemetry radioed to Earth after the arrival burn showed signs the spacecraft’s computer saw some particle hits as the probe passed in and out of the radiation belts.

“With the instruments off, we don’t get a lot of data,” Nybakken said. “We did see an increase, for a very short amount of time near the beginning of the burn and the end of the burn, kind of as expected, in the single bit errors that showed up on our spacecraft flight computer. They were all corrected.”

Juno’s most sensitive electronics are housed inside a titanium box on the spacecraft’s main body to protect systems from Jupiter’s radiation. Juno’s three large solar panels, each extending nearly 30 feet (about 9 meters) long, are covered in an extra-thick layer of glass to shield the craft’s power-generating solar cells from the energetic electrons lurking around Jupiter.

Managers want to learn about the radiation around Jupiter, both for scientific research and for practical, engineering purposes to help plan Juno’s mission.

“That’s part of what we’re going to be looking at,” Nybakken said. “Models are just that. They don’t give you highly accurate predictions, and, of course, the data we will be obtaining will help us refine environmental models as we go through the mission.”

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