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All eyes are on Jupiter

A 1/4 scale model of the Juno spacecraft.
A 1/4 scale model of the Juno spacecraft.David McNew/Getty Images

As NASA’s Juno spacecraft closes in for its July 4 arrival at Jupiter, many other eyes are also staring at the solar system’s largest planet.

Data from about 25 observatories, including some of the largest on Earth, like the W.M. Keck Observatory in Hawaii, and in orbit around Earth like the Hubble Space Telescope, will aid scientists in interpreting the data that Juno is expected to gather as it swoops close to the cloud tops of Jupiter over the next 20 months.

It has taken Juno nearly five years to reach this point in its journey.

“In just a few days, we’re about to arrive at Jupiter, and it’s hard to believe,” Scott Bolton, the mission’s principal investigator, said at a NASA news conference on Thursday.

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Around last Friday, Juno crossed from interplanetary space into the magnetic bubble surrounding Jupiter that deflects the stream of particles from the sun known as the solar wind. “Inside that magnetosphere is Jupiter’s domain that’s filled with its particles,” Bolton said. “It has blocked out the sun’s particles.”

Auroras created by high-energy particles are seen on a pole of the planet Jupiter.
Auroras created by high-energy particles are seen on a pole of the planet Jupiter.NASA/ESA/Handout via Reuters

One of Juno’s instruments measures radio emissions from the charged particles, and those radio waves are easily converted into sound waves, which sharply changed in tone as the spacecraft crossed what is called the “bow shock,” where Jupiter’s magnetic field plows through the solar wind.

“Just the sound of it can tell you it’s nontrivial to go into Jupiter,” Bolton said, as NASA released the recording of it.

A couple of days ago, the propulsion system was pressurized in preparation for the firing of the spacecraft’s engine, which will take 35 minutes on July 4. On Thursday, the last set of instructions before the arrival were sent to Juno.

“Then it’ll be hands-off from the team,” said Edward Hirst, the mission manager. “The spacecraft is on its own, and it’s designed to take care of itself.”

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When in orbit, Juno’s instruments will be able to peer deep into Jupiter, but only for a narrow swath. The faraway observations by telescopes will “fill in the blanks to get the big picture story,” Leigh Fletcher of the University of Leicester in England, said in an interview. “We’ll have the best observational data set of Jupiter that we’ve ever had.”

This week, Fletcher and his colleagues released infrared photographs of Jupiter seemingly on fire. “You can see the internal glow of Jupiter coming out,” Fletcher said. “Those clouds are colder and are absorbing in the infrared.”

Once Juno begins orbiting Jupiter, each time the spacecraft zips in close, the team will take similar photographs using the European Southern Observatory’s Very Large Telescope in Chile “to place the Juno close-up observations in their broader spatial context.”

For example, Fletcher said he wanted to piece together Juno’s measurements of ammonia and water — key constituents of Jupiter’s clouds — with the weather patterns seen in the Very Large Telescope pictures.

With Juno providing the first good look at Jupiter’s poles, Fletcher is curious as to whether huge hurricane-like storms rage, similar to what has been observed in Saturn’s polar regions. The auroras — glowing patterns powered by charged particles slamming into the atmosphere — might also affect the weather.

To gain more data on the auroras, the Hubble Space Telescope has been staring at Jupiter for 45 minutes every day for the past month. At Earth, auroras light up when solar wind particles slam into molecules of air near the polar regions. At Jupiter, the charged particles mostly come from a different source: the volcanoes of Io. Jupiter’s powerful magnetic fields then accelerate the particles into the planet’s atmosphere.

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At times, the auroras are subdued. At other times, they are “almost like fireworks going off in the ionosphere of Jupiter,” said Jonathan Nichols, a University of Leicester scientist who is leading the Hubble study.

One of the things that is unknown is how much the solar wind contributes to Jupiter’s auroras. “We’ll be working on that over the next few weeks and months,” Nichols said.

On Earth, auroras appear over an area about as large as the United States. Jupiter’s auroras, like everything else on Jupiter, are vastly larger, about as wide as five Earths.

“But we’ve never been able to get up close and really observe these processes,” Fran Bagenal, a professor of astrophysical and planetary sciences at the University of Colorado and one of the mission scientists, said at the news conference. “So we can then compare them with what we see at Earth, what we seen at Saturn.”

Bagenal said scientists were curious about whether Jovian auroras were simply larger versions of Earth’s auroras. “Or do we have to really go back to the fundamental physics and work out what’s really going on here?” she wondered.