An artist's rendition of Juno flying by Jupiter. NASA NASA's Juno space probe crossed a momentous threshold in the solar system last week.

As the spacecraft closed in on 5.56 million miles from the largest planet in the solar system, Juno crossed into Jupiter's magnetic field.

Inside this domain, the density of charged particles bombarding Juno in space dropped about a hundredfold, because the planet's magnetosphere is protecting it.

"We can actually listen to what it's like to leave the Sun and enter Jupiter," Juno's principal investigator Scott Bolton said at a NASA press conference June 30. "Just the sound of it will tell you that it's non-trivial to go into Jupiter."

You can hear the eerie sound 10 seconds in. Listen:

The sun fills outer space with charged particles (ions, electrons) called solar wind, Bolton explained. Here on Earth, our magnetic field protects us from the brunt of this galactic force.

Jupiter also has a magnetic field, and it's much, much larger than Earth's. Jupiter's magnetosphere is actually the biggest thing in the solar system.

A visualization of Jupiter's magnetosphere. You can see it extends all the way to Saturn on the right. NASA

NASA expects the density in charged particles to climb again as Juno gets closer to the planet, since it will then be bombarded with particles from Jupiter.

Bolton joked that the scientists argued about when Juno technically crossed the threshold, but the sound data confirmed it was on June 24.

The most difficult part of the mission is ahead as Juno attempts to get closer to Jupiter than we ever have before. Then they'll have the planet's wicked gravity to contend with.

The space probe will start orbiting Jupiter July 4.