"We've just crossed the boundary into Jupiter's home turf," Juno Principal Investigator Scott Bolton of Southwest Research Institute said in a statement.
The probe has entered Jupiter's magnetosphere — the area of space where particles are influenced not by solar wind, but by Jupiter's own magnetic field. The charged particles zipping relentlessly around Jupiter give it a magnetic field some 20,000 times more powerful than the one surrounding our own planet. As a result, its magnetosphere is the largest structure in the solar system — which is fitting, because if every other planet in our solar system teamed up to form one massive monolith of a world, mostly-gas Jupiter would still be 2½ times heavier.
"If Jupiter's magnetosphere glowed in visible light, it would be twice the size of the full moon as seen from Earth," mission scientist William Kurth of the University of Iowa said in a statement. And the portion that we'd be able to see is puny compared to the tail that streams out behind Jupiter.
According to data from Juno, the spacecraft passed through the bow shock — the sonic-boom-like disturbance created when Jupiter's magnetosphere plows into the solar winds around it — on June 24. By the next day, it was working its way into the fringes of the magnetosphere itself. Here's that data translated into an audio "boom":
Now Juno is recording the calm before the storm. At the edge of the sun's influence, the spacecraft encountered some 16 particles per cubic inch of space it traveled through. Once it entered the magnetosphere, the particle density dropped by about one hundred times. But that will change as the orbiter approaches its tumultuous target.
"It's like a spray of radiation bullets," Heidi Becker of NASA's Jet Propulsion Laboratory, who serves as Juno's radiation monitoring investigation lead, said of Jupiter's radioactive particles during a news conference last month. The magnetic field of the planet could quite literally fry a space robot that got too close, and Juno is slated to get very close during its 32 planned orbits — close enough to probe under Jupiter's roiling cloud cover for the first time ever. But the probe has been given a veritable suit of armor to protect it. Its most valuable computer components are encased in 400 pounds of titanium.
The power of Jupiter's magnetic field isn't just terrifying. It also creates gorgeous light shows in the form of aurorae. On Earth, aurorae like the Northern Lights form when particles from the sun interact with our magnetic field and become excited. The excited particles spewed by Jovian moon Io's volcanic activity help to make these displays much more dramatic on Jupiter than they are at home. Some of them cover areas larger than our entire planet.
Juno will help us understand these gorgeous space storms and much more. On Thursday, scientists sent a signal to switch Juno over to autopilot — beginning the probe's insertion sequence. The probe is expected to enter Jupiter's orbit just before midnight Eastern time on July 4. Scientists should be able to confirm its success soon after.