Johns Hopkins APL Instruments Aboard Europe’s Jupiter Mission Successfully Hum to Life

Two months after their successful launch from French Guiana, the pair of particle instruments built by the Johns Hopkins Applied Physics Laboratory (APL) on behalf of NASA for the European Space Agency Jupiter Icy Moons Explorer (JUICE) spacecraft successfully hummed to life.

The gilded instrument JENI sits on a white table
The Jovian Energetic Neutrals and Ions (JENI) instrument that will capture images of plasma around Jupiter and three of its icy moons. (Credit: Johns Hopkins APL/Ed Whitman)

On June 5, spacecraft operators in Germany switched on the instruments, called Jovian Energetic Neutrals and Ions (JENI) and Jovian Energetic Electrons (JoEE). Together, they form the high-energy Particle Environment Package, or PEP-Hi, which is part of the six-instrument suite PEP.

For just over a week, operators alongside instrument team members who called in from or near APL’s campus in Laurel, Maryland, gradually ramped up the instruments’ electrical juice. JoEE and JENI became fully operational June 14, with all systems running nominally. Operators shut them down a few days later as part of the planned testing sequence of onboard systems, but in that short time frame, the instruments already displayed their sophisticated detectors’ capabilities.

JoEE, a small, gray metallic instrument with a row of small punctured plates on its front, sits on a white lab table
The Jovian Energetic Electrons (JoEE) instrument is an electronic spectrometer that will help scientists understand how Jupiter accelerates charged particles to near-light speed. (Credit: Johns Hopkins APL/Ed Whitman)

At Jupiter, both instruments will not only help researchers understand the gas giant’s three icy, ocean-bearing moons — Europa, Callisto and Ganymede — but also elucidate the physical mechanisms that allow Jupiter to whip particles to near relativistic speeds and heat them close to 2 million degrees Fahrenheit (more than 1 million degrees Celsius).

JoEE specializes in collecting electrons zooming around, while JENI focuses on detecting ions (charged atoms) and energetic neutral atoms. According to PEP-Hi Principal Investigator Pontus Brandt, a space physicist at APL, both instruments have already successfully detected their respective particles, including electrons, protons and helium ions. To the team’s pleasant surprise, the instruments already saw substantial signals from the solar wind — the stream of hypersonic particles that flows from the Sun.

“The tests couldn’t have gone better,” said APL’s Stephen Jaskulek, the PEP-Hi Lead Systems Engineer who led the team from Germany during the instrument commission. “The teamwork in flight control was excellent.”

The successful tests are a good sign for what’s to come years from now, after JUICE has wound around Earth and Venus a few times to gain enough speed to catapult out to Jupiter. “Few other spacecraft have visited the region of space between Earth and Jupiter with these types of instruments,” Brandt said. When JUICE finally begins to approach Jupiter in 2031, JENI should be able to provide an unprecedented wide-scale view of the solar wind colliding with Jupiter’s magnetic field, the largest astrophysical object in the solar system.

“From JENI’s view, it should burst alive with light,” Brandt said. “It should be incredible.”

The instruments’ next operational checkout when they’ll switch on again is scheduled for next year.

A graphic of several colorful graphs showing data the JoEE and JENI collected during its first operation
Visualizations and descriptions of the first data returned from the JENI and JoEE instruments, including signals from the solar wind. (Credit: Johns Hopkins APL/Pontus Brandt)

Launched in April 2023, JUICE will explore Jupiter’s three large moons for at least three years, characterizing them as both planetary objects and possible habitats for life. The mission will also closely examine Jupiter’s complex radiation environment and study the wider Jovian system as an archetype of gas giants across the universe.

To learn more about JUICE and the PEP-Hi suite, visit

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