Using data collected by the Advanced Stellar Compass (ASC) and Stellar Reference Unit (SRU) aboard NASA’s Juno orbiter, scientists have created the first complete 3D radiation map of the Jupiter system. The map not only describes the intensity of high-energy particles near the orbit of the icy moon Europa, but also shows how the radiation environment is shaped by the smaller moons orbiting near Jupiter’s rings.
This graphic shows Juno’s model of radiation intensity at various points in the spacecraft’s orbit around Jupiter. Image credit: NASA / JPL-Caltech / DTU.
“On Juno, we are trying to find new ways to use our sensors to learn more about nature, and have used many of our science instruments in ways they were not designed for,” said Juno lead investigator Dr. Scott Bolton, a planetary scientist at the Southwest Research Institute.
“This is the first detailed radiation map of the region at these higher energies and an important step toward understanding Jupiter’s radiation environment.”
“It’s a big deal that we were able to create the first detailed map of the region because we don’t have an instrument specifically designed to look for radiation.”
“The map will help plan observations for the next generation of missions to the Jupiter system.”
Juno’s ASC instrument consists of four star cameras on the spacecraft’s magnetometer boom and takes images of stars to determine the spacecraft’s orientation in space.
The instrument is also a valuable detector for high-energy particle fluxes in Jupiter’s magnetosphere.
The cameras record “hard radiation” – ionizing radiation that hits a spacecraft with enough energy to penetrate the ASC’s shielding.
“Every quarter of a second, the ASC takes a picture of the stars,” said Juno scientist Dr. John Leif Jørgensen, a researcher at the Technical University of Denmark.
“High-energy electrons that penetrate the shielding leave a telltale signature in our images that looks like the trail of a firefly.”
“The instrument is programmed to count the number of these fireflies, allowing us to accurately calculate the amount of radiation.”
Because of Juno’s ever-changing orbit, the spacecraft has traversed virtually all regions of space near Jupiter.
The ASC data suggest that there is more high-energy radiation relative to low-energy radiation near Europa’s orbit than previously thought.
The data also confirm that there are more high-energy electrons on the side of Europa facing its orbit than on the far side of the Moon.
This is because most of the electrons in Jupiter’s magnetosphere overtake Europa from behind due to the planet’s rotation, while the very high energy electrons drift backward – almost like fish swimming upstream – and crash into the front of Europa.
The radiation data from Jupiter is not the first scientific contribution of the ASC to the mission. Even before arriving at Jupiter, the ASC data were used to make a measurement of the interstellar dust impacting Juno.
Using the same dust-detection technique, the camera also discovered a previously unknown comet, identifying small pieces of the spacecraft ejected by microscopic dust that collided with Juno at high speed.
Like Juno’s ASC, the SRU can be a radiation detector and a low-light imager.
Data from both instruments suggest that the small shepherd moons that orbit within or near the edge of Jupiter’s rings – and help maintain the shape of the rings – also appear to interact with the planet’s radiation environment, like Europa.
When the spacecraft flies on magnetic field lines associated with ring moons or dense dust, the radiation number on both the ASC and the SRU drops rapidly.
The SRU is also collecting rare images of the rings in low light from Juno’s unique vantage point.
“The formation of Jupiter’s rings is still a great mystery, and very few images have been taken by previous spacecraft,” said SRU lead co-investigator Dr. Heidi Becker, a researcher at NASA’s Jet Propulsion Laboratory.
“Sometimes we are lucky and one of the little shepherd moons can be captured in the picture.”
“These images will allow us to determine more precisely where the ringed moons are currently located and how the dust is distributed relative to their distance from Jupiter.”
The results appear in the journal Geophysical Research Letters.
