Using data gathered by the Advanced Stellar Compass (ASC) and the Stellar Reference Unit (SRU) onboard NASA’s Juno orbiter, scientists have produced the first complete 3D radiation map of the Jovian system. Along with characterizing the intensity of the high-energy particles near the orbit of the icy moon Europa, the map shows how the radiation environment is sculpted by the smaller moons orbiting near Jupiter’s rings.
This graphic shows Juno’s model for radiation intensity at different points in the spacecraft’s orbit around Jupiter. Image credit: NASA / JPL-Caltech / DTU.
“On Juno we try to innovate new ways to use our sensors to learn about nature and have used many of our science instruments in ways they were not designed for,” said Juno principal investigator Dr. Scott Bolton, a planetary researcher at the Southwest Research Institute.
“This is the first detailed radiation map of the region at these higher energies, which is a major step in understanding how Jupiter’s radiation environment works.”
“That we’ve been able to create the first detailed map of the region is a big deal, because we don’t carry an instrument designed to look for radiation.”
“The map will help planning observations for the next generation of missions to the Jovian system.”
Consisting of four star cameras on the spacecraft’s magnetometer boom, Juno’s ASC instrument takes images of stars to determine the spacecraft’s orientation in space.
But the instrument is also a valuable detector of high-energy particle fluxes in Jupiter’s magnetosphere.
The cameras record ‘hard radiation’ — ionizing radiation that impacts a spacecraft with sufficient energy to pass through the ASC’s shielding.
“Every quarter-second, the ASC takes an image of the stars,” said Juno scientist Dr. John Leif Jørgensen, a researcher at the Technical University of Denmark.
“Very energetic electrons that penetrate its 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, giving us an accurate calculation of the amount of radiation.”
Because of Juno’s ever-changing orbit, the spacecraft has traversed practically all regions of space near Jupiter.
The ASC data suggest that there is more very high-energy radiation relative to lower-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 orbital direction of motion than on the moon’s trailing side.
This is because most of the electrons in Jupiter’s magnetosphere overtake Europa from behind due to the planet’s rotation, whereas the very high-energy electrons drift backward, almost like fish swimming upstream, and slam into Europa’s front side.
The Jovian radiation data are not the ASC’s first scientific contribution to the mission. Even before arriving at Jupiter, the ASC data were used to determine a measurement of interstellar dust impacting Juno.
The imager also discovered a previously uncharted comet using the same dust-detection technique, distinguishing small bits of the spacecraft ejected by microscopic dust impacting Juno at a high velocity.
Like Juno’s ASC, the SRU can be a radiation detector and a low-light imager.
The data from both instruments indicate that, like Europa, the small shepherd moons that orbit within or close to the edge of Jupiter’s rings — and help to hold the shape of the rings — also appear to interact with the planet’s radiation environment.
When the spacecraft flies on magnetic field lines connected to ring moons or dense dust, the radiation count on both the ASC and SRU drops precipitously.
The SRU is also collecting rare low-light images of the rings from Juno’s unique vantage point.
“There is still a lot of mystery about how Jupiter’s rings were formed, and very few images have been collected by prior spacecraft,” said SRU lead co-investigator Dr. Heidi Becker, a researcher at NASA’s Jet Propulsion Laboratory.
“Sometimes we’re lucky and one of the small shepherd moons can be captured in the shot.”
“These images allow us to learn more precisely where the ring moons are currently located and see the distribution of dust relative to their distance from Jupiter.”
The findings will appear in the journal Geophysical Research Letters.
Source : Breaking Science News