It’s time for the close-up, Ganymede.
On Monday, NASA’s Juno spacecraft passed within 645 miles of Ganymede, the largest of Jupiter’s 79 known moons and makes it the largest moon in the entire solar system. It was the first in-depth examination of Ganymede since the passage of an earlier NASA probe, Galileo, in December 2000.
NASA released two images of the flyby on Tuesday, revealing in remarkable detail craters, possible tectonic faults and distinct light and dark terrain.
One image, taken by the main camera, JunoCam, captured most of Ganymede’s daytime side. For now, the image is in black and white. But when additional versions of the same view, taken through red and blue filters, are returned from the spacecraft, the images can be combined into a color portrait.
The second image was captured by a navigation camera called the Stellar Reference Unit which can operate in low light conditions and was able to get a clear view of Ganymede’s night side as Juno passed.
“It will be fun to see what the two teams can put together” with the images to come, said Heidi Becker, head of radiation monitoring for the Juno mission.
The spacecraft will continue to return its observations over the next few days.
Juno, who arrived in Jupiter on July 4, 2016, has just completed its main mission to probe the deep interior of the largest planet orbiting the sun. He discovered that storms like the Great Red Spot penetrate deep into the gaseous atmosphere of the giant planet and that Jupiter’s core is larger and more diffuse than expected.
But instead of ending the mission by sending Juno to dive to death in Jupiter, NASA has extended the mission until 2025. Juno will now perform 42 additional orbits of Jupiter and some of those orbits will include close flyovers of Ganymede and two of the other great moons, Io and Europe.
“We are very fortunate that the spacecraft is in good health,” said Scott Bolton, the mission’s principal investigator, “and able to produce such great science and all the amazing results and images all these years.”
Ganymede, over 3,200 miles wide, is larger than the planet Mercury and is the only moon large enough to generate its own magnetosphere – a bubble of magnetic fields that trap and deflect charged particles from the sun.
“We are well equipped, probably better equipped to measure Ganymede’s magnetosphere and its interaction with Jupiter’s magnetosphere than any spacecraft has ever been,” said Dr Bolton.
The data Juno collects will help a few future missions. Next year, the European Space Agency will launch JUICE – the Jupiter Icy Moons Explorer – which will perform multiple overflights of three large moons – Ganymede, Europa and Callisto – before entering orbit around Ganymede in 2032.
Another NASA mission, Europa Clipper, is set to launch later this decade and will focus on Europa, one of the most intriguing worlds for planetologists looking for life elsewhere in the solar system. Europe has a deep ocean beneath its ice-encrusted surface, with the heat from the moon’s core perhaps providing enough energy for organisms to live in the waters.
“We’re going to kind of fill the void a bit,” Dr Bolton said.
Jupiter’s immense pull of gravity steadily tilts Juno’s orbit so that it is now approaching Jupiter in the northern hemisphere. This was not ideal for some of the sightings during the main mission, but it will now allow planetologists to get a better look at Jupiter’s North Pole and the enigmatic storms in the area.