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The European Space Agency’s Juice mission made two historic firsts this week on its long journey to study the potential habitability of Jupiter’s icy moons.
The Juice, or Jupiter Icy Moons Explorer, spacecraft — launched in April 2023 — conducted the first flyby of the moon and Earth in succession, and the first double gravity assist maneuver, on Monday and Tuesday. This process served as a slingshot, allowing Earth’s gravity to put the brakes on Juice’s current trajectory and redirect it for a flyby of Venus in August 2025.
Then, Juice can take a shortcut through the solar system and be on track to arrive at Jupiter and its moons in July 2031.
Juice flew by Earth’s moon on August 19 at 5:15 p.m. ET. Next, it zipped by Earth nearly 25 hours later at 5:56 p.m. ET on August 20. The space agency provided animated tracking of the spacecraft, and some people with binoculars or telescopes in Southeast Asia may have been able to see Juice as it flew directly overhead.
All the while, Juice’s two cameras took photos during the flyby and sent them back to Earth.
An image captured by one of Juice’s cameras on Monday shows color differences in large-scale features on the lunar surface.
The pioneering maneuvers were also incredibly daring and challenging, and one small mistake could have knocked Juice off course and ended the mission. But the careful planning of the mission team paid off.
“The gravity assist flyby was flawless, everything went without a hitch, and we were thrilled to see Juice coming back so close to Earth,” said Ignacio Tanco, Juice’s Spacecraft Operations Manager, in a statement.
High-risk, high reward
On average, Jupiter is located 497 million miles (800 million kilometers) from Earth, so reaching it without an enormously powerful rocket or thousands of pounds of onboard propellant requires some careful strategizing by mission planners. And the plan for Juice’s journey was in the works for 20 years.
Using the gravity of planets like Earth and Venus can make just the right adjustments to Juice’s trajectory to put it in the proper direction and at the optimal speed to reach Jupiter and go into orbit without whizzing right by the planet.
Gravity assists can speed up or slow down spacecraft, depending on how they are used, and they conserve fuel and allow spacecraft to be outfitted with a multitude of scientific instruments.
The double flyby of Earth and the moon slowed down Juice just enough so it can fly by Venus for an energy boost next year before looping around our planet twice for more boosts in 2026 and 2029, according to the agency.
Minor adjustments to Juice’s flight pattern put the spacecraft on track to arrive at the moon and Earth at the right time and speed while coming extremely close to both. Juice first passed 434 miles (700 kilometers) from the moon’s surface and then it flew 4,250 miles (6,840 kilometers) from Earth’s surface.
The moon’s gravity slightly bent Juice’s path so it received a much larger gravity assist from Earth.
The flyby of Earth reduced Juice’s speed by 10,737 miles per hour (4.8 kilometers per second) to set it on the path toward Venus and will save the mission around 220 to 330 pounds (100 to 150 kilograms) of fuel.
A boost from the Ariane 5 rocket used to launch Juice last year gives the spacecraft extra propellant to get closer to Jupiter’s moon Ganymede than initially planned, and the success of the double flyby has safeguarded this “bonus science” opportunity.
“Thanks to very precise navigation by ESA’s Flight Dynamics team, we managed to use only a tiny fraction of the propellant reserved for this flyby. This will add to the margins we keep for a rainy day, or to extend the science mission once we get to Jupiter,” Ignacio said.
Operators using ground stations around the world closely tracked Juice’s data before, during and after the flyby to make any necessary adjustments at a moment’s notice.
The flight control team trained and rehearsed for the flyby in case any anomalies occurred so they could quickly get Juice back on track.
A chance opportunity
Telescopes and observatories have been tracking Juice’s close approach throughout of the summer.
On July 6, an object triggered the automated NASA and ESA warning systems that monitor for potentially hazardous asteroids. The object was estimated to be 164 feet (50 meters) in diameter and on track to pass very close to Earth and the moon.
But it was just Juice, and given that it has large, reflective solar arrays, the spacecraft appeared much larger and brighter, like an asteroid. The agencies confirmed that Juice posed no risk to Earth or the moon during its flyby.
Juice also activated all 10 of its scientific instruments during the double flyby to calibrate them before arriving at Jupiter. In addition to testing the instruments, the mission team could also make some discoveries about Earth and the moon.
The RIME team, in charge of the Radar for Icy Moon Exploration instrument, is eager to gather data on electronic noise within the spacecraft that seems to disturb the instrument. This could be one of only a few chances to measure any impacts to the instrument before it arrives at Jupiter.
During the flyby, RIME took eight minutes to make observations while the other instruments shut off or went into quiet mode, and that data could help the mission team fix the noise problem.
Images and data collected during the flyby will be shared by the agency in the coming weeks.
“The timing and location of this double flyby allows us to thoroughly study the behaviour of Juice’s instruments,” said Claire Vallat, Juice Operations Scientist, in a statement.
“It happens early enough in Juice’s journey that we can use the data to prepare the instruments for arrival at Jupiter. And given how well we know the physical properties of Earth, the Moon, and the surrounding space environment, it’s also the ideal location to understand how the instruments respond to a real target.”