Scheduled to lift off Thursday, April 13, the Jupiter Icy Moons Explorer (JUICE) will embark on an eight-year journey to Jupiter, with the spacecraft expected to arrive at the gas giant in July 2031.
The European Space Agency (ESA) spacecraft will carry out observations of Jupiter and three of its large oceanic moons – Ganymede, Callisto and Europe – characterizing the Jovian system in detail.
Jupiter orbit, on average (opens in a new tab), about 444 million miles (715 million kilometers) from Earth, so it’s clearly more than a hop, skip and a jump to the giant planet. But other missions made the trip from Earth to Jupiter much faster than JUICE will be.
Related: Facts about the European JUICE Jupiter Mission
It turns out that one of the main factors determining travel time to Jupiter is whether a spacecraft will fly over the gas giant or slot into orbit for a relatively long-term mission.
The first spacecraft to travel from Earth to Jupiter was NASA’s Pioneer 10, which was launched on March 3, 1972 and flew past Jupiter on December 3, 1973, meaning it reached the gas giant in just 640 days. Pioneer 11 was even faster on its flyby path, reaching Jupiter in just 606 days.
Then there are orbiters, which have to be more deliberate than hovercraft because they have to go slow enough at the end of the journey to be caught by Jupiter’s gravity. JUICE will travel for eight years in deep space. from NASA Galileo probe took about six years to get to Jupiter, launching in October 1989 and arriving in orbit in December 1995. The agency Juno spacecraft launched in August 2011 and reached its destination in July 2016.
Such variations in Jupiter’s orbiter travel times stem from several factors. For example, the distance between Earth and Jupiter varies greatly over time. Additionally, orbital missions do not take a direct path to the giant planet; they travel a scenic route around the inner solar system, flying over other planets to gain “gravity assists” that boost speed and sculpt trajectory.
For example, Galileo traveled about 2.5 billion miles (4 billion km) to get to the gas giant, taking gravitational aids from Venus, Earth and even the asteroid Gaspra along the way. JUICE will use a similar strategy.
The mission “will perform a complex sequence of gravitational assist maneuvers, beginning one year after launch, taking lunar and terrestrial gravitational assists, both bodies at the same time to obtain the additional energy,” the scientist said. ESA Alessandro Atzei during a JUICE press briefing on April 6. “And then we have a Venus flyby gravity assist [and] two more Earth gravitational assists in 2026 and 2029 to arrive at Jupiter in distant July 2031.”
Related: How gravity assists labor
What is gravity assist?
Gravity assists are a hovering technique that can increase a spaceship’s momentum. Mission planners use gravity aids to send spacecraft on paths that would not be feasible with a direct route; they would need too much fuel, for example.
Gravity assist can also be used to “steal” momentum from a spaceship. The Galileo mission is an example; the NASA craft reduced its energy compared to Jupiter while flying in front of the volcano Jovian Moon Io. Propellant is used to both speed up and slow down spacecraft, so an energy-stealing gravity assist can reduce the amount of propellant a spacecraft must use to fit into orbit.
A gravity assist works similar to a ball rolling down a hill into a valley, via a switch between gravitational potential energy and kinetic energy, the Planetary Society explains (opens in a new tab). As a spacecraft approaches a planet, it falls into the celestial body’s gravity well and gains kinetic energy, accelerating while losing gravitational potential energy. The spacecraft then leaves the planet on a new trajectory, which can be adjusted by changing the distance the craft passes in front of the planet. Planetary flybys can therefore have a “slingshot effect” on a spacecraft, helping it to make the most of its limited fuel reserves.
JUICE is one such spacecraft.
“We just don’t have enough energy [at launch] go into a more direct orbit [around Jupiter]so we have to do all these gravity maneuvers in order to gradually increase the energy of the spacecraft in order to reach Jupiter,” Atzei said.
The extra energy transmitted to the spacecraft must come from somewhere – the motion of the planet. The difference in mass between a spacecraft and a planet, however, is so great that the resulting slowing down of the planet is so small as to be practically immeasurable.
For example, the The planetary society says that when NASA’s Voyager 1 performed its gravity flyby of Jupiter in 1979, the gas giant slowed in its orbit by about 0.00000000000000000000000010 kilometers per second while Traveler 1 got a speed increase of 10 kilometers per second.
Inner Planets Helping Out: JUICE’s Gravity Helps
To get to Jupiter, JUICE will receive a total of four gravitational assists from the inner solar system, from the moonearth and Venus. (In 2017, the ESA proposed a fifth and final gravitational assist from Mars about three years before reaching Jupiter, but this is no longer planned.)
JUICE will head for its first gravity assist in August 2024 after completing its first solar orbit, and it will be very special for the ESA craft and for space exploration in general. This will be a combined lunar-terrestrial gravity assist, officially called Lunar-Earth Gravity Assist (LEGA), the first time such an operation has been performed.
“The first one will be very difficult, because it won’t just be terrestrial gravitational assist – lunar-terrestrial gravitational assist, which means we have to pass through the Earth and the Moon at the same time,” Atzei added. “So it will be the most precise gravity assist maneuver ever.”
The next gravity assist for JUICE will take place a year later, in August 2025, when the spacecraft will receive a boost from the second planet from the sun, Venus.
The spacecraft’s next two gravitational assists will both come from Earth. JUICE will fly past our planet in September 2026 and then make a final visit to Earth in January 2029, when it will receive another gravitational “kick”, its last before hitting Jupiter.
That won’t be the end of gravity assists for JUICE, though. Even after beginning its science mission, the spacecraft will need to perform gravitational assists to travel to the moons Ganymede, Europa, and Callisto. (JUICE will initially orbit Jupiter, performing multiple flybys of these three moons. It will then orbit Ganymede in 2035, becoming the first probe to orbit a moon other than Earth’s.)
“The gravity assists have to be precise, but we have very capable people in the mission control unit, and they are used to these maneuvers,” Atzei added.
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