Why is our trip to Jupiter taking so long?

At a glance, it seems rather perplexing: five months post-launch and the spacecraft named Juice has covered 370 million kilometers, which represents a mere 5% of its journey to Jupiter. 

To explain the extended timeline, the European Space Agency breaks down the science of flight dynamics and orbital mechanics.

Complex dance of planets

Every celestial body in our universe – planets, moons, stars, or galaxies – orbits another entity. 

Earth orbits the Sun at a 30 km/s. When a spacecraft is launched from our planet, it inherits this orbital energy, effectively placing it in a similar orbit around the Sun as Earth.

Challenges for Juice 

One might assume the most efficient route to Jupiter would be a straight line, but this approach has its challenges. 

Not only would it require immense fuel to propel the spacecraft in such a direction, but an even greater fuel reserve would be essential to decelerate and orbit Jupiter upon arrival, lest the spacecraft overshoot its target.

Moving targets

Earth and Jupiter, in their eternal celestial journey around the Sun, are constantly changing positions relative to each other. 

Their distance varies, reaching a maximum of 968 million kilometers and a minimum of just under 600 million. 

Given this perpetual motion, aiming a spacecraft directly at Jupiter is not feasible. 

Instead, flight engineers must anticipate where Jupiter will be when the spacecraft arrives. 

“The planets are all moving at different rates in their orbits around the Sun,” said ESA. 

“Imagine throwing a ball at a moving target from a moving vehicle. Engineers must calculate the ideal time to make the jump on a circular path from Earth’s orbit to where Jupiter will be when the spacecraft arrives, not where it is when the spacecraft leaves Earth.”

Flyby missions

Historically, missions like Voyager, Pioneer, and New Horizons have managed to reach Jupiter in under two years. 

However, these missions were mere flybys, using Jupiter as a gravitational slingshot to other destinations.

Energy and mass

To maintain an extended presence around Jupiter, the approach must be slower. 

A spacecraft’s mass plays a pivotal role here. More mass implies more fuel, which in turn affects the launch complexity. Juice, weighing in at over 6,000 kg, is one of the heaviest interplanetary probes to date. 

Even with the immense power of the Ariane 5 rocket, a direct two-year trajectory to Jupiter wasn’t viable.

The solution lies in gravity-assist or flyby maneuvers. These maneuvers utilize the gravitational pull of celestial bodies to boost the spacecraft’s speed, a cosmic dance that trades energy between the spacecraft and planets.

Trading energy 

Juice’s itinerary includes a series of flybys involving Earth, the Moon, and Venus. These maneuvers will position it for its grand rendezvous with the Jovian system in July 2031.

The journey, filled with an unprecedented 35 flybys of Jupiter’s moons, will end with Juice entering the orbit of Ganymede, marking a historic first for humankind.

Precise maneuvers 

But such intricate space choreography is not without challenges. Every maneuver must be precise. 

Even slight miscalculations could lead to the probe being lost in space or requiring extensive fuel to correct its course.

The overarching goal of Juice’s mission is to study the potentially life-harboring oceans beneath the icy surfaces of Europa, Ganymede, and Callisto. 

Through this grand exploration, insights into the formation of planets and moons across the universe might be gleaned.

For updates, you can follow the mission’s progress on Twitter via @ESAJuiceBar.

Image Credit: ESA

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