Spacecraft use multiple gravity assists when a single launch cannot provide the speed and direction needed for the trip to Jupiter. Each flyby changes the spacecraft’s velocity relative to the Sun, and a carefully planned sequence can build the trajectory a mission needs. The tradeoff is often a longer, more demanding journey; not every Jupiter mission needs multiple assists.
How a gravity assist changes a spacecraft’s path
A gravity assist is an encounter among a spacecraft, a moving planet and the Sun. In the simplified view of the spacecraft relative to the planet, the craft speeds up as it approaches and slows by about the same amount as it departs. Its speed relative to that planet is therefore approximately unchanged, but its direction has changed.
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Because the planet is moving around the Sun, that change in direction alters the spacecraft’s velocity and energy relative to the Sun. The spacecraft exchanges a tiny amount of momentum and energy with the planet; energy is not created from nothing. The flyby’s geometry determines whether the craft gains or loses Sun-relative energy, as well as which direction it travels next. NASA explains the basic maneuver in its Galileo mission overview and notes that a flyby can also reduce a spacecraft’s energy when that is useful in its planetary-flyby explanation.
Why use a sequence of assists?
A mission may need a larger overall change in speed and direction than its launch vehicle and onboard propulsion can provide by themselves. Instead of obtaining all the required energy at launch, planners can target a sequence of planetary encounters. Each encounter is designed as part of the whole route: an assist is not automatically a boost, and the next planet or destination determines what the spacecraft needs from it.
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- Launch capability: A sequence can make Jupiter reachable when the available launch vehicle cannot send the spacecraft directly onto the required trajectory.
- Trajectory design: Flybys can shape both the spacecraft’s energy and its direction for the next leg or arrival.
- Mission tradeoffs: Detours can add travel time and distance, and a route closer to the Sun can require additional thermal protection.
Galileo: three assists made Jupiter reachable
Galileo was originally intended to travel directly to Jupiter using a more powerful launch configuration. After the Shuttle-Centaur combination was canceled following the Challenger accident, the mission was reconfigured to use the less powerful Inertial Upper Stage. NASA’s account says the spacecraft then needed a Venus-Earth-Earth sequence, commonly called VEEGA, to attain the energy needed to reach Jupiter. The altered route also brought Galileo closer to the Sun than planned, so the spacecraft needed additional thermal shielding. See NASA’s Galileo mission overview.
NASA reports that the change extended Galileo’s trip from two years to six. The assists solved a launch-capability problem, but the price was a longer journey and additional spacecraft design demands.
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Juno: a Jupiter mission with one Earth assist
Not all Jupiter missions use several planetary flybys. Juno launched in 2011, traveled out beyond Mars, then returned to Earth for one gravity assist before continuing to Jupiter. NASA reports that the Earth encounter increased Juno’s velocity by 16,330 mph (about 7.3 km/s). NASA says that without the assist Juno would have needed a more powerful launch vehicle or a more time-consuming voyage. The figure is specific to Juno’s Earth flyby, not a general amount that other spacecraft can expect. Details are on NASA’s Juno mission page.
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1Clear out junk files and repair common Windows errors2Fix the driver behind crashes, sound loss and screen glitches3Repair Windows errors before they cause bigger problemsWhat determines the number of assists?
There is no fixed number of flybys required to reach Jupiter. Mission designers choose a route based on the spacecraft’s launch opportunity and vehicle, its propulsion, the desired arrival conditions, and the operational limits it can tolerate. One mission may use a single assist, another may use several, and a mission may use a flyby to shed energy rather than gain it. Galileo and Juno illustrate different solutions to different mission constraints.
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