What are the scientific and engineering goals and outcomes of gravity assist missions?

Let’s explore this concept via the ‘JUICE’ mission, which involves sending a spacecraft to study the Jovian system. It has been 11 months since launch and yet there’s an additional 8 years of travel ahead. Guess what? This is still the shortest trip we can manage, and that’s made possible by more than 25 gravity assists planned along the way. Without these crucial manoeuvers, it would be most unlikely for the spacecraft to achieve the necessary velocity to reach Jupiter in the designated timeframe. Also, conserving propellant is imperative for the entirety of the mission as we have limited resources on board. So, in short, you can think of gravity assists as I like to call them, “cosmic hitchhiking”.

Gravity assist, also known as gravitational slingshot or swing-by, is a technique used in spacecraft navigation to alter the trajectory and velocity of a spacecraft by utilizing the gravitational pull of celestial bodies, typically planets or moons. When a spacecraft approaches a massive body, such as a planet, it interacts with the body's gravitational field, which bends the spacecraft's trajectory. By carefully timing the encounter and adjusting the spacecraft's trajectory relative to the body's motion, engineers can harness the body's gravitational energy to either accelerate or decelerate the spacecraft. This allows spacecraft to gain speed without expending propellant or to alter their trajectory to reach distant destinations.

By utilizing the gravitational pull of planets or other celestial bodies, spacecraft can gain speed or alter their trajectory without expending additional fuel. This technique enables missions to reach distant destinations more efficiently and at higher velocities than would be possible solely through propulsion. Furthermore, gravity assists allow spacecraft to navigate complex trajectories, such as reaching outer planets or exploring multiple destinations within a single mission, while conserving onboard resources. Additionally, gravity assists can enable missions to conduct flybys of interesting celestial objects, gathering valuable scientific data and imagery along the way.

One notable example is NASA's Voyager missions, which utilized gravity assists from Jupiter and Saturn to explore the outer planets of the solar system. Another prominent example is the Cassini-Huygens mission, which employed multiple gravity assists from Venus, Earth, and Jupiter to reach Saturn and study its moons and rings. Additionally, the New Horizons mission to Pluto utilized a gravity assist from Jupiter to increase its speed and reach its distant target more efficiently. These missions demonstrate the effectiveness of gravity assists in maximizing spacecraft efficiency and enabling ambitious exploration of the solar system.

In case of gravity assist maneuvers, precision in timing and trajectory is crucial as even the smallest of miscalculations can lead to significant deviations (like in Chaos Theory). Navigational uncertainties, planetary atmospheric drag, and the need for accurate data on celestial bodies' positions complicate these maneuvers. Unintended and unforeseen celestial bodies like asteroids could also affect the trajectories as a whole and alter the expected outcomes. Instances where assists didn't work as intended include the Mars Climate Orbiter in 1999, which failed due to navigation errors, leading to its loss, Mariner 10 whose trajectory had to be adjusted multiple times due to Mercury affecting its path more than expected during its flyby

One challenge is accurately predicting the trajectory and timing of the spacecraft's encounter with the assisting celestial body, which requires precise calculations and modelling of complex gravitational interactions. Additionally, gravity assists are limited by the availability of suitable planetary alignments and the locations of target destinations, constraining the timing and feasibility of mission opportunities. Furthermore, the gravitational slingshot effect can only provide a finite amount of velocity change, limiting its utility for missions requiring significant alterations in trajectory or velocity.

Technically, as spacecraft and mission planning capabilities continue to evolve, the utilization of gravity assists offers a cost-effective means to optimize trajectories, conserve fuel, and achieve ambitious mission objectives. Future missions may leverage advanced trajectory optimization algorithms to capitalize on multi-body gravitational interactions, enabling spacecraft to traverse vast distances within our solar system and beyond. Moreover, with ongoing developments in propulsion technologies and mission planning, gravity assists may play a pivotal role in facilitating interstellar travel concepts, potentially revolutionizing our ability to explore distant star systems.