The moment we bring a satellite into the sphere of influence of a celestial body such as a planet, it can use the gravitational force to change its absolute velocity. The increase or decrease in its own velocity is called a”gravity assist,” “flyby,” or “hyperbolic encounter”. The gravitational action in question succeeds in rotating the direction of the velocity vector while keeping its modulus unchanged.
Flyby or Gravity assist: Let’s try to go into more detail
Let’s take the satellite’s velocity with respect to the Sun Vs, which is equal to the satellite’s velocity with respect to the considered planet Vsp added to the planet’s velocity with respect to the Sun Vp. We can have two possible transitions in the planet’s “sphere of influence”.
The “downstream” passage of the planet occurs when the outgoing Vs is less than the incoming Vs and thus we have a loss of kinetic energy in the motion of the satellite relative to the Sun. This operation is often referred to as a “flyby in front of the planet”
The “upstream” passage of the planet occurs when the outgoing Vs is greater than the incoming Vs and thus we have a purchase of kinetic energy in the motion of the satellite relative to the Sun. This operation is often referred to as a “flyby behind the planet”.
This effect just mentioned is often used for speed modification of any spacecraft in interplanetary missions. The vehicle in question, is purposely guided through the spheres of influence of one or more celestial bodies.
Let’s look at an example of the application of a gravitational slingshot
A recent example we can cite is NASA’s Cassini mission.This mission started with the launch of the probe in 1997. It took several fly-bys to get, using as little propellant as possible,into the orbit of Saturn. The Cassini probe implemented two gravitational slingshots around Venus, one around Earth and, finally, one around Jupiter to arrive after 7 years around the planet Saturn.
How do we determine if a flyby is “upstream” or “downstream”?
Before answering this question we must keep in mind what the pericenter of a planetary orbit is.The pericenter of an orbit is the point of maximum approach to the focus of the orbit. So, if we want to determine whether a gravitational slingshot will increase or decrease the kinetic energy of our probe relative to the Sun,we must observe the position of the pericenter of the hyperbolic orbit of approach.
If the pericenter of this orbit is in the downstream half-space, then we have a “downstream” approach.If the pericenter of this orbit is in the upstream half-space, then we have an “upstream” approach.
The BepiColombo mission and its Flyby or Gravity assist
BepiColombo is an international mission consisting of two spacecraft traveling together to Mercury. These aim to enter Mercury’s orbit and study the planet from unique vantage points. The European Space Agency (ESA) provided the first probe. The Japan Aerospace Exploration Agency (JAXA), meanwhile, provided the second probe.
Below is a list of all the gravitational slingshots that the two probes will go on:
- Ottobre 20, 2018 Lancio
- Aprile 13, 2020 Flyby intorno alla Terra
- Ottobre 16, 2020 Flyby intorno a Venere
- Agosto 11, 2020 Flyby intorno a Mercurio
- Ottobre 2, 2021 Flyby intorno a Mercurio
- Giugno 23, 2022 Flyby intorno Mercurio
- Giugno 20, 2023 Flyby intorno Mercurio
- Settembre 5, 2024 Flyby intorno Mercurio
- Dicembre 2, 2024 Flyby intorno Mercurio
- Gennaio 9, 2025 Flyby intorno Mercurio
- Dicembre 5, 2025 Inserimento nell’orbita di Mercurio
