Gravity assist. A gravity assist is a planetary
flyby technique that's used by spacecraft to get to where they're going.
It's often used to speed up the spacecraft's velocity
and propel it outward in the solar system, much farther away from the Sun
than the rocket it launched on would have been capable of doing
so on its own.
The technique has been used
for multiple missions, from the Voyagers to the Parker Solar Probe.
So how does it work?
A gravity assist can change the velocity of a spacecraft
by entering the gravitational influence of a large object, like a planet.
The orbit of a planet around the Sun is used as a source of energy.
The spacecraft falls in toward the planet under the influence of gravity,
and then it flies back away.
A gravity assist can either increase or decrease
the energy of the spacecraft's orbit.
If the spacecraft moves around the planet
in the same direction as the planet's orbit around the Sun,
it'll gain a bit of energy, as done with the Voyager missions around Jupiter.
The Lucy spacecraft will need
two gravity assists to get on trajectory to the Trojan asteroids.
The second assist will happen in December 2024, giving Lucy the energy
it needs to cross to the main asteroid belt, where it will observe the asteroid
Donaldjohanson, then travel into the leading Trojan asteroid swarm.