ESA Science
Gravity assist
Stealing a fraction of a planet's orbital momentum to go faster for free.
Fly a spacecraft past a planet and it leaves with the same speed relative to that planet as it arrived with — but the planet is itself moving around the Sun, and relative to the Sun the spacecraft can gain or lose a great deal. The energy comes out of the planet's orbit, which is slowed by an utterly unmeasurable amount. Voyager 2 reached Neptune only because a rare alignment let it use Jupiter, Saturn and Uranus in turn. Parker Solar Probe uses repeated Venus flybys in reverse, shedding speed to fall closer to the Sun. The technique costs nothing but patience: the alignments dictate the launch window, sometimes decades apart.
Where it has come up lately
Related terms
Regolith
The layer of broken rock and dust covering an airless world — and a serious engineering problem.
Sample return
Bringing a piece of somewhere else back to a laboratory on Earth.
Opposition and conjunction
The two alignments that decide when a planet is worth looking at and when it is unreachable.
Tidal locking
When a body's spin has been dragged into matching its orbit, so one face never turns away.
Questions
What is Gravity assist?
Stealing a fraction of a planet's orbital momentum to go faster for free.
Why does it matter?
Parker Solar Probe uses repeated Venus flybys in reverse, shedding speed to fall closer to the Sun. The technique costs nothing but patience: the alignments dictate the launch window, sometimes decades apart.