Concept

Hyperbolic flyby — where it appears

An unbound passage close to a body, on which the approach and departure speeds relative to that body are equal and only the direction changes. The turning angle depends on the closest approach and the approach speed, and the change of frame is what makes it a gravity assist.

Named by 2 essays across one field — each of them below, with the objects they name alongside it.

A gravity assist with a 70° turn. The velocity triangle of a flyby. In the planet's frame the spacecraft's speed is unchanged and only its direction turns; adding the planet's own velocity converts that turn into a gain in speed measured from the Sun.

Stealing speed from a planet, which does not notice

A flyby cannot change a spacecraft's speed relative to the planet. It changes its direction — and adding the planet's own motion back turns that into free velocity.

spaceflight · Gravity assist $How small a sphere of influence is. Each planet's sphere of influence as a fraction of its own orbital radius, against that radius, both logarithmic. The largest belongs to Jupiter at 6.19% and the smallest to Mercury at 0.194%. The patched-conic method treats a trajectory as heliocentric everywhere outside these, and the figure is the argument for why that costs so little: they are thousandths of the journey.$

One trajectory, stitched from three two-body problems

An interplanetary flight is a problem with no closed solution. It is flown by cutting it into pieces that each have one, and the seams are places where the model is knowingly false.

spaceflight · Patched conics

Named alongside it

The objects these essays reach for when they reach for this one.

Gravity assist Patched conics Anomaly Characteristic energy Eccentricity Hyperbolic orbit Launch window Mass ratio Momentum exchange Oberth effect Reference frames Sphere of influence

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