SLINGSHOT
GRAVITY ASSISTS & THE GRAND TOUR
A flyby changes a spacecraft's speed around the Sun without propellant.
BEGIN — CHAPTER 01 →The outer planets are far, and the cheap way there is slow
The cheapest route is a Hohmann transfer.
Voyager 2 reached Neptune in twelve years.
The planet only changes the direction — the Sun sees a new speed
Relative to the planet, speed in equals speed out; only the direction turns.
Cassini's 1999 Earth flyby: computed 5.47 km/s, announced about 5.5.
|vout|² − |vin|² = 2 V·Δv
Presets use published v∞ and altitude.
Not free energy: the planet pays
Relative to Earth, Juno's speed did not change; relative to the Sun it gained 3.9 km/s.
The ball's extra momentum comes out of the train.
Exact one-dimensional elastic collision.
Behind the planet to speed up, in front of it to brake
MESSENGER needed six flybys to brake into Mercury orbit.
Galileo's Io pass saved about 90 kg of propellant.
Which side decides faster or slower; how close decides how much.
One pass of Jupiter turns a closed orbit into an escape
One Jupiter flyby can push a probe past solar escape speed.
New Horizons gained about 14,000 km/h at Jupiter.
Model, not a flown trajectory.
From a lunar loop to a planetary relay
Luna 3 swung past the Moon in 1959; Minovitch championed the technique in the early 1960s.
Flandro found the 175-year alignment in 1965; Mariner 10 flew the first gravity assist in 1974.
Four planets, twelve years, one spacecraft
NASA cancelled the Grand Tour in 1972; Voyager replaced it.
Voyager 2 visited all four giants.
Direct times are Hohmann transfers.
Half a century of borrowed speed
Every spacecraft heading out of the Sun's grip took some speed from Jupiter.