On September 28, ESA’s JUICE spacecraft flew past Earth and picked up a 3.5 km/s speed boost. But how does flying near a planet make a spacecraft go faster?

The trick is called a gravity assist. As the probe falls toward Earth, the planet’s gravity pulls it in and speeds it up. The spacecraft then swings around and leaves with extra energy borrowed from the planet’s motion around the Sun.

Think of it like a slingshot. The probe steals a tiny bit of Earth’s orbital energy. Earth is so massive that it does not notice, but for the small spacecraft the boost is huge.

JUICE passed at about 8,640 km above Earth. Engineers plan each flyby precisely, since a small error can send the probe off course.

The boost saves fuel and shortens the trip. JUICE will repeat the trick in January 2029 before reaching Jupiter in July 2031, where it will study the planet’s icy moons.