Many spacecraft bounce among the planets and moons of the solar system like they’re inside an old pinball machine. They use the gravity of one object to bat them toward another. That’s like having an extra rocket booster at no extra cost.

The first gravity assist took place 50 years ago today. Mariner 10 flew past Venus, the next planet in toward the Sun. It used Venus’s gravity to help it reach Mercury, the closest planet to the Sun. That made Mariner 10 the first probe to explore two planets beyond Earth. It eventually flew past Mercury three times.

Today, gravity assists are common. A spacecraft might use many of them during its mission. The Juno spacecraft, for example, is using the gravity of Jupiter’s big moons to allow it to explore some of the other moons. And later this year, a mission called JUICE will use the gravity of Earth and the Moon to help push it toward Jupiter. Once there, it, too, will get gravitational kicks from Jupiter’s big moons to help it explore the Jovian system.

There’s no way for a craft to carry enough propellant to do that kind of maneuvering on its own. So without help from the planets and moons, we’d know a whole lot less about the many worlds of the solar system.

Venus, the planet that provided the first helping hand, is the brilliant “morning star.” It’ll stand well to the lower left of the Moon at dawn tomorrow, and closer to the Moon on Wednesday.

Script by Damond Benningfield