In 1977, NASA’s Voyager 2 space probe began its Grand Tour of the solar system, looping around Jupiter, Saturn, Uranus and Neptune and using the power of gravity from each planet to slingshot away to the next.

The European Space Agency’s Rosetta spacecraft took similar gravity slingshots from Earth and Mars before its rendezvous with a comet in 2014. And this past July, a NASA spacecraft named for the Roman goddess Juno began settling into orbit around Jupiter after a five-year journey and one gravity slingshot from Earth.  

Gravity is powerful. On Earth, it keeps our feet firmly on the ground and prevents our possessions from flying off into space. That is why spacecraft bound for the planets (or a comet) require powerful rockets to launch them away from Earth’s gravitational pull. Out in space the Sun’s gravity pulls on them, so spacecraft are often sent into a planet’s gravity field to gain some speed – a maneuver called a gravitational “slingshot” or gravity assist.

“The fundamental idea is to use a close flyby of a large body, such as a planet, to bend the trajectory [path] of a spacecraft,” says Sylvia Miller, a retired mission design engineer who worked at NASA’s Jet Propulsion Laboratory (JPL) for 40 years. The planet’s gravity causes the trajectory to bend, explains Ms. Miller, which can have various effects on the spacecraft depending on its “aimpoint” or the point at which it enters the gravity field. The spacecraft’s velocity will increase if it enters behind the planet (the “trailing” side), and will decrease if it enters in front (the “leading” side). If it enters at one of the two poles, the angle or “inclination” of the spacecraft’s path will change. “In each case,” says Ms. Miller, “a desired result is achieved without the expenditure of an unacceptably or impossibly large amount of propellant.”

Ms. Miller explains that a conventional spacecraft launched from Earth with NASA’s most powerful rockets would not have enough energy to reach Saturn because of the Sun’s gravitational pull. Halfway to Saturn the spacecraft could make a targeted pass close to Jupiter for a nip of some of that massive planet’s strong gravity field. (NASA’s Pioneer 11, Voyager 1 and Voyager 2 missions were the earliest to tap Jupiter for a speed bump toward Saturn.) The planet gives up some of its orbital speed (how fast it orbits the Sun) to the spacecraft – only a tiny amount, says Ms. Miller, but enough to give the spacecraft a significant boost.

When Voyager 1 flew past Jupiter in 1979 for a gravity assist to Saturn, Jupiter’s energy loss amounted to just one foot per trillion years in orbital speed, according to retired NASA aerospace engineer Charley Kohlhase. In a Planetary Radio interview, Mr. Kohlhase explained that Jupiter’s tiny loss gave Voyager 1 a speed gain of 16 kilometers per second, or 35,000 miles per hour.

The net increase or decrease in a spacecraft’s velocity after a gravity assist flyby is only with respect to the Sun – there is no net change in the spacecraft’s speed relative to Jupiter, says Ms. Miller. To Jupiter, the spacecraft’s speed before and after the flyby is the same. From the Sun’s perspective, the spacecraft gains considerable velocity, which helps speed our space missions along nicely.

Gravity assist involves three “bodies,” explains Ms. Miller – the central body, the gravity assist body, and the spacecraft.  For interplanetary missions the central body is the Sun and the gravity assist body is a planet, but they can also be a planet and its moon. The Cassini spacecraft has used multiple gravity assists from Saturn’s moon Titan, and Galileo used Jupiter’s moon Io to help slow it down.

Calculating gravity assist for a mission to Saturn via Jupiter would factor in the gravity fields of the Sun and Jupiter, the relative velocity of the spacecraft and Jupiter, the closest approach point to Jupiter, and the most favorable alignment between Jupiter and Saturn – a factor that can narrow the dates during which gravity assist is most effective. The planetary alignment that enabled Voyager 2’s outer solar system Grand Tour, for example, occurs only once every 176 years.

The Soviet Union used gravity assist first – employing the moon’s gravity to sling its Luna 3 probe around the dark side of the moon and back to Earth in 1959. In the early 1960s, mathematics graduate student Michael Minovitch discovered that solar system missions could use gravity from one planet as a slingshot to another planet, greatly reducing travel time. A decade later, in 1973, NASA’s Mariner 10 probe was the first spacecraft to use gravity assist for planetary exploration. Mariner 10 flew around the Sun to Venus, whose gravity boosted it back around the Sun to Mercury. It used Mercury’s gravity to slingshot around the Sun two more times for additional Mercury flybys.

Mr. Minovitch also determined that Jupiter’s strong gravity field was the key to reaching Saturn, Uranus, Neptune, and Pluto (which was still considered a planet at the time), and he foresaw the mid-1970s alignment of those five planets, which enabled NASA’s Grand Tour using the two Voyager spacecraft.

One of the most well known gravity assists occurred during the Apollo 13 moon mission in 1970, which was jeopardized when an oxygen tank exploded in the spacecraft’s service module. Three nerve-wracking days after astronaut Jim Swigert famously uttered, “Houston, we’ve had a problem here,” NASA and the Apollo 13 crew harnessed the moon’s gravity to slingshot their damaged spacecraft around the moon and return safely to Earth.