Why we fly by planets, moons and asteroids

Flybys? Nothing new. But two flybys of the same planet a day apart? That's special. In a first for space, two probes flew by Venus in August — BepiColombo, headed for Mercury, and Solar Orbiter on its way to the sun. They wouldn't make it to their goals without flybys and gravity assists. Sadly, these two didn't snap each other. They were 575,000 kilometers (357,000 miles) apart!

BepiColombo took this cool image of Venus. But the shot is secondary. The probe flew by Venus to help it slow down. It needs to match its "orbital energy" with Mercury's to get into that planet's orbit. Bepi started out with Earth's (greater) orbital energy and is trading off the excess. Simply put, it's giving it to Venus, and it's being assisted by gravity ― a slingshot, planetary swingby.

It was the Cold War when it started — the first space race. The Soviets were the first to try a Venus flyby in 1961, but failed. It must have hurt when the US did it a year later with their probe Mariner 2. By the time the Soviets bagged their first success in 1978, the Americans had done Mercury, Mars and Jupiter. But the Soviets were the first to land on the moon, and that was a bigger deal.

Launched in 1977, the Voyager 1 & 2 spacecraft were sent out to explore the outer solar system. They each carry a Golden Record of Earth sounds: Our story told for aliens. Flybys include Jupiter, where V1 snapped the Great Red Spot (storm raging for hundreds of years), Saturn, Uranus and Neptune. They are now beyond our solar system in interstellar space and thus the "farthest human-made objects."

People often speak of our solitary, cheesy moon with a sense of love and wonder. And why not? It wouldn't be the same if we had a mass of 79 moons like Jupiter. Voyager 2 discovered one of them (plus five at Neptune). It also discovered that Jupiter's moon Europa may host some form of life beyond Earth. We're intrigued by its salty oceans. NASA wants to find out more with its probe Europa Clipper.

If you think 79 moons is cool, try 82. That's Saturn for you. The Cassini spacecraft was a joint American and European mission to probe Saturn and its moons. It featured 162 targeted flybys of Saturn's moons, including Titan and Enceladus, where it found ocean worlds. After 13 years exploring the planetary system, Cassini took one final dive into Saturn, filing observations until the very end.

Voyagers 1 & 2 have company at the edge of our solar system: New Horizons. Having swung past Jupiter for a gravity boost, it did a six-month flyby to study the dwarf planet Pluto. It then ventured to the Kuiper Belt, where it captured Voyager 1 on camera. Pioneers 10 and 11 are the only other probes to have gone this far. These missions help us answer questions about the geology and life in space.

There are many other notable flyby missions — Rosetta, which did Earth and Mars flybys before heading to comet Chury, Giotto at Halley's comet, Deep Space 1, Deep Impact, Stardust, the first sample return mission to a comet… And in the future: Hera, which will be humankind’s first probe to rendezvous with a binary asteroid system, Didymos. Why? It's all about who and where we are in the universe.