 Gravity assists, sometimes called flybys, are a big part of space exploration. These precision maneuvers involve harnessing a planet's gravity to accelerate and direct a spacecraft to some far-fung destination, so the spacecraft can go further without a big and heavy propulsion system. Most people know gravity assists from the twin Voyager spacecraft that visited the outer planets with a series of flybys in the 1970s and 80s. But how exactly do they work? Hello everyone, I'm Amy, welcome to The Vintage Space, my little corner of the internet, where we talk about all things mid-century history that frankly fascinate me. And you know mid-century space exploration is high on that list. In a previous video I talked about the late 1960s proposals NASA studied to send humans to Mars and Venus. In that video, which you can watch right up here, I mentioned that given the state of the art at the time, it was easier to go to Mars via Venus using a flyby than going directly to Mars. The gravity assist is a pretty important concept, so I thought I'd make a dedicated video about it. I actually did this video like six years ago, but it was short and needed an upgrade, so let's dive in. A quick sidebar on terminology before we begin. A gravity assist flyby differs from a flyby mission. The New Horizons mission did a flyby of Pluto in that it flew past the planet to take its images and gather data, rather than going into orbit. A gravity assist flyby is a flyby that uses the planet's gravity to affect its trajectory. Gravity assists our maneuvers that allow a spacecraft to gain velocity, or delta V, from passing by a planet, and in some instances use that plant's gravity to bend its trajectory to change direction. No matter the specific flyby's outcome, it negates the spacecraft's need to carry a big engine to make that trajectory modifying burn. Let's talk about New Horizons again to see why this is useful. When the mission did its close encounter with Pluto, a lot of people asked why it wasn't going into orbit. It was, in effect, going too fast to stop. The crude but accurate explanation? It was launched on an Atlas rocket, so it would need the same thrust in reverse to slow down enough at Pluto. It would, in essence, need another Atlas rocket strapped on its back to stop. But you know what isn't going to be able to launch a fully loaded Atlas rocket off the Earth? Another same-sized Atlas rocket. This is exactly why mass is so important. Every mission is in the same basic position. To make a big change to its velocity or path, a spacecraft needs a big engine, but it can only take as big an engine with as much fuel as its launch vehicle can get off the ground in the first place. The assists mean a spacecraft can do the same thing with a much smaller onboard engine. The mission is lighter. An excellent illustration of why gravity assists are helpful is the 1960s proposed Apollo applications program mission looking at Venus or Mars. In this case, the spacecraft was an upgraded Apollo stack with an unused S4B as the habitat module. The rocket was the Saturn V. Because the program was aiming to find new uses for this hardware, this was the study's technological limitation. With this technical limit in mind, let's look at our solar system. We can think of our solar system as a gravity well, the Sun pulling everything towards it, but the planet's moving fast enough to be in a state of continual free fall. A spacecraft launching towards Mars needs a lot of speed or delta V to fight against that gravity well, and a lot less delta V to go towards it since the Sun is already pulling it in. If you imagine that gravity well like a drain going towards Venus, you're going with the flow of the water, going to Mars you're fighting against it. So in this Venus mission concept, if your upgraded Apollo stack weighs X, let's present that's an actual number right now. You would need a bigger, more powerful rocket to launch it to Mars than to Venus, but you only have one rocket, in this case the Saturn V. With that rocket, you can launch a lighter spacecraft to Mars or a heavier spacecraft to Venus. The difference in mass comes because you need more fuel for the Mars trajectory than for the Venus trajectory. So if you have one rocket and one big spacecraft, it makes sense to take advantage of the gravity well and launch to Venus, and use Venus to slingshot you to Mars. Harnessing the power of a gravity assist comes from a gravity transfer. A planet's gravity is far greater than a spacecraft's, but both do have mass, so both have gravitational poles. When a spacecraft passes by a planet, that planet pulls the spacecraft towards it, but if the spacecraft's Delta V is just right. If it's going fast enough to dip into the planet's gravity well, without going slow enough to fall into orbit, it can gain energy. The planet transfers a little of its own momentum to the spacecraft. The effect on the planet is miniscule. There is no appreciable difference in how fast it's traveling, but the effect on the spacecraft is significant. It will not only gain a lot of Delta V, but that dip into the planet's gravity will also bend the spacecraft's trajectory. Planets aren't static. They orbit around the sun and rotate on their own axes. So when a spacecraft passes by the planet, the planet's orbit helps bend the spacecraft's trajectory. It's in this way that a perfectly timed Venus flyby can not only give the spacecraft the Delta V it needs to fight the gravity well and go to Mars. It can put it on the right course, too. And because the initial mass leaving Earth was smaller to take advantage of the gravity well going towards Venus, the bigger spacecraft on the smaller rocket can now get to Mars. Perhaps the most famous use of gravity assist is the twin Voyager missions to explore the outer planets in the 1970s and 1980s. Voyager 1 and Voyager 2 launched in the fall of 1977. Each visited Jupiter, then Saturn to complete their primary missions before flying off in different directions. Voyager 1 flew north from the plane of the ecliptic, so made no further encounters. But Voyager 2 was able to pass by both Uranus and Neptune. Like the manned Venus-Mars concepts, the Voyager missions came out of post-Apollo study plans in the mid-1960s. The National Academy of Sciences Space Science Board urged NASA to shift its focus from the moon to the planets, focusing on Mars and Venus without ignoring the other planets. Various ideas arose, including a multi-planet flyby mission launched on a single rocket. This would deliver a lot of data for the relatively low cost of one launch. And the timing was right. Just on the horizon was a once-in-a-175-year planetary alignment that would facilitate a spacecraft visiting Jupiter, Saturn, Uranus, Neptune, and Pluto with one launch. The four-year launch window was between 1976 and 1980. NASA's outer planets working group established in 1969 fleshed out the multi-planet mission into two twin missions, each of which would visit three planets, a Jupiter-Saturn-Pluto mission launched in 1977 and a Jupiter-Uranus-Neptune mission launched in 1979. Another mission profile emerged from NASA's Jet Propulsion Laboratory. This plan envisioned four launches, two Jupiter-Saturn Pluto missions in 1976 and 1977, and two Jupiter-Uranus-Neptune missions both launched in 1979. As the concept evolved, a four-launch profile was ultimately deemed prohibitively expensive, as was the idea of one spacecraft large enough to visit all four giants. NASA was also constrained by its shrinking budget in the post-Apollo era. The spacecraft got smaller. NASA's 1973 budget request included funding for a pair of Mariner-class spacecraft. Mariner was a smaller class of spacecraft that had taken big steps in the early space age. Notably, Mariner-4 returned the first ever close-range images of Mars. Two proposed Mariner-Jupiter-Saturn spacecraft would launch in 1977 with the possibility of going on towards Uranus and Neptune if this first set of encounters were successful. The missions were approved on May 18 of 1972. The Mariner-Jupiter-Saturn spacecraft developed at JPL with extra subsystems designed to increase the mission's longevity. The team was giving the spacecraft every chance possible to visit all four outer planets, including plutonium batteries that could last more than 10 years. An additional $7 million enabled more sophisticated elements, including a reprogrammable computer. The science payload was also developed with longevity in mind, covering all kinds of data, including imaging, spectroscopy, magnetometry, studying charged particles, cosmic rays in addition to gathering specific data about each planet's environment. The spacecraft were renamed Voyagers 1 and 2 on March 4 of 1977. Voyager 2 launched first on August 22, and Voyager 1 followed on September 5, both on Titan 3E rockets. The launch vehicle had enough power to send each Voyager to Jupiter. At that point, Gravity Assist would give each spacecraft the velocity and major trajectory adjustments they needed to visit more distant planets. Voyager 1 launched towards Jupiter where it did a Gravity Assist to fly onwards to Saturn. The Assist at this planet sent it out of the ecliptic so it couldn't visit any other bodies. Voyager 2 had a more interesting path, hitting all four giants from favorable Gravity Assist. Gravity Assist like this for deep space missions are incredibly common. New Horizons flew by Jupiter to gain some extra velocity to reach Pluto. With only minor adjustments, its trajectory was so fine-tuned it was able to fly between Pluto and its largest moon, Sharon. The Cassini mission to Saturn used two Venus flybys, an Earth flyby, and a Jupiter flyby to reach Saturn. So Gravity Assist are extremely useful for accelerating a spacecraft to a distant destination. But they can also be useful for the opposite, slowing a spacecraft for a destination closer to the sun. Or in the case of the Solar Parker probe, slowing it into a highly eccentric solar orbit to study our star. This mission used Gravity Assist from seven repeated Venus flybys to get the spacecraft into an orbit that dipped within 8.86 solar radii from the sun's surface. Another example is NASA's Galileo spacecraft, which flew by Jupiter's moon Io to slow down before the retrofire burn that put it into Jupiter's orbit. The flyby was part of the overall braking maneuver. So while it makes some sense to go to Venus on the way to Mars, we don't see this path taken. That's because there's another way to get to Mars, or anywhere really, called a Homan transfer. Let's take Mars as an example again. With a Homan transfer, the spacecraft is essentially launched into an elliptical orbit with Earth's distance as its periapsis and Mars' distance as its apoapsis. This trajectory takes into account that a spacecraft already has momentum from the Earth orbiting around the sun and harnesses that energy. The Earth's orbit's energy, in conjunction with the rocket at launch, imbue the spacecraft with the Delta V it needs to fight the gravity well to go outwards, in this case, to Mars. With a Homan transfer, the launch is timed such that the spacecraft crosses Mars' orbit when the planet is at the right spot. Then the spacecraft can do its braking maneuver to orbit or land, whatever the mission's particular objective. Earth and Mars align for a Homan transfer every 26 months. Mars and Venus align for a Venus assist Mars every 12 months. So why don't we see this profile? For one thing, modern missions strike the right balance between rocket power and spacecraft mass to where they can take advantage of the Homan transfer. It ends up being a simpler trajectory and a shorter mission, which is preferable for spaceflight. Also, it means a spacecraft doesn't need to be designed to survive both the near Venus environment and the Mars environment. A Homan transfer at the end of the day simplifies a spacecraft's design. I hope this explained any questions you may not have known you had about gravity assists. I've got a couple of unflown mission concepts coming up before we dive into our mini-series on X-planes. Until then, I want to remind you guys that my new book, Fighting for Space, is available however you like to consume books, including now in paperback, which has been lately updated with a little bit of new research. My first book, Breaking the Chains of Gravity, is also still widely available. I have links for both of those in the description below. A special shout out to all my Patreon supporters and YouTube members. You guys truly make these videos possible, so thank you so much for your ongoing support. I honestly could not do this without each and every one of you guys. If you would like to help keep the vintage space up and running, I've got the link you need in the description below, as well as links to connect all across social media. Thank you guys so much for spending part of your day with me today. I really do appreciate it, and I hope to see you next time.