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-flung destination so the spacecraft can go further without a big and heavy propulsion system. It’s a key element in both human and robotic space explotration.
What is a Flyby?
A flyby, also called a gravity assist, is a manoeuver that allow a spacecraft to gain velocity or delta-V from passing by a planet, and in some instances, use that planet’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 a trajectory-modifying burn. This is different than a flyby mission; a flyby mission is one that sees the spacecraft photograph and measure its target without stopping in orbit.
We can look at the New Horizons mission to Pluto to understand one of the basic issues affecting all spaceflights. When the spacecraft 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 apt explanation: it was launched on an Atlas rocket, so it would need the same thrust in reverse to slow enough to stop at Pluto since the small planet’s gravity wasn’t enough to trap the spacecraft itself. It would almost need another Atlas rocket strapped on its back to stop t Pluto, but a fully loaded Atlast rocket isn’t able to launch another, same sized, fully loaded Atlas off the Earth to begin with. The mission was constrained in part by its launch mass.
Every mission is in the same basic position. To make a big change to its velocity or path, it needs a big, powerful 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. Gravity assists mean a spacecraft can do the same thing with a much smaller onboard engine.
The Venus Mission
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 uprated Apollo stack with an unused S-IVB 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 the solar system. We can think of our solar system as a gravity well, the Sun pulling everything towards it but the planets moving fast enough to be in a state of continual free fall. A spacecraft launching towards Mars needs a lot of speed to fight against that gravity well, and a lot less 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 water. Going to Mars, you’re fighting against it.
If your uprated Apollo stack weighs X, 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 the Venus trajectory. So if you have one rocket and a big spacecraft, it makes sense to gravity well and launch to Venus, then 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 and therefore gravitational pulls. When a spacecraft passes by a planet, that planet pulls the spacecraft towards it. But if the spacecraft’s speed 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 minuscule — there is no appreciable difference in how fast its travelling. But the effect on the spacecraft is significant. It will not only gain a lot of speed, that dip into the planet’s gravity well will also bend and the spacecraft’s trajectory. And let’s not forget that planets aren’t static; they orbit around the Sun and rotate around their own axis. So when a spacecraft passes by the planet, the planet’s orbit helps bends 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 get 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.
Gravity Assists in Practice
Perhaps the most famous use of gravity assists is the twin Voyager missions that explored the outer planets in the 1970s and 1980s.
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, focussing on Mars and Venus without ignoring the outer 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-175-years 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 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 the first set of encounters were successful. The missions were approved on May 18, 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 ten years. An additional $7 million enabled more sophisticated elements, including a re-programmable computer. The science payload, too, was developed with longevity in mind, covering 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, 1977. Voyager 2 launched first on August 22 and Voyager 1 followed on September 5, both on Titan IIIE rockets. The launch vehicle had enough power to send each Voyager to Jupiter. At that point, gravity assists 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 favourable gravity assists.
Gravity assists 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 that it was able to fly between Pluto and its largest moon Charon; it passed 7,750 miles (12,500 kilometres) from Pluto and about 17,900 miles (28,800 kilometres) from Charon. The Cassini mission to Saturn used two Venus flybys, an Earth flyby, and a Jupiter flyby to reach Saturn.
So gravity assists 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.
The Solar Parker Probe’s trajectory is fascinating and worth noting designed by the same specialist who got New Horizons to thread the needle in Pluto’s system, Yanping Guo. This mission used gravity assists from seven repeated Venus flybys to get the spacecraft into an orbit that dipped within 8.86 solar radii from the Sun’s surface. NASA’s Galileo spacecraft flew by Jupiter’s moon Io to slow down before the retrofire burn put it into Jupiter’s orbit — a flyby as breaking manoeuver.
So while it makes 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 Hohmann transfer. Let’s take Mars as an example again. With a Hohmann transfer, the spacecraft is essentially launched into an elliptical orbit with Earth’s distance at its periapsis and Mars’ distance at its apoapsis. This trajectory takes into account that a spacecraft already has momentum from the Earth orbiting 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 that gravity well to go outwards to Mars.
With a Hohmann 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 breaking manoeuver to orbit or land… whatever the mission’s objective.
Earth and Mars align for a Hohmann transfer every 26 months. Mars and Venus align for a Venus assist to 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 Hohmann transfer. It ends up being a simpler trajectory and a shorter mission, which is preferred for spaceflight. It also means a spacecraft doesn’t need to be designed to survive both the near-Venus environment and the Mars environment. A Hohmann transfer simplifies the spacecraft design.
Gravity Assists in Human Missions
Though we typically see gravity assists in robotic missions, there’s one very famous example of a gravity assist saving a human mission. On Apollo 13, after the crew lost the ability to land on the Moon, they also didn’t have enough power to abort the flight and turn around; the Service Propulsion System engine didn’t have enough power to counter their velocity going to the Moon. So they did a gravity assist around the Moon, using the Moon’s gravity to slingshot them home safely. It was a backup method every mission could take advantage of with only small adjustments to its trajectory.
Sources, in addition to those linked in the text: My old blog on PopSci because I wanted to revisit this for a video and updated the old article. Special thanks to Con Tsang and Lyle Tavernier for walking me through some things on this one!