Fiction Science: Orbital Slingshot

‘Serina, plot a course that takes us into the sun.
We’re going to slingshot around it.’

’Threading a needle while accelerating around an exploding star inside a planet that’s falling apart?
Well sure, why not?’

’Serina, can you do it?’

’It’s done.’
— UNSC AI Serina talking to Captain James Cutter, February 25th, 2531

This topic was suggested to me a while ago by Matt Blankenship, fan of the show, this blog apparently, and now Podcast Evolved patron. It was something that hadn’t occurred to me to look at, but I realized that it is both an interesting topic, is very scientifically relevant, and most importantly has good examples from the lore we can examine. I chose four examples that either I found or other community members pointed me towards, though there is a good chance there are more that I missed. If you find some good examples of an orbital slingshot maneuver being used in Halo that I didn’t cover, let me know and I will look into adding it to this article. Thanks again to everyone who enjoys this blog, and if you have any pressing science-related questions from the Halo universe, let me know and I may just cover it. With that, I hope you enjoy my analysis of the orbital slingshot maneuver.


While minor, the following article makes reference to and could potentially contain spoilers for Halo Wars, Halo: The Fall of Reach, Halo: Ghosts of Onyx, and the short story The Impossible Life and the Possible Death of Preston J. Cole from the anthology Halo: Evolutions.




Because there are several examples of the use of gravitational assist in the Halo universe, I have decided to break this article up slightly differently than normal. First, I will break down what a gravitational assist is including all its forms and how it is used today, then I will go into some of the most prominent examples of gravitational assists in Halo media and see how close they come to reality. As usual, I will give as much leeway to the fiction as possible, though in each instance I will be comparing the portrayal to the specific form of gravitational assist claimed to be utilized.


The term gravitational assist, or orbital slingshot as it tends to be called more commonly, comes from the idea that a spacecraft can use the gravity well of a large body in space like a star or planet to adjust its trajectory, accelerate it towards its target, or decelerate it to make a soft approach or prepare it for orbital insertion.  This is a very real concept, and is used regularly by spacecraft today, from the Apollo Program in the 1960s and 1970s to the Voyager missions in the 1970s to the New Horizons mission just a few years ago in the mid to late 2000s.  I will explain a bit further, but before that, check out this short video on how gravity assist works:



The key to the entire maneuver is “velocity relative to the sun”.  A spacecraft can’t just create energy out of nothing, so we have to either bring it along in the form of propellant, or in the case of gravity assist, steal it from the planet.  If you imagine a large planet like Jupiter or Saturn floating out in space and a spacecraft approaching it at some speed, you would imagine that the spacecraft (assuming we aren’t using the engines) would start accelerating towards the planet due to gravity.  As we got closer and closer the spacecraft would keep accelerating and be travelling faster and faster.  If you angle the spacecraft’s trajectory just right, it will fly very close to the planet and bend around at some angle, then start flying away from the planet at some slightly (or significantly) different trajectory.

But gravity is now working against you, and the spacecraft starts slowing down.  You would think that the laws of motion would slow down the spacecraft at the same rate it sped it up, and you’d be correct!  We can’t just make free energy.  So then how the heck does gravity assist work?  Going back to the last paragraph, the important part is “velocity relative to the sun”.  The velocity relative to the planet is the same on the approach as it is during the departure.  Assuming the planet is standing still, a spacecraft travelling at 10,000 kph as it closes in would be travelling at the same speed away from the planet at the same distance.  But the planet isn’t standing still, at least not in respect to the sun.  It is in orbit around the sun.  This means if the launch time and angle and speed are all planned out perfectly, a spacecraft can accelerate its speed, relative to the sun, by using a gravity assist maneuver.  The following video has a little math in it, but it is fairly straightforward I think and can probably explain better than I can with just words.


I think this video is the best I have found that really shows how the heck gravity assist works without defying the laws of physics. It is all about the speed and direction the planet is travelling relative to the sun and to the spacecraft. In the example above, the spacecraft approaches the planet headed opposite its trajectory and leaves much faster at an angle mostly with the direction of the planets motion. In reality, due to the fact that everything in the solar system, Earth, the other planets, and nearly every piece of space debris is rotating around the sun in the same direction, a spacecraft usually wouldn’t approach a planet at such a sharp angle, and instead have a path through the solar system more like this, courtesy of Wired:



Hopefully the idea of a gravity assist makes some sense to you now if it didn’t before. The concept isn’t some intrinsic property of gravity itself, but of the rotation of the solar system. So it isn’t breaking the laws of physics, but neither is it some magical free energy machine. It is a useful tool for us here on Earth, which is already rotating, and is sending spacecraft either to other rotating planets or out of the solar system altogether. And as was mentioned in the first video I linked, it is all about energy efficiency, not speed. It would be faster for us to strap a larger rocket to any spacecraft and shoot it directly at the planet we wanted to go to, but that would cost way more money and quickly become impractical from a design standpoint.

Very briefly before we move on, I would like to mention that there are really three different uses of gravity assist which I mentioned at the very beginning: speed up, slow down, and turn around. The first is the most common and the one we have been talking about. The second is the same idea as the first, but in the opposite direction. That is, should you want to slow down relative to the sun, a spacecraft could approach a planet moving with it and leave moving away from it, transferring some of the spacecraft’s energy to the planet, rather than the other way around. Its the same concept, just with the opposite results.

The third method is probably the most common, though it usually isn’t mentioned when talking about gravity assist because there is no net energy transfer. That would be the turn around method, and while it doesn’t change the overall speed of the craft, it is much more efficient than trying to use propellant to reverse your trajectory back to where you came from. This method is used regularly with flybys of the Moon, which is moving at essentially the same speed and trajectory to the sun as the Earth.


So with at least a basic understanding of how gravity assist works, how does its use stack up in the Halo universe? As always, I will make every attempt to be generous to the fiction unless something is explicitly stated, so if there is some wiggle room available to make it work, I will absolutely make use of it.

During my research I found four good examples (with the help of other community members) of a gravity assist being used. One of which, the use of it during Halo Wars, was the original depiction that spurred this article, while the others were pulled from the books. If there are other mentions of gravity assist that could be added to this analysis, let me know and I will take a look and consider adding it. I have taken these in lore-chronological order because I felt like it.



‘Are there any planets currently aligned with our entry trajectory and Eridanus Two? I want to pick up a gravitational boost so we can move in-system ASAP.’

’Calculating now, Doctor Hal—…’

’…Entering normal space in five minutes, plus or minus three minutes.’

With a sudden deceleration, they entered normal space. The main view screen flickered and a million stars snapped into focus. The Han dove toward a cloud-swirled gas giant dead ahead…

…The ship rumbled and sped faster toward the gas giant. The pull of the harness increased around the Lieutenant’s chest, making breathing difficult. They accelerated for sixty-seven seconds … the storms of the gas giant grew larger on the view screen—then the Han arced up and away from its surface.

Eridanus drifted into the center of the screen and filled the bridge with warm orange light.

’Gravity boost complete,’ Toran chimed. ‘ETA to Eridanus is forty-two minutes, three seconds.’
— Halo: The Fall of Reach, Chapter One

So from a lore perspective this section bothers me quite a lot, but scientifically I don’t see a lot wrong with it, or at least nothing that is breaking the laws of physics. Lore-wise, slipspace is being treated like it is in the rest of Halo media, inconsistently. It is generally accepted that slipspace travel before and during the Human-Covenant War was slow and more importantly inaccurate. Yet somehow Halsey knows their trajectory while still in slipspace and they managed to drop back into normal space perfectly positioned to use the gas giant to slingshot around and get a boost to Eridanus II. It seems unlikely at the least, but considering this is one of the first bits of Halo media ever, I’ll give it a pass.

We aren’t here to argue the finer points of traversing slimstream space, however, we are here to look at the physics of gravity assist. Based on what little bit of information Nylund presents, there is nothing that stands out as physics-breaking. The speed and timing of everything seems incredibly fast, but since we know nothing else, I’m going to have to call this one science. We are one for one.




So this is where we start to have problems. Videos and gameplay are tough, because they intrinsically give us a lot of information even if the creator didn’t intend to. We know from the video and the game that the Spirit of Fire is literally in the planet, meaning its velocity relative to the planet itself is 0. Doing a slingshot maneuver around the artificial star at the center would reverse their direction, but it wouldn’t actually impart any more energy onto the ship than if they had just turned around and fired their engines. It really buys them nothing, and considering they were already stationary relative to the star to start, it just adds more time to the maneuver. If the ship were already moving toward the star for some other reason, the maneuver would have helped a little, but considering they were trying to save time, not fuel, it doesn’t really make any sense.

The only thing I can think of that makes this different than the examples from earlier is that they are accelerating towards the star and that the gravitational field of the star is expanding (for some unknown reason). The former wouldn’t help them out more than just accelerating away to start, however, and the latter would only make their situation worse as the gravity going towards the star during their acceleration would be less than the gravity going away during their departure. So while it is a cool scene, it is unfortunately total fiction. We are one for two.



The moon grew large on the central viewscreen.

’I’m afraid I had no chance to double-check the trajectory,” Dr. Halsey told Kelly through gritted teeth. “It’s my best guess at a slingshot approach.’

’It is quite accurate, ma’am,’ Jerrod chimed in…

…The aft section of the Beatrice shuddered and metal groaned from uneven stress.

The twilight region of the planet’s moon filed the viewscreen with canyons of blue ice and methane geysers…

…’From our emergent position on the far side on the satellite, I now only detect one hundred forty-seven vessels. Two minutes until they are again within weapons range.’

’Only a hundred and forty-seven?’ Kelly muttered. ‘Lucky break for us.’

A blue-green planet appeared centered on the viewscreen.

’What was Dr. Halsey’s last course correction?’

’Planetary insertion,’ the AI said.

’We can’t insert at this velocity,’ Kelly said. ‘The only things that reenter this fast and touch down are meteors.’
— -Halo: Ghosts of Onyx, Chapters Seventeen & Eighteen

This is a pretty easy one, mostly because it doesn’t try to do anything drastic, and we aren’t given much. Halsey positions the Beatrice to slingshot around the moon of Onyx, but it appears to be mostly to avoid the pursuing sentinels rather than accelerate the ship. Even so, it does seem that the ship comes out of the maneuver travelling much faster than before. Considering we aren’t given much concrete information such as actual speed or trajectory, there is nothing that says that wasn’t possible. It would prove difficult for the ship to gain much velocity when their exit trajectory was toward Onyx, but their primary concern was avoiding the sentinels, not accelerating the ship. Guess that puts us at two for three.



Battle Group India one by one moved into slipstream space, the preliminary Shaw-Fujikawa calculations having been done a week previously by Cole himself.

And the entire fleet then reappeared two seconds later, one hundred thousand kilometers on the opposite side of Viperidae—positioned so the gas giant blocked the enemy sight line.

UNSC FTL technology, however, was not perfect—especially over short intersystem hops near gravity wells. A dozen UNSC ships reappeared, scattered from the main group.

The Covenant fleet angled toward the stragglers and accelerated to attack speed.

Cole‘s fleet split into two wings, both using the gravity of the gas giant to slingshot around either side of the planet—and toward the onrushing enemy armada.

In response, the Covenant fleet also split to track each portion of the UNSC forces on either side of the Viperidae.

The starboard wing of the UNSC fleet, however, shifted its orbital burn—arced up and over the gas giant and angled back to meet the rest of the battle group.

Engine cones flaring with the power from overloaded reactors, the human ships reunited and rocketed toward the port-side breach in the Covenant line.
— The Impossible Life and the Possible Death of Preston J. Cole, Section 7

Once again Eric Nylund makes use of the gravity assist maneuver, this time by Vice Admiral Preston Cole’s Battle Group India at Psi Serpentis around the gas giant Viperidae. The plan here was to perform a slipspace jump to the far side of Viperidae, slingshot around the planet, and attack the Covenant fleet head on. One thing we know about slipspace is, aside from the inaccuracy of slipspace jumps performed by UNSC ships, is that any ship will maintain the velocity and trajectory it had in normal space once it transitions from slipspace, so considering the ships were travelling towards Viperidae prior to the jump, it would make sense that they would need to slingshot around the planet in order to face the enemy. This maneuver doesn’t adjust the relative velocity of the ships in any meaningful way either, so using gravity assist to turn around is a totally plausible. And like the other written examples above, there is just enough information given to make this entire scene sound awesome but not enough to dig its own grave with physical impossibilities. That gives us one more for science, putting the analysis at three for four.



In looking at four different uses of the gravity assist maneuver, we see that three of them are science, or at least not total nonsense, and one is just that, fiction. The three examples from the books seem to be at least decent attempts to keeping true to the nature of the gravity assist maneuver, though I think a lot of that is to do as much with Nylund’s attempts at accuracy to physics as it does with the lack of any specific information provided. If you were to take the scene from Halo Wars above and novelize it, I doubt much of what we know from just looking at the video would translate to text. This would probably provide enough wiggle room to turn that fiction rating into at least a partly science, if not a straight science. But we work with what we are given, and what is presented in Halo Wars doesn’t make any sense from an orbital mechanics perspective. That isn’t to say it isn’t an awesome scene, but at best you could say the maneuver they performed is physically possible, but just wouldn’t have any benefit to getting the ship out of the Etran Harborage and faster than just turning around and flying away. Considering the goal was to escape as quickly as possible, I had no choice but to give it a full fiction rating.

At a minimum I hope this helped make some sense of the gravity assist maneuver and how it isn’t just a theoretical idea, but has actually been performed many times since the start of the space program. I hope it also makes clear the limitations of the maneuver, so the next time you inevitably see this performed in another Halo or science fiction medium, you know whether they made any attempt to stick to science. Gravity assist is one of the most important methods we have for getting the spacecraft of today to the outer reaches of the solar system in reasonable time frames, so knowing how it actually works can help you better understand actual science as well as science fiction.


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