Scenario: A gas giant knocked off orbit is projected to make an extremely close approach to our planet. Close enough to cause miles high tides, continent wide earthquakes, and send our planet on an escape trajectory out of the solar system. While many will stay on our planet, many want to try their luck surviving in space or on another planet. However not everyone can afford the fuel needed to get to space. In a desperate maneuver, many attempt to use the gas giant itself to leave the planet.


  • You'd launch when the gas giant is making it's closest approach
  • You'd need to launch from sea as all land on that side of the planet will be flooded
  • You'd still need fuel to actually reach orbital speeds
  • The gas giant may be close enough to overpower the gravity on the planet surface if it helps
  • Survival of the planet does not matter

Question: Could you use the gravity of an extremely close gas giant to assist in leaving a planet? How close should the gas giant be and how much fuel would such a maneuver actually save you?

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    $\begingroup$ If the gas giant is close enough to have an noticeable effect it has already ripped the planet apart. $\endgroup$
    – John
    Feb 5 at 21:59
  • $\begingroup$ If you are getting mile high tides then pretty much everybody on the planet is dead. It's going to be a challenge to live long enough to launch. It's going to be a challenge to avoid having the rocket splattered before launch. $\endgroup$
    – Boba Fit
    Feb 6 at 19:11
  • $\begingroup$ @BobaFit My question assume a high tech purpose built ship could ride the tides and survive. I also assume the tides would be fairly gradual as they appear over a long period of time and the ship could stay on the high point of the tides. Are these bad assumptions? $\endgroup$
    – Goose
    Feb 6 at 19:49
  • $\begingroup$ @Goose Tides move at the speed of dawn. They come twice per day. youtube.com/watch?v=GrsMxy7VRMY These are only a few hundred yards and they're in water. Mile high tides would pretty much kill everything. $\endgroup$
    – Boba Fit
    Feb 6 at 20:34
  • $\begingroup$ @BobaFit I think you're right. Tides move at 1,600 km an hour. That much additional water moving that fast will be violent. I wonder if the north or south pole would be easier to sail in assuming you had near future technology. $\endgroup$
    – Goose
    Feb 6 at 23:16

1 Answer 1


Simple answer: yes

Your rocket would need to lift off on the side of the planet facing the gas giant. If you launch from the other side you're fighting the gravity of both planets.

Having said that, here's the simple lowdown.

  • If the gas giant is close enough that its gravity overwhelms Earth's — meaning that "down" to some small degree is in the direction of the gas giant, then you'd need almost no fuel at all. But this also means that everything is being pulled toward the gas giant, making navigation a blooming nightmare if not impossible.

  • If the gas giant is close enough to negate Earth's gravity then things on Earth start to float. Basically the same benefits of overwhelming Earth's gravity without as much navigational debris.

  • With every meter separation after those first two conditions, your fuel requirements increase until you get to the normal escape velocity.

So, the trivial answer to your question is "sure, you can use it to boost a launch."

Complex answer: when the gravity boost comes around, it's probably too late to matter

@John's also right. The closer you get to Earth, the more destructive the situation becomes. If a gas giant's gravitational field is strong enough to significantly counter Earth's own gravity, you're deep into wreck-the-planet mode. You're not just pulling up water, you're pulling up everything else, too, which means substantial vulcanism and a noticeable wobble to Earth's rotation due to all the water being pulled to the side of the planet facing the gas giant.

But, if "realism" is to rule the day (I'm not a fan...), then realistically, by the time you wait for a substantial fuel reduction due to the gravitational influence of the gas giant, the rocket is either already destroyed or can't get away from Earth due to all the chaos.

And just to add insult to injury...

Remember when I said the simple answer is "yes?" Well... if you save the fuel getting off Earth, you still need a boat load of fuel to escape the gas giant's gravity well. One could say that a launched rocket could try to skirt around the gas giant, even perhaps using it as a sling shot... but if it's close enough to nullify or all but nullify Earth's gravity, then you're too deep into the gas giant's gravity well to do that. In other words, "realistically," even if you got off the planet cheaply, you'll burn more escaping the gas giant than you ever would simply launching before the gas giant grew close enough to affect the Earth.

But does that matter?

How "realistic" do you need to be? Take that simple yes and run with suspension of disbelief.

  • $\begingroup$ Excellent answer. Realism doesn't necessarily rule the day, but it does make a good starting point. $\endgroup$
    – Goose
    Feb 6 at 3:37
  • $\begingroup$ @Goose Thanks for the compliment. Please note that as a policy we recommend waiting 24-48 hours before rewarding the coveted green check mark. We have users around the world and generally human nature is to lose interest in posting answers once the green check shows up. You not only might miss a better answer than my own, but you may also miss answers that enrich your understanding. $\endgroup$
    – JBH
    Feb 6 at 16:03
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    $\begingroup$ Good advice. I'm more familiar with stack overflow which moves much quicker but I'll remember that going forward. $\endgroup$
    – Goose
    Feb 6 at 19:45
  • $\begingroup$ @jhb I may have found a solution. This point where gravity offsets is essentially the L1 Lagrange point. Your depth in each gravity well will be the same. Does this mean you have to escape both gravity wells? Actually the opposite! You can ride the Lagrange point as the two objects separate. Eventually you will be out of both and on a trajectory between the two trajectories. This all requires very little fuel. At this point, unlike the doomed planet, you can strategically prograde and retrograde at Apoapsis from the sun and potentially even make use of gravity assists. $\endgroup$
    – Goose
    Feb 7 at 0:25
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    $\begingroup$ @Goose Oh.... the idea of riding the Lagrange point is excellent! I agree that the idea represents the lowest fuel consumption point with the highest chance for success. Good idea! $\endgroup$
    – JBH
    Feb 8 at 3:34

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