# Using a brown dwarf for a gravity-assisted slingshot

Assuming the feasibility of utilizing a brown dwarf for a gravity assisted slingshot, or Oberth Maneuver, what sort of hazards/difficulties could one expect during such a maneuver?

EDIT: To make this more plausible, would it make a difference if the brown dwarf were part of a binary system, and would thus have a slightly higher velocity (depending on its radius from its solar partner) for the passing ship to "borrow" some of it's velocity?

More specifically, if a fictional starship were to need a large boost in velocity to reach its destination (either due to low fuel reserves or somesuch scenario), and as I understand it, the closer to a celestial body that one can get, the faster the periapsis speed will be, what would be the likelihood of survivability of such a maneuver, in lieu of volatile storms, molten iron condensing in the atmosphere, lightning, x-ray bursts, etc.? Put another way, how limiting would those factors be to the success of the maneuver?

(Acknowledging of course, that diving too deep into its atmosphere would produce drag, and also assuming that the craft is protected by both an ablative shield as well as a superconducting magnetic shield, and also assuming that the ship has no other option available.)

• Brown Dwarf [can be pretty cold] (space.com/25659-coldest-brown-dwarf-near-sun-discovery.html) - could you be more specific which one you have in mind? Or how hot and massive is yours? Sep 5, 2016 at 21:59
• @ Molot: I assumed it would have to be a Class M or L brown dwarf; didn't think a Class T would be hot enough to condense the iron. Sep 5, 2016 at 22:02
• If you’re arriving at relativistic speeds, swining by a brown dwarf while firing retro rockets won’t help much. You are near the body for a very short time. Sep 5, 2016 at 23:25
• Right, but I'm talking about acceleration, not deceleration. Sep 6, 2016 at 1:05
• I was going to answer "what do you call all the times we did this with Jupiter?" and then I read this over at physics.se. Seems I had the same misconception, and have yet another reason to lament how sadly my teachers failed me yet again. Thank goodness there is StackExchange to fill in these gaps in my education. Sep 9, 2016 at 17:54

I don't think flying through the actual atmosphere would be useful - certainly not the part dense enough to encounter clouds, lightning, and storms.

A brown dwarf is going to have a radius comparable to Jupiter's, but its mass is much greater (13 to 80 times or so). Gravity goes by square of distance from the center, so the gravity in the cloud tops won't be much stronger than that in vacuum above the atmosphere. (and the atmosphere will drop off to vacuum much quicker than Jupiter's since the gravity is so much stronger.)

• So do you mean that a brown dwarf isn't useful for a gravity assist at all because of its low density, despite its mass? Or is it a matter of diving deeper into the atmosphere? Sep 6, 2016 at 1:03
• No, the point is that you don't gain any meaningful extra velocity change by going through part of the atmosphere as opposed to going by just outside the atmosphere. (And, of course, going through the atmosphere would slow you down due to drag.) Sep 6, 2016 at 11:10

A starship is not going to get a meaningful velocity boost from an encounter with a brown dwarf. Let's look more carefully at a gravitational slingshot--specifically from the vantage point of the massive object. In this context the spacecraft simply does a parabolic flyby with effectively no velocity change. From the viewpoint of the spacecraft it can get a boost that is a portion of its velocity relative to the massive object. Thus your boost is always less than the object's velocity--and no star is going to be moving at a velocity that's even a small fraction of the velocity of a starship.

The only starship I have ever seen suggested below .01c is in Vernor Vinge's Long Shot.

There is also the problem that a gravity assist is a turn--if you do an unplanned slingshot you end up somewhere far from your original destination.

Now, if you want to do an unplanned gravity maneuver and get substantial velocity out of it we need to look much deeper into the gravity well. Unfortunately, we will need three degenerate (or black holes, whatever they're made of) objects in close orbits. The thing is each turn will be less than 180 degrees, thus we need three turns to get back to our original course.

To actually find the requirements would be exceedingly unlikely.

• "Let's look more carefully at a gravitational slingshot" The question is about an assisted slingshot, where you burn fuel at closest approach. Sep 9, 2016 at 17:37