# What is the smallest body in which a sling shot maneuver can be performed?

This question asks about the smallest sling shot maneuver performed. What was the smallest intentional, acknowledged slingshot maneuver?

I'm asking how small of an object be to perform a sling shot maneuver around it?

Update I didn't think of Magnestars and micro-blackholes when I wrote this question but the replies are great.

Within our solar system I would guess that an object lacking an atmosphere a spacecraft could get closer to the surface of it during the sling shot.

• This question is a little different than mine. I've asked for a real documented maneuver in a real spacecraft's planned or executed trajectory (1, 2). This is asking each person to judge for themselves what counts as a slingshot maneuver which leaves things more open to interpretation about sizes and threshold. So this is going to be more difficult to answer without expressing an opinion. – uhoh Mar 25 '19 at 10:01
• Technically, any close flyby is a slingshot maneuver. With small objects, the trajectory change just becomes too small to be significant. In this question, the calculation is done for a really close flyby of Pluto, yielding a 1.4° change in course. So the question becomes, what is the smallest course change you want to consider? – Hobbes Mar 25 '19 at 10:45
• The object also doesn't have to be lacking an atmosphere, I don't think that portion has anything to do with this and should be removed. Also the tag identify-this-object definitely doesn't belong here. – Magic Octopus Urn Mar 25 '19 at 16:25

How small do you want to get? $$F=G{Mm \over r^2}$$ applies regardless of size. If you remove enough disturbances from other bodies you can get two neutrons to orbit a common barycenter on gravity alone - or send them against each other on a near-miss trajectory and they'll pass influencing each other gravitationally in essence performing a slingshot against each other.

That's considering mass. Considering size as radius - a singularity is dimensionless, zero size, and can easily slingshot planets or smaller stars... but if you're going to slingshot against one, better stay well clear of the event horizon, which may span many kilometers past the dimensionless singularity point.

• I think considering a singularity as zero size is misleading, since anything that crosses the event horizon cannot get out, so there are no slingshot trajectories that get closer than that. The event horizon is, effectively, a (very hard) surface. – Davidmh Mar 25 '19 at 12:01
• @Davidmh: I disagree. It's like adding 5mln km to the Sun radius (on top of its 700,000km) because entering the corona is bound to fry the spacecraft. – SF. Mar 25 '19 at 13:44
• @SF.: Davidmh didn't say it's wrong, he said it's misleading. For all intents and purposes, anything that touches the event horizon is dead (orbitally speaking) as if it smacked into a planet's surface. For your solar example, you're hinging on what you find a reasonable limit on temperatures a spacecraft can handle. Different spacecraft, different limit. You can build better spacecraft (or send something that doesn't disintegrate), but an event horizon doesn't care about what you send, it will (definitively) capture whatever you choose to send. – Flater Mar 25 '19 at 14:55
• @SF. a sufficiently shielded spacecraft going sufficiently fast can survive the corona; the specifics are an engineering detail. Coming back out after crossing the event horizon is a mathematical impossibility. General Relativity shows that, once you cross the horizon, the only possible trajectories are going towards the centre every step of the way. – Davidmh Mar 25 '19 at 15:26
• @SF. depends on your model. You could model the black hole as a 2-brane coincident with its event horizon, with some extra strings stuck onto it. The only observers able to tell the difference would be those that have already reached the event horizon. The neat fact about this brane is that it would have exactly one bit of entropy per Planck area. – John Dvorak Mar 25 '19 at 17:52

We've done a slingshot maneuver with the Moon. That's essentially what Apollo 13's free return trajectory was when the spacecraft became crippled and had to be returned to Earth.

I would like to address some comments this answer has drawn. First, some have said that this dies not answer the question of what the smallest object suitable for a slingshot is. But that question has no clear answer because technically any gravitational deflection that does not result in capture is a slingshot. [This reference] describes a (very low angle) slingshot-type maneuver past the Martian moon Phobos used to nail down it's mass and density (thus, it's porosity). Thus the above is intended as a practical example of a much more significant slingshot involving an object smaller than the planets.

• This is interesting trivia, but does not answer the question about what body is the smallest with which a slingshot maneuver can be performed. I am not even sure Luna is the smallest body ever used for a slingshot maneuver in practice. What about Cassini and its fly-bys of several Saturn moons, for example? – Philipp Mar 25 '19 at 10:28
• I'm not sure that was done for the slingshot effect -- it was more a case of "this is the simplest path requiring the least thruster use and lowest risk of return failure" . I remember listening to the news every day during A-13's journey. – Carl Witthoft Mar 25 '19 at 14:54
• I know. What I'm saying is, at the beginning of the return we used the Moon as we would for a slingshot effect, basically setting the turn angle to 180°. – Oscar Lanzi Mar 25 '19 at 17:49
• @CarlWitthoft Which does not say it wasn't a slingshot maneuver. The Apollo rockets specifically launched into a path that would slingshot back to Earth if no further burns were done. – Loren Pechtel Mar 25 '19 at 18:39
• @OscarLanzi, most of the turnaround on an Apollo free-return trajectory was because the spacecraft was near the apoapsis of its Earth-centered orbit. Yes, the lunar flyby provided a bit of braking, but it was nowhere near as dramatic as it seems. – Mark Mar 25 '19 at 20:43