<p>If you pass an object with negligible mass, your your path will remain unchanged and your deflection will be zero degrees.</p>
<p>if you pass a planet, your path will be bent in the direction of the planet. You'll speed up and the planet slows down.</p>
<p><a href="https://i.sstatic.net/5195WO8H.gif" rel="nofollow noreferrer"><img src="https://i.sstatic.net/5195WO8H.gif" alt="enter image description here" /></a></p>
<p>My question is, what's the maximum angular deflection you can get from this operation? Can swinging around a planet change your path by 90 degrees?</p>
<p>How about a black hole? Can I whip around real close and leave moving in the opposite direction, 360 degrees?</p>
<p>I guess I want to understand the geometry of geodesics.</p>

CC BY-SA 4.0
2024-09-04T15:19:14.120Z
A deflection of 180° sends you back where you came from: a "hairpin" bend.
PM 2Ring
CC BY-SA 4.0
2024-09-04T15:23:47.323Z
The maximum deflection (in Newtonian gravity) is 180°, in the frame of the planet. But that calculation ignores the size of the planet. And in a gravity assist we want to know the deflection in the Sun frame.
PM 2Ring
CC BY-SA 4.0
2024-09-04T15:31:14.117Z
In GR, things get more interesting. Trajectories can loop multiple times around a black hole or neutron star. See my https://physics.stackexchange.com/a/805213/123208 & https://physics.stackexchange.com/a/810981/123208 for some diagrams of photon trajectories. And see rob's excellent answer for massive particle trajectories: https://physics.stackexchange.com/a/774043/123208
PM 2Ring
<orbit><gravity><space-probe>
What's the maximum deflection angle from a gravitational velocity boost flyby?
Miss Understands
# What's the maximum deflection angle from a gravitational velocity boost flyby?

(Asked by: username_0 on 2024-09-04)

If you pass an object with negligible mass, your your path will remain unchanged and your deflection will be zero degrees.

if you pass a planet, your path will be bent in the direction of the planet. You'll speed up and the planet slows down.

[![enter image description here](https://i.sstatic.net/5195WO8H.gif)](https://i.sstatic.net/5195WO8H.gif)

My question is, what's the maximum angular deflection you can get from this operation? Can swinging around a planet change your path by 90 degrees?

How about a black hole? Can I whip around real close and leave moving in the opposite direction, 360 degrees?

I guess I want to understand the geometry of geodesics.

**Tags:** orbit, gravity, space-probe

> **Comments:**
> **username_1**: A deflection of 180° sends you back where you came from: a "hairpin" bend.  
> **username_1**: The maximum deflection (in Newtonian gravity) is 180°, in the frame of the planet. But that calculation ignores the size of the planet. And in a gravity assist we want to know the deflection in the Sun frame.  
> **username_1**: In GR, things get more interesting. Trajectories can loop multiple times around a black hole or neutron star. See my https://physics.stackexchange.com/a/805213/123208 & https://physics.stackexchange.com/a/810981/123208 for some diagrams of photon trajectories. And see rob's excellent answer for massive particle trajectories: https://physics.stackexchange.com/a/774043/123208  

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