[JumpCrisscross]

> non-propulsive gravity assists are essentially the same as elastic collisions with balls of non-equal mass

From the Earth’s frame of reference there is no change in delta-v other than a change in direction. It’s only from the Sun’s frame of reference that there is velocity added in the speed component (v_infinity, commonly). If you can find a single measurement to the contrary, that’s novel enough to be worth publishing.

That’s why you can’t gravity assist around the Sun to get around the Solar System faster.

[nick238]

I don't know what you mean by "gravity assist around the Sun":

1. Using the perihelion in an orbit "around the sun" as a gravity assist?: spacecraft usually care about their speed relative to the sun (characteristic energy, C3), and a (free) gravity assist around the sun won't do much. Dropping close to the sun to perform a powered bi-elliptic transfer could be a thing if you wanted to travel extreme distances (e.g. put a telescope at 500 AU to use the solar gravitational lens)

2. Using other bodies that are "around the sun" to get a gravity assist?: spacecraft do this all the time.

Also "get around the solar system faster":

1. Decrease the orbital period (lower orbits orbit faster): This is exactly what Messenger and Parker Solar Probe is doing flying by Venus/Mercury. They're 'bouncing' off of the planets, trading orbital energy and raising the planets' orbit around the sun while dropping their own.

2. Get to places faster: This is what outer planets probes (Voyagers 1/2, Cassini, New Horizons) do. If Jupiter wasn't there, these missions might not be possible.

[JumpCrisscross]

> If Jupiter wasn't there, these missions might not be possible

If Jupiter weren’t there and moving relative to the destinations. The gravity isn’t the critical piece, it’s the relative motion.

Just having a massive object does nothing because gravity isn’t doing any work, it’s just coupling you to a moving object.

[nick238]

Oh, sorry, I didn't specify which Jupiter I was referring to, the real, moving Jupiter that orbits the sun vs. the stationary, straw-man Jupiter that jumps out at you in bad faith retorts.

[JumpCrisscross]

> I didn't specify which Jupiter I was referring to, the real, moving Jupiter that orbits the sun

This is why I said you can't gravity assist around the Sun to travel around the Solar System. The Sun is moving around the galaxy at a terrific speed. But so is the Solar System. Dropping into and out of the Sun's gravity well does nothing other than change your trajectory.

[ahazred8ta]

If you fire rockets at closest approach to get more delta-V, that's an Oberth Maneuver - https://en.wikipedia.org/wiki/Oberth_effect

And as you say, an unpowered planetary flyby is a gravity assist.

[bgirard]

> From the Earth’s frame of reference there is no change in delta-v other than a change in direction.

But no one is talking about the Earth's frame of reference.

> But “the gravity of Earth” imparts no net delta-v and wouldn’t on its own allow the craft to reach Venus.

That's statement is untrue. Gravity assists with planets can provide net delta-v allowing spacecrafts to reach other planets. See the Voyager 2 gravity assists for one example.

[JumpCrisscross]

> Gravity assists with planets can provide net delta-v allowing spacecrafts to reach other planets. See the Voyager 2 gravity assists for one example

If you were on Jupiter measuring Voyager’s speed before and after it interacted with Jupiter, you wouldn’t measure a net effect. It’s only if you’re standing on the Sun (or somewhere else where you can see Jupiter revolving) that you see Jupiter “pull” the spacecraft along, thereby imparting velocity.

Gravity doesn’t do any work. The gravitational potential energy of the Voyager-Jovian system is entirely conserved in a flyby. Jupiter’s orbital energy about the Sun is what’s stolen.

This is a common misconception when it comes to gravity assists. It’s why I think that language could be tighter.

[bgirard]

> If you were on Jupiter measuring Voyager’s speed before and after it interacted with Jupiter, you wouldn’t measure a net effect.

Again that's not something I'm claiming.

[saagarjha]

You’ve retreated to a different argument nobody was making.

[JumpCrisscross]

> You’ve retreated to a different argument nobody was making

Respectfully, I have a background in aerospace engineering. This is the number one popular misconception about gravity assists/slingshot maneuvres.

[nick238]

If jessriedel hadn't said

> The gravity of the Earth absolutely changes the speed of the probe.

but instead

> The gravity of the Earth absolutely changes the characteristic energy with respect to the sun of the probe.

Would you be happy?

[JumpCrisscross]

No. The gravity of the Earth doesn't do any work on the probe. It's a coupling mechanism. The work that is done is by Earth's orbital energy around the Sun. That is the energy that is less after the spacecraft encounters the Earth than before. The Earth is "dragging" the spacecraft along with it around the Sun. Not trebucheting it.

[jessriedel]

The gravity of the Earth absolutely does net work done on the probe. That’s an unambiguous mathematical statement. It’s just that this work is frame dependent. It sounds like you want it to frame-independent and to be given by the value in the center-of-mass frame. But it’s not.

[nick238]

How about:

> The gravitational coupling of the probe to the Earth absolutely changes the characteristic energy with respect to the sun of the probe.

[jessriedel]

Your first paragraph is exactly what I already said in my comment.