Interstellar Flight: Perspectives and Patience

> those of us with an interstellar bent naturally start musing about ‘sundiver’ trajectories, using a solar slingshot to accelerate an outbound spacecraft, perhaps with a propulsive burn at perihelion. . The latter option makes this an ‘Oberth maneuver’ and gives you a maximum outbound kick.

You can't do a solar slingshot like you can with (say) Jupiter because the sun is essentially at rest with respect to the rest of the solar system. You could still do an Oberth manoeuvre.

If you unfurl a solar sail after perihelion, presumably you get more energy while nearer the sun, giving more of a "kick", and lower in the gravity well (would oberth still apply for solar sail)

Your speed once you get to 1AU would I assume be far higher than if you had simply started at Earth

But getting to the sun in the first place (from Earth) is a massive hassle as you have to lose the Earth's significant orbital speed to 'fall' inward [0]. Perhaps better just to use that fuel to head out. Operating a solar sail really close to the sun would also be challenging because of the massive heat.

[0] https://www.nasa.gov/solar-system/its-surprisingly-hard-to-g...

> the sun is essentially at rest with respect to the rest of the solar system

But not with respect to other star systems.

If you start in our solar system this doesn't help. You could do a gravity assist from a planet to help with an interstellar trip (like the Voyagers) but not a gravity assist from the sun, starting from one of 'our' planets.

> could do a gravity assist from a planet to help with an interstellar trip (like the Voyagers) but not a gravity assist from the sun, starting from one of 'our' planets

Of course you can. Galileo, Cassini-Huygens and Giotto are Earth-launched spacecraft that used Earth for a gravity assist. If you need to accelerate with reference to the galaxy, you can use the Sun’s motion through it to slingshot.

Earth isn't going in a straight line. If it were, it would be useless as a gravity assist.

No, you can't. Gravity assists require the assisting body (Jupiter) to be moving, relative to the body that needs the assist (Voyager), in the direction the assisted body wants to go.

They have an unfortunate name because it's not just the assisting body's gravity that provides the assist. It's the combination of gravity and velocity.

As Voyager nears Jupiter, Jupiter's gravity grabs it. Then, Jupiter's velocity relative to Voyager drags Voyager - that is, transfers energy to it. Without that relative motion, all Voyager's gravity does is add energy on the way in and then remove exactly the same energy on the way out.