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Comment by benjoffe | Hacker News Reader

That "trick" would indeed work and is a well known orbital manoeuvre: http://en.wikipedia.org/wiki/Bi-elliptic_transfer (without the final circularising burn).

This manoeuvre would be preferable to the article's alternative (a Hohmann transfer reducing periapsis to the sun's radius without the final circularising burn) in all cases where the initial semi-major axis (relative to the sun) is roughly greater than 11.94 solar radii (ignoring minor factors that make this case slightly different to the wikipedia example).

edit: to make it clear, I'm talking about a heliocentric bi-elliptic transfer after Earth escape, where the initial orbit is equivalent to Earth's orbit, and the final inner orbit can be imagined as one scraping the surface of the sun, but since the sun would destroy the craft, the final circularising burn is not required (in sloppy terms: hitting the sun on the edge would be easier than hitting it straight on).

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The downside, of course, is that your nuclear waste spends an awfully long time in an orbit that crosses, or at least passes very close to, Earth's. Better not lose control of your rocket, or you'll have a politically inconvenient situation on your hands.

More important than politically inconvenient: a technically inconvenient situation, as that waste could be on a trajectory back to earth.

https://youtu.be/YIqXql6LLz0?t=6m50s

thats not how this manoeuvre works.

the burn to fall into the sun happens way before any possible return to earth if that orbit were not altered... all that happens is that you start near the earth - thats unavoidable because the rocket comes form the earth.

it will of course cross the earth's orbit on the way into the sun, but, with the exception of very small ranges of orbits the chance of hitting the earth on return is tiny, and easily avoided with some timing.

I'm aware of how a bielliptic transfer orbit works. See the long gap between burns 1 and 2 on the Wikipedia diagram? That interval is necessarily going to be more than half a year long, and potentially much, much longer. Spacecraft are pretty reliable once they're in space, but that's still a lot of time for something to malfunction. And if burn 2 doesn't happen, now you have tons of nuclear waste on an elliptical orbit that re-visits Earth's path every so often. At any given perihelion, the earth has only a small chance of being in the right part of its orbit, but eventually...

I'm not saying it's a big risk; the chance of a catastrophic launch malfunction is probably a lot greater. But it's definitely a long-term potential hazard that needs to be accounted for.

unless burn 1 has a very specific effect that thing is not going to return to earth, despite crossing the orbit. hitting a resonance by accident is pretty unlikely... one could say astronomically so :P

its hard enough to hit things when we want to

Sorry, your link doesn't mean what you think it means.

What you're missing is that both waste and the earth are already in orbit around the sun whereas the maneuver your link describes involves switching between orbits around different smaller bodies.

Edit: Which is to say as the other have mentioned, this doesn't send the waste into the sun (as the parent post implied) but merely on some orbit similar to the earth as the article described.

No, he's right

A bi-elliptic transfer is used for transferring between two orbits around the same body. In some situations, this is more efficient than a straightforward Hohmann transfer, at the cost of taking much, much longer.

For a Hohmann transfer, you reduce the ship's orbital velocity relative to the sun, reducing the orbit's perihelion to somewhere inside the sun. That requires cancelling out almost all of the Earth's orbital velocity.

For a bi-elliptic transfer, you first increase the ship's orbital velocity relative to the sun, increasing the orbit's aphelion to somewhere in the outer solar system. That's your first Hohmann transfer, going apparently the wrong direction, but requires less than 40% as much energy as dropping it into the sun did. Then, at aphelion, when the ship's orbital velocity is lowest, you reduce the ship's orbital velocity to drop it's perihelion inside the sun. From that far out, and at such an eccentric orbit, that requires only a fraction of the energy it did from Earth's orbit.

The entire manuever, potentially, could be far more efficient than a direct transfer. It just takes far longer, and if the ship fails to make the second maneuver, it'll be left in an orbit that crosses Earth's orbit.

you don't understand the link yourself... there are no bodies mentioned.

its about transferring from one orbit to another with a large elliptical orbit inbetween, its very well established. (and e.g. something i am familiar with before googling it or looking at a wikipedia page)

I believe they're referring to a Solar radius orbit as the destination given by the article. Doesn't sound wrong to me.

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