# Dyson slingshot & Neutron stars

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1. Nov 5, 2014

### Iulian Ionescu

Hi,

I am currently working on a Sci/Fi short story where a spaceship leaving a planet will perform a gravitational assist sling through a binary neutron star system. The problem is that I have difficulty finding information about how close a planet could be to a binary neutron star system? I need to figure out the distance and speed I would need for the ship to enter the gravitational field and then figure out the possible exit speed. In a nutshell, I need a planet that is *close* to the stars, so that when the ship is thrown out by the gravitation it goes very far away very fast... I hope it makes sense. I just have difficulty knowing how close "close" could be for this to be possible...

Any ideas or hints into where to look would be appreciated.

Thank you!

Iulian

2. Nov 5, 2014

Hi,

You need to read or at least be aware of Larry Niven's 'Neutron Star' short story. Synopsis here. Pay particular attention to the "... Afterthoughts ..." comment from Niven in the Notes section towards the end of the page. Sorry I can't help with the mathematics.

I'd recommend reading it anyway - they don't write them like that any more!

Last edited: Nov 5, 2014
3. Nov 5, 2014

### Bandersnatch

Hi Iulian, welcome to PF!

There are two main issues with planets being close to neutron stars, that stem from the way a neutron star is formed.

Since it requires a massive(>10M) star going through its supergiant phase before exploding as a supernova, there will be a period in the life of the stellar system when the star will have swollen to huge proportions. 1AU can be thought as minimum radius. Any planet closer than that is going to evaporate when engulfed by the star.

Furthermore, as the star finally explodes leaving the collapsed core, it loses large portion of its mass. This in turn means the dynamics of the planetary system change, and all the planets migrate to new orbits with greater radii.

Do note that supernova explosions are not very conductive to planetary habitability, so I hope you're not planning on making the planet a cradle for a civilization.

However, I'm not sure why is it you need the planet to be close. I don't see what does the orbit of the planet have to do with the maneuver you're planning for the ship to perform.

4. Nov 26, 2014

### Khashishi

Could the planet be captured after the neutron star forms? The probability is perhaps small, but I don't think it's too far fetched.

5. Dec 8, 2014

### Danger

That's certainly possible, however low the odds might be. I'm more concerned with the fact that space is uninhabitable within about 100 light-years of a supernova for some considerable time after. If your ship needs a slingshot boost, it clearly isn't using anything close to a light-speed drive (cudos on you for that), which means that it must be originating nearby. I''m pretty sure that no one will even be aware of that, though, let alone care enough for it to detract from their enjoyment of the story. (I know that it won't bother me any.)

6. Dec 21, 2014

### Khatti

If I remember my (casual) studies of thirty years ago correctly, dense stars have a much shorter life than a star like the sun. In fact the lifetime of such a sun is so short that life would probably not have time to evolve on any planet orbiting it. What would be a point of interest in a story written using this background is that any civilization you would find in this star system would have migrated there from somewhere else.

7. Dec 25, 2014

### Staff: Mentor

Neutron stars could still be useful if you plan for really long distances. Tidal gravity of a neutron star with 1 solar mass would allow to get as close as ~3000km for ~1g per meter where you have an escape velocity of ~10 000km/s. If the stars orbit each other with a similar velocity, you can get a few percent of the speed of light for free. Typical relative star velocities are ~20km/s or 1 light year in 15000 years. The neutron stars can be old at the time they come close to the system where your life developed, then their origin does not matter.