Interstellar Flight: Perspectives and Patience

>To get around thousand-year generation ships, we are examining some beamed energy solutions that could drive a small sail to Proxima in 20 years.

The odds of a spacecraft hitting a single particle of dust while in space are 100%.

A spacecraft hitting a single particle of dust at 0.2c will impart tens of millions of joules into the body of the spacecraft, the equivalent of getting hit with hundreds of pulses from the most powerful laser ever created by humanity-- simultaneously.

Or concentrating several kilogram's worth of TNT into the size of a particle of dust and detonating it.

Only true if the dust grain is stopped by the craft. For a thin lightsail the grain will probably pass right through without depositing much energy

So it passes through the sail and then hits the spacecraft attached to the sail. Now what? kaboom? small holes in the hull would not be good for the occupants.

Ten megajoules sounds like a lot, but a single kilo of TNT produces about 4 megajoules of energy. And the size of particles of dust and how often you’re likely to hit one in the interstellar medium is quite speculative.

alternatively you ionize that particle, may be make in into plasma by laser for ease of "digestion" down in the engine, direct in into the engine where it is used as working mass for your ion thruster, kind of similar to scramjet.

As far as i see with today's tech - like Starlink's ion thruster + classic nuclear reactor - we can get to 300km/s in about 4 stages. Straightforward improvement of ion thrusters - mainly voltage increase and associated engineering (which will immediately happen once we start flying to Mars and beyond as ion thruster currently our best/fastest option inside the Solar system) - can get us to 1000-2000km/s, i.e. under 1000 years to Alpha Centauri (that for a large populated spacecraft, and for just tiny probe to announce our existence (and to send back photos which we'd receive using Sun's gravitational lensing) we can do even better). And using interstellar gas and dust scramjet-style will improve on those numbers (as such ship is mostly limited by the working mass it starts with while the reactors would be able to continue produce the energy much longer).

I wasn't even considering the sail.

Most of the designs for a system like this are "chip" designs where a single 1cm x 1cm silicon wafer is towed by the sail.

This design prevents the need for lasers so large that they create enough ozone to kill the entire human race.

The contents of the chip vary, based on who is speculating, but tend to contain exotic, uninvented, circuitry capable of both harvesting energy from the laser and doing "something" of use besides zipping by the target at 0.2c deaf, dumb, and blind. Sometimes it's even an AI-enhanced swarm! (Shoulda figured out how to work blockchain in there, post-doc guy)

Regardless, during the 40 trillion kilometer voyage to Proxima Centauri, that 1x1cm silicon wafer (and the sail) will hit space dust, and numerous other atoms and molecules (including carbon rings) because empty space... isn't.

Are there any real proposals that deal with this issue for a vehicle that would carry humans and go fast? Something that's not "energy shields".

Edit to add: we basically understand the physics of accelerating something to a high speed, what it would need to be made from, etc., afaik all within the realm of possibility- if we could gather and direct that much energy and then wait long enough to decelerate at the other end.

It seems like the questions that are completely unaswered are: keeping people alive and healthy for that long, and how the ship could survive if it hit something.

> the size of particles of dust ... in the interstellar medium is quite speculative

Technically yes. I think there's a significant variety of sizes of dust or larger-than-dust particles in interstaller medium but I don't really have much to back that up.

> how often you’re likely to hit one in the interstellar medium is quite speculative.

Also technically yes. But unless you can map every single particle of dust, and their trajectories, I think the risk is absolutely real.

The sail is much, much larger than the craft. The odds of that happening are tiny.

In any case, we should launch more than one.

"A 0.1 µm interstellar dust grain (≈10⁻¹⁴ kg) striking a spacecraft at 0.2 c carries ≈20 J of kinetic energy—millions of times below “tens of millions of joules.” Reaching 10–50 MJ would require a ≈0.14 mm grain (≈10 µg), vastly rarer than ordinary dust, and even that impact equals only a few shots from the world’s highest-energy laser (~2 MJ per pulse), not “hundreds.”"

scifi answer (the particle warps with space around the vehicle)

the other thought is a space ram jet sucking in particles heard of some idea like this

> Are there any real proposals that deal with this issue for a vehicle that would carry humans and go fast?

Whipple shields [1].

> Something that's not "energy shields"

The interstellar medium contains lots of charged particles [2]. Electromagnetic deflection is perfectly realistic.

[1] https://en.wikipedia.org/wiki/Whipple_shield

[2] https://www.space.com/interstellar-space-definition-explanat...

> alternatively you ionize that particle, may be make in into plasma by laser for ease of "digestion" down in the engine, direct in into the engine where it is used as working mass for your ion thruster, kind of similar to scramjet

This is a Bussard ramjet [1]. The interstellar medium is too thin to make it work. (Maybe we'll find the husk of an ancient ramjet from an earlier era of the universe floating around one day...)

[1] https://en.wikipedia.org/wiki/Bussard_ramjet

more than one basket for those eggs will be important.

Bussard would collect gas to be used as fuel. For ion thruster it is only a working mass.

I used something something 10^-10 for my dust. To reach ~50MJ.

As far as the laser goes, ~2MJ is the total output. Energy that reaches the fuel pellet due to inefficiencies throughout the path of the laser, the actual "hitting power", is hundreds-ish kJ.

When it passes through the sail, enough energy is deposited in the grain to explode it, so if there's sufficient distance to the hull the vapor deposits sufficiently low energy/area to be tolerable.

https://en.wikipedia.org/wiki/Whipple_shield

Perhaps, some fraction of the laser energy beamed to move the sail is redirected directly in front of the spacecraft to vaporize any dust particles in its path. Fragments of a circular sail could be angled in the right way so as to take some fraction of the light's momentum in its desired direction with the rest reflected at an angle to clear the opposite side of the sail. For a circular sail you'd then get a circular "clearance beam" of sorts.

You're avoiding the energy that would be imparted by hitting the dust spec by... adding energy to the dust spec? Perhaps splitting it so it damages a larger area? It seems better to let it plow as small a hole as possible in the sail.

1. Dust specs would respond to momentum imparted by the light much more quickly, which would reduce the momentum difference between the ship and the specs.

2. Enough incident light could vaporize the specs entirely leaving only gas or plasma, which doesn't carry the same danger to the ship.