You don't stop this type of craft, it's strictly accelerate and coast type of thing.

Also note that "solar sail" is a bit misleading, the (now apparently dead) Breakthrough Starshot design was a big reflector "sail" in space and very many lasers on Earth to power it, it's not actually driven by a stellar wind directly.

This suggests ejecting a secondary mirror in front of the craft to reflect light to brake the original craft: https://arxiv.org/pdf/1604.01356:

"...or by ejecting a reflector that is then used as a braking system (similar to thrust reversal on jets) but this only works if the payload is still within illumination range of the primary laser system"

Flipping the sail around would probably be the lightest option, though tricky because the larger sail designs would not be rigid.

Solar sails aren't powered by solar wind but by light reflecting off like the probe. But the probe would be powered by laser so not really "solar" sail. Light sail is the generic term.

I don't think we can just go near speed of light. Even hard vacuum out there contains particles. Heliosphere is chock full of them, then Oort cloud has stuff way bigger than that (or any probe), even if sparsely spread out. Then there is cosmic stuff outside, as Voyager found out.

Getting hit by some random molecule when orbiting Earth or just travelling say 30,000 kmh is one thing. Getting hit by swarms of molecules with say 0.5c can be catastrophic to the material. Now imagine wading through some space dust cloud, or even plasma cloud (ie remnant of some bygone supernova).

Star trek had shields, and for good reasons. Super strong magnetic field may divert some charged particle, but helium molecule is just a helium molecule, no extra charge to play with.

Nit: shields were just for battle, for this they used the deflector.

> Or does it strand you halfway between stars in the doldrums where the force on both sides of your sail equals out from two stars?

This is actually I "love" to think about:

What would it be like, to be "stranded" in the space far from any stars?

or in the "voids" where there are relatively very few stars/galaxies to begin with?

There must be things drifting there right now...

It would also be the perfect place to HIDE something :)

If intelligent life evolved on a planet of a brown dwarf — a “failed” star — that was ejected from its original galaxy deep into intergalactic space, then that species would be spectacularly isolated.

Note that the “naked eye” stars we see in our night sky are all big, bright stars in our immediate vicinity.

Outside of a galaxy the night sky would be black, other than some fuzzy smudges of other galaxies.

It would be a long time before any such species would figure out what galaxies are, what stars are, and their own relationship to those things.

Their study of astronomy would take a wildly different path even assuming they end up at the same conclusions!

And then what? What missions could they envisage, tens of thousands of light years away from the next nearest… anything?

> If intelligent life evolved on a planet of a brown dwarf — a “failed” star — that was ejected from its original galaxy deep into intergalactic space, then that species would be spectacularly isolated.

Even better, (or worse): A species that evolved on a rogue planet! Without any star!! (heated by it's core or nuclear elements or space magic or whatever)

> It would be a long time before any such species would figure out what galaxies are, what stars are, and their own relationship to those things.

Humans are bad enough with our "We're unique and special!" complex, imagine theirs!! lol

Do we have a good estimate for the density of intergalactic stars? Or how far away from a star will you be on average, when you are, say halfway between the Milky Way and Andromeda?

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

Apparently this is very new science, the information is still being collected as cutting edge research.

One thing that is clear is that the intergalactic medium has a highly variable density. In the vicinity of a recent galactic merger or near-miss, there would be a smear of stars fading off into the distance.

Conversely, even a fairly quiet and passive galaxy like our own is expected to eject stars at a rate of one every few hundred years from the core region immediately nearby the black hole there.

It’s where the future hides :)

You would fold the sail?

That only stops you accelerating, it doesn't put the brakes on

Deceleration is the same as acceleration. You use the light of the star you're approaching to slow down.

But if you're going near light speed, the light / particles would be too faint to have any significant effect until you get very close. You'd basically just fly backwards straight into it. Unless your sail is very large and/or the total mass is very small.

That would not stop the probe from continuing to glide further. He's making a good point here.

Gliding is fine. We whizzed past Pluto with New Horizons. Never stopped, just a photo flyby.