> a swarm of unguided relativistic projectiles

In a sense, this is essentially what the universe is.

I don't think you'll find 0.2c relative velocities except near very energetic phenomena, especially not of things that are too small to easily see coming.

Cosmic rays are far too small to easily see coming, and we get those all the time here.

I think it should be clear from the context that this discussion was about macroscopic objects. Cosmic rays don't carry the kinetic energy equivalent of a medium size nuke.

True, the universe is generally a pretty unfriendly place that tries to kill you in any possible way.

To be fair, the universe was pretty friendly and created us in the first place.

Technically, we are the universe trying to understand itself.

Technically, we may be the universe trying not to kill itself.

Technically, we may be the universe trying to kill itself.

Their speed isn't particularly relevant, what matters more is their energy. As big as that number is to us, it's still pretty insignificant; there's a lot of objects already traveling around with us in our own solar system with 70kT (per PaulHoule in other comments) of energy. A looooooot of objects. In the grand scheme of things, this is a rounding error on a rounding error. We are small.

It would be rather anticlimatic if it hit one of them before reaching Alpha Centauri and no one noticed.

That's one of the reasons they want to send thousands of them.

Ten thousand years from then, some alien on some planet somewhere might have a very bad day.

"That is why, Serviceman Chung, we do not eyeball it!"

https://www.youtube.com/watch?v=4tIk-vUtLBs

"Cool, an iPhone."

Maybe it's the backstory for Seveneves.

It would be nice if they could use solar wind to decelerate as they approached. I know there's not enough energy there to insert them into an orbit, but they might get time for a few more pictures?

You can --- you use a thing called a magsail.

The short version is: remember the old Bussard ramscoop idea? You use a magnetic field to collect interstellar hydrogen which you then fuse for thrust? Turns out that in our part of the galaxy, you get more drag from the sail than you do from the fusion thrust, so the idea was scrapped.

An embarrassingly long time later people finally realised that they'd invented a fuelless brake, and the idea was resurrected (but without the fusion drive). The maths are quite plausible and the sail itself trivially simple --- just a wire loop.

http://www.centauri-dreams.org/?p=22138

However, I don't think they'd be compatible with this idea --- I suspect you wouldn't get one big enough to be useful in a one gram package. But estimating the numbers is beyond me. Here's the paper if you want it. http://www.niac.usra.edu/files/studies/final_report/320Zubri...

Very fascinating! I had no idea we had such a viable option for braking.

Two questions for you if you're up for it.

Could we use this concept for braking during earth re entry to depend less on complex heat shields?

Why is the idea of a bussard ramscoop not viable?

Why decelerate? At 0.2c it takes around 40 minutes to cross 1AU. (Ignoring time dilation which is not super significant at 0.2c.) That gives ample time to take photos from a moderate distance.

Just flip the battery in the laser around and it will pull instead of pushing.

You mean we should... reverse the polarity?

excuse me, but I think you forgot to take the opportunity provided by that ellipsis to put on your visor.