I was just wondering if we did build something like the Starshot initiative, how we could slow the spacecraft approaching their interstellar targets – the drag on the solar sail! All the sail has to do is impact enough gas travelling thru interstellar space equivalent to the mass of the spacecraft – because the relative momentum between spacecraft and gas will cancel out.
The question then is simply how much gas a sail of a given area will impact.
The sun is actually travelling thru a slightly less dense region of space right now called the Local Fluff.
But the density is still 0.3 atoms/cm3, or 106 * .3 atoms/m3.
12.04×1023 atoms in 1 mole of hydrogen molecules which is, conveniently, 1 g. ( the mass of our spacecraft.)
Basically, the sail would have to pass thru 12*1023 / 3*105 = 4*1018 meters before it slows the spacecraft to a stop.
That might seem like a monstrously huge number but there are 1016 meters per light year! So about 400 light years of dust impacting a 1 square meter sail to slow it to a stop.
Clearly the sail needs to be 100 square meters to slow down in 4 light years!
The next problem is the mass of solar sail. So far, the solar sails tested in space have been absurdly heavy for this project. A potential new material makes it feasible – graphene films. They calculate a single molecule thick film would weigh only .00074 g/m2. A more robust solution for instellar travel might be 100 layers with a shock-absorbing bond between layers.
Starshot hopes to accelerate a 1g spacecraft to .1c with multiple ground-based lasers focused on it. After the sail is used to accelerate the craft, coasting some distance and then returning the sail tangential to the flight path will eventually slow it down to a hopefully more lengthy encounter with a distant star system.
Yes, even the smallest dust particle encountered will punch a hole thru the nano-film, but 99% of interstellar matter is simply atomic or molecular hydrogen atoms.