Perhaps on the 'todo' list is pulsing over another 100GW laser to slow future objects back down from the other direction.
By the time the original craft arrives, we will have solved high speed interstellar travel and will have been there for a couple generations with plenty of time to build the optical catcher's mitt.
Hopefully anti-depressants will have made similar gains, because there is going to be nothing so depressing as waking up from your 300 year interstellar nap to find out there's a housing crunch in your new star system.
Besides, let's not get ahead of ourselves with interstellar missions. Scaling this all down for intrasystem propulsion would be great as well. Anyone liked what New Horizons has shown? How about sending one small probe to Pluto per month (with travel time measured in months/weeks instead of years)?
* Base laser "pushers" on moon, Lagrange points, etc.
* Use moon/earth pushers to push 1st wave lightsail craft(s) towards destination. 1st wave craft are also themselves equipped with laser pushers aimed back towards earth.
* Use moon/earth pushers to push 2nd wave lightsail craft towards destination
* 1/2 way to arrival, 1st wave starts pushing _back_ 2nd wave and moon/earth pushers stop pushing.
* 2nd wave "stops" at destination or slows enough to be captured in destination orbit.
- the current plans for Breakthrough Starshot[0] are to send a centimeter-sized, ~1 gram mass spacecraft.
- even such a tiny spacecraft would require a 4x4-meter solar sail and 100 GW of laser power, because photons carry such small amounts of momentum
- a ground based nuclear power plant produces a couple GW of power. 100 GW of continuous laser power would require the entire output of 50+ nuclear power plants
- that amount of power is difficult enough on the ground, trying to put 50+ nuclear power plants worth of energy generation on a space craft would alone be a tremendous challenge
- assuming we could do that, it would weigh unfathomably more than a gram. Just the fuel alone in a ground based station is several tons; the infrastructure to turn that fuel into power is many thousands of tons more… times 50+. It would not be impossible for that amount of power generation to weigh a megaton.
- assuming it could be launched from earth or built in space, and pushed by solar sail, the solar sail needed for a ship weighing a megaton would be another level of challenge. If 16 square meters of sail are needed for every gram, then a megaton would require 16x10^12 square meters. That would be a manufactured object 4,000km on a side, or roughly the size of Russia.
- We haven’t accounted for the increased power needed to power this craft by sail from the ground. Momentum (let’s ignore relativity even though we are talking about reaching speeds where that starts to be important) is mass*velocity, so in order for our megaton-class spacecraft to be pushed with the same performance as a 1 gram spacecraft, we’d need to scale the momentum delivered by photons with the mass of the craft. That would require 10^12 (one trillion) times more laser photons, which would require one trillion times more energy to produce and 50+ trillion ground-based nuclear power plants.
- I think it’s safe to say that pushing a megaton-class starship with lasers and a solar sail is unfeasible. But wait, if we’ve managed to assemble and launch the craft, we don’t need the ground based lasers or the sail, we can just power the ship using all those power plants on board. [1]
- with a ship that large, we could take a city worth of people along for the ride, so is there still a use for the giant laser we’ve brought with us? Maybe interstellar package delivery?
[0] https://en.wikipedia.org/wiki/Breakthrough_Starshot [1] https://en.wikipedia.org/wiki/Project_Daedalus
And for the deceleration with a backward laser idea, could the initial sail store some percentage of the laser energy that's used to propel it, along with solar energy from the target system's star, and use that for the deceleration laser? I'm sure that's still prohibitive in terms of mass, but probably much better than nuclear or any other fuel-based option.