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Comment by leecarraher | Hacker News Reader

Jane, Stop this crazy thing! or in other words, how do you decelerate the spacecraft once you near Mars orbit. You'd need either a ton of conventional propulsion fuel , or a mars based laser there too. Worse is that you have a limited energy sources and no one to mine them. Solar might help, but it'd take a helluva alot of planning to install them via some automated system.

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I had the same question. One thing that crossed my mind is that if speeds like these are possible, you could let the spaceship fly a gigantic loop and slow it down on the way back to earth. Same laser for acceleration and deceleration. (Things I've learned from the Martian #1.)

Did Hermes use a solar(laser) sail?

I know the ISV Venture Star from Avatar did (with antimatter engines for the Pandora side of the trip.)

Hermes used an ion engine, so no sailing, just steady (but slow) acceleration.

In the story "Flight of the Dragonfly", which used the same propulsion system, they simply detached a portion of the sail, and used it to reflect the beam back at the spacecraft to decelerate it.

This doesn't make sense to me. The detached portion would be subject to radiation pressure from the ground-based propulsion lasers, and would quickly be pushed away from the spacecraft it was meant to act on.

Yes, but it's reflecting the light back at the spacecraft. So, even though it would accelerate quickly away from the spacecraft, the light reflected back at the spacecraft would be used to decelerate the spacecraft. So long as the daughter sail never hits C (thanks, physics), it's always reflecting back photons back at the spacecraft (though red-shifted).

You'd still have to account for the light coming from the source as well as from the daughter sail, but a black coating on the back of the spacecraft's reflector would take care of that; it would give you n force pushing you away from the original light source, and 2n pushing you towards the source.

The coordination and planning required for such a device would not be trivial, but it would be possible.

Regarding slowing down upon arrival, could they enter a low orbit and skim the atmosphere of Mars to reduce velocity?

Definitely not. Back of the napkin shows that to get to Mars in 3 days would need a speed in excess of 200 km/s. That is an order of magnitude larger than Mars escape velocity. Your only chance of being captured by Mars is by pancaking.

Make the first vehicle nothing but a giant mirror, crash it into Mars and then reflect the laser off the debris to slow down the second vehicle?

I think you're having trouble grasping exactly how fast 200km/s actually is.

The "debris" would a fine dust scattered over millions of square kilometers at that speed.

Mars is actually a pretty bad planet to try landing on. It has just enough atmosphere to burn up a spacecraft that enters too quickly, but not enough to use for air-braking (skimming or parachutes).

Given that aerocapture and parachutes have been successfully used for Mars probes in the past, that seems unlikely to be true.

What jobu probably means, is that Mars' atmosphere is too thin for parachutes to slow a reasonably sized craft down to low speeds (that's why MSL's landing required additional rocket propulsion [1]). Compare with Earth: Here, the terminal velocity of a reentering capsule is slow enough for parachutes to take over, which in turn slow it down enough for touchdown.

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

It works great for lighter unmanned missions, but becomes less useful as you increase in mass. NASA has had to come up with some interesting solutions in order to land their rovers. Mars Pathfinder used a giant inflatable airbag to scrub the remaining speed it had when hit the martian surface(Lithobraking.) MSL Curiosity, after it jettisoned it's heat shield and parachutes, used a rocket-powered 'sky-crane' in order to slow down the last 200 MPH for a soft touchdown.

This is one of the reason SpaceX is focusing on rocket-powered landings of boosters and capsules vs. parachutes. This is a vital technology required to be able to put large amounts of mass on the surface of Mars

Landing under a parachute is extremely problematic, but they can still be useful for braking before landing, and there's plenty of atmosphere to brake into orbit from an interplanetary trajectory.

Doing that last bit from a 200km/s arrival speed is going to be challenging to put it mildly, but because of the massive deceleration required and the massive heating.

I assume that all fuel taken with you could be used to stop, though at those speeds it might not be a wash. So perhaps atmospheric braking?

Still I could see the value for sending problems out of the system without having to do the orbital boost tricks with gravity

Turn the craft around halfway to your destination and burn to decelerate at the same rate you accelerated. Keeps a nice 1G on the body the entire time.

Propulsion is provided from the earth /LEO/GEO (imagine blowing on a kite with giant fan).

turning the kite around will not make it slow down/return to you while you keep blowing on it.

Sail boats are able to sail into the wind. This, however, requires the drag of the keel to work. It might be possible to use a traditional rocket engine to act like a keel to allow using the laser energy to slow down the craft.

You use 2 kites on board the spacecraft. One gathers the transmitted beam and reflects it to kite #2, which then reflects forward again. Careful design is important to ensure the spacecraft in the middle isn't struck by the photons. And you will have significant losses by the double-reflection, so you have to turn earlier than 50% of the way there.

Wouldn't your idea, at best, null-out the forward propulsive force, but not actually decelerate the ship?

Not my idea - it's been around for decades.

It works because you can treat the two kites as one system, because they're both connected to the ship.

> Not my idea - it's been around for decades.

> It works because you can treat the two kites as one system, because they're both connected to the ship.

Care to supply some citations?

Physicist Robert L. Forward mentioned it in his book Rocheworld (later republished as The Flight of the Dragonfly)in the early 1980's, with the concept work being done in the 1970s.

https://en.wikipedia.org/wiki/Solar_sail#Interstellar_flight

That's a science-fiction book, though. I'm still not getting how the ship could basically sail straight into the laser that's pushing it (e.g. decelerate). The main sail is still getting pushed forward by the laser, even while the second sail is pushing backward (with the reflected light from the main sail).

His credentials in the field are pretty good, and his books are always hard-science (no elves or wizards, and very little hand-wavy stuff).

https://en.wikipedia.org/wiki/Robert_L._Forward

If I recall the design correctly (forgive me - it's been over 15 years since I read it), there's a significant size difference in the sails. The larger sail collects photons from the distant laser and focuses them on the smaller "backwards facing" deceleration sail.

Yeah, for this idea to work as transportation, you really need lasers at both ends: one to accelerate the ship, and other to decelerate.

So it doesn't really solve any problems of initial colonization (beyond fly-by surveys), but it could be used for commerce once a colony has already been established.

You'd also probably want the lasers based on the moon: no atmosphere but large enough not to fly off in the opposite direction.

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