How does a laser provide thrust for a probe?

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Offline thedoc

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How does a laser provide thrust for a probe?
« on: 22/09/2016 09:53:02 »
Luke Pullar asked the Naked Scientists:
   In relation to the proposed probes to be sent to Proxima Centauri b, how does a laser provide thrust? And why are the velocity estimations limited to 0.25c?
What do you think?
« Last Edit: 22/09/2016 09:53:02 by _system »


Offline evan_au

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Re: How does a laser provide thrust for a probe?
« Reply #1 on: 22/09/2016 10:57:55 »
I haven't seen any specific proposals for a probe to the rocky planet thought to orbit Proxima Centauri b.

But from your brief description, I assume that:
- They are proposing to use laser propulsion to get far higher speeds than you can obtain from chemical fuels or even electric ion propulsion
- The problem with any sort of propulsion is that it requires a lot of power.
- Power sources are very heavy, whether they are chemical, solar panels or nuclear power
- Reaction mass is very heavy, for chemical rockets or even for the relatively small amount of Xenon needed for electric ion propulsion.
- It is extremely difficult to accelerate a high mass to a high velocity

One solution is to have a small passive spacecraft, with a large, low-mass reflective "parachute"
- There is no heavy engines or reaction mass on the spacecraft
- The lasers are mounted on the ground or in Sun orbit
- They get their fuel from nuclear power or solar power
- This heavy infrastructure does not need to be accelerated to high velocity (you just need to get them into orbit)

The limitations are:
- The laser intensity declines rapidly as the distance increases
- Once you accelerate the space probe to a high speed, it rapidly gets far away, so you can't accelerate it any more.
- Of course, you can't accelerate anything up to the speed of light
- It's a matter of economics - how many lasers can you afford to build, with what power source, and where can you put them (out near Jupiter?)

At least the lasers should be reusable for later interstellar space probes - Sirius is only twice as far as Proxima!
- The biggest risk is whether we can really trust politicians with the power to obliterate each other?