Reaching for the stars, finally - will the young ones among us get pics of Alpha C.?

[Alex K]

This is pretty cool: shooting small probes across  space by pushing them with lasers positioned near earth will allow to reach velocities unconstrained by the rocket equation. Near light speed becomes thinkable.

http://www.nytimes.com/2016/04/13/scienc....html?_r=0

Within two minutes, the probes would be more than 600,000 miles from home — as far as the lasers could maintain a tight beam — and moving at a fifth of the speed of light. But it would still take 20 years for them to get to Alpha Centauri. Those that survived would zip past the star system, making measurements and beaming pictures back to Earth.

[SteelCurtain]

I'm assuming the laser source would be at like L2 or L1, or even multiple sources launched intermittently to different solar orbits. The problem is going to be power, it'd likely have to be nuclear if we're pushing that much wattage, right?

I wonder what the beam width would have to be, and how much attenuation would result over those distances?

ETA: there's no way we would have an array of lasers on earth's surface, right?

[TubbyTubby]

This is pretty cool: shooting small probes across  space by pushing them with lasers positioned near earth will allow to reach velocities unconstrained by the rocket equation. Near light speed becomes thinkable.

Yes, this was on the BBC news last night plus an interview with Hawking. Presumably the receiver would need to redirect the force of the laser otherwise it would get pushed all over the place as the earths position in space moves. I wonder if they have any estimates of the chances of a tiny probe not hitting anything as it escapes the solar system.

The engineering of this would be one of the greatest achievements in our history.

[SteelCurtain]

I just read the article. So it's an initial impulse, not a continuous beam.

[Alex K]

I just read the article. So it's an initial impulse, not a continuous beam.

Yes, but it has to go for a while to get the object up to speed. Initially, only a minuscule fraction of the energy of the beam gets converted to actual movement. Almost all of it will be reflected. I've done a back of the envelope calculation:

[Alex K]

and I get that to reach 1 g acceleration with a 1 kg probe, one needs to reflect roughly 1.4 Gigawatt off of it.

1 g is of course a lot. To reach 1% of light speed, one takes a bit more than three days.

This all scales linearly, so if you have just 100 megawatts and a 10 kg probe, you'd need 300 days of boosting.

I hope I haven't messed up the calculation.

[SteelCurtain]

The article says it'll be 600k miles away after minutes. That's a large acceleration.

[Alex K]

The article says it'll be 600k miles away after minutes. That's a large acceleration.

Yes, if you constantly accelerate with 1 g, you reach 1 million kilometers distance at the 240 minute (four hour) mark because distance goes quadratic with time. But by that time you will go 140 km / second. That's twice as fast as the fastest probe ever, but still a factor 2000 away from light speed. That's why to get to 1 percent light speed, I got 80 hours rather than four.

I don't understand the two minutes. That must be with a very light probe or a very big laser, or I have a mistake in my calculation

[Alex K]

Edit:

Reading the article again, it seems that they aim for a probe which is much lighter than 1 kg. Still, the number they mention (1 million km in 2 minutes) requires a ridiculous g factor. I get like g 14000 . Can anyone confirm this?

Building a sail which doesn't burst if you do this sounds hard. If the pressure is just a bit uneven, it is going to go poof.

[SteelCurtain]

The animation showed a laser array of maybe tens or hundreds of radio telescope sized lasers.

It still seems more rational to have an array of lasers in space that continuously accelerate the probe, but the logistics of that would likely be impossible.