Light Beamer | Atmosphere

The atmosphere introduces two effects: absorption (or ‘reduction of transmission from unity’), and loss of beam quality (or ‘blurring of the beam spot’). The transmission of the atmosphere at a wavelength of 1 micron is extremely good, exceeding 90% at high altitude ground-based sites. Going to a high altitude site also significantly reduces atmospheric blurring, which would allow an adaptive optics system to achieve performance near to the diffraction limit.

The effects of atmospheric turbulence on the beam include a broadening of the beam footprint (equivalent to image blurring for telescope observations), random jitter of the beam spot, and intensity fluctuations (or ‘scintillations’). The blurring depends on turbulence and wind profile in the atmosphere. The turbulence amplitude is reduced by a factor of approximately 4 between sea level and an altitude of 5km.

The quality of an image is measured by the Strehl ratio, which reflects the ratio of the peak image intensity from a point source to the diffraction limit of an ideal optical system. The ratio measures phase deviations caused by lens aberrations and atmospheric turbulence. 10m-class telescopes, such as the Large Binocular Telescope (LBT), comprising two 8.4m telescopes, have demonstrated image resolution of 40 milliarcseconds and Strehl ratio of 80% at a wavelength of 1.6 microns.

Breakthrough Starshot aims to achieve the diffraction limit for an optical system of laser beams across 0.2-1km, which is 1-2 orders of magnitude beyond existing demonstrations. There are no fundamental physics limitations to achieving this improvement. A beacon on the nanocraft or near its launch point (for instance on the mothership) could be used to correct for phase variations in real time. The effect of the light beam on the atmosphere could be studied, and corrected for, by adaptive optics, again in real time. Additional beam focusing may also be explored to reduce the beam spot size using pulsed laser filamentation techniques.

Jun 02, 2016 04:18 Andrew Palfreyman Posted on: Centauri Dreams

As many have noted, placing the beamer on the far side of the Moon both removes political objections about weaponisation, and radically simplifies the very difficult atmosphere/phase distortion issue. It's a win-win predicated on minimising a single number - the launch cost in dollars per kilogram.

We can do this, but we have to swallow hard to eat the upfront capital cost of StarTram and/or Skylon in order to enjoy launch costs around $50/Kg to LEO. Sorry to say that even reusable rockets will never get close to that level of cheapness.

Jun 10, 2016 18:50 Carl Hinton Posted on: Breakthrough Initiatives

Producing huge amounts of energy in orbit (or on the moon / Mars) within short time scales feels almost an insurmountable problem with current technologies. Which calls for an alternative. The logical alternative is to charge something up over a period of time such that a huge amount of energy can be released when it is needed quickly. For an earth based comparison see pumped-storage hydroelectricity – the Bath County pumped storage station has been described as the “largest battery in the world” - a generation capacity of 3,003 MW. We need something around 5 times as big, out in space. We don't think we have water out there (yet / in the quantities needed) and the moon has little gravity to help us out. What we do have on Mars is a lot of small grained sand which can act like a liquid under certain conditions - think egg timer. We could (theoretically, with a lot of imagination) create a sand powered generator on Mars, something like what the ancient Egyptians may have used. On Mars we have Olympus Mons which is practically speaking a nature built space lift - it has a height of nearly 22 km (13.6 mi); at the top you are practically speaking already in outer space.

The highest earth based spot that I know of is Mt. Everest (elevation: 8,848 m) - creating some kind of elevator up to this height would be a major engineering feat in its own rite. But, it would be another great place to put the beamer. This conceivably could be accomplished with carbon nano-tube cabling (CNTs - invented at Rice University) which have been identified as possibly being able to meet the specific strength requirements for an Earth space elevator. CNTs have the additional benefit of being highly conductive, they have greater capacity (up to four times) to carry electrical current than copper cables of the same mass, according to new research.

However, in my opinion the closest technology is actually geothermal, and the ideal location sits above the clouds of Venus. Venus is a potentially amazing power source. The difference in both temperature and pressure between surface and above the atmosphere could potentially provide a planet sized generator. The temperature at the equator averages around 450 °C (723 K; 842 °F), higher than the melting point of lead. The atmospheric pressure on the surface is also at least ninety times greater than on Earth, which is equivalent to the pressure experienced under a kilometre of water. Venus presents several significant challenges to human colonization. Venus's atmosphere is made mostly out of carbon dioxide, which after filtering from sulphuric acid, can be used to grow food. Because nitrogen and oxygen are lighter than the carbon-dioxide that makes the atmosphere, breathable-air-filled balloons will float- at a height of about 50 km (31 mi). At this height, the temperature is a manageable 75 °C (348 K; 167 °F); or 27 °C (300 K; 81 °F) if we could get 5 km (3.1 mi) higher then this is a viable solution. The geothermal industry and the U.S. government are looking at ways technology can change the hot rocks game and give a boost to an often-overlooked resource. Legendary venture capitalist Vinod Khosla, (who made a pile of money in information technology and computers), is making another big bet, on an advanced energy technology. He too has put his money down to the promising but long-lagging field of geothermal energy.

Jun 13, 2016 14:13 michael.million@sky.com Posted on: Centauri Dreams

It may be possible 'prepare' the atmosphere with another laser that heats and disperses the absorbant components allowing the main beam more transmittance.

Jul 16, 2016 00:21 Lincey Murray Posted on: Breakthrough Initiatives

I have never thought something like this!

Jul 16, 2016 00:22 Lincey Murray Posted on: Breakthrough Initiatives

i'm sorry I have posted twice

Jul 23, 2016 21:18 Breakthrough Initiatives Posted on: Breakthrough Initiatives

Excellent questions. Thank you.

We have chosen to focus on a ground based system for Starshot as the costs of going into space are vastly higher. While much of our critical technology is on an exponential growth curse “Moore’s Like Law” and hence highly favorable to reducing our costs and increasing our performance, the cost of launching into orbit is not on an exponential curve. In the longer term such systems as we propose for Starshot will likely be placed in orbit on the moon (for example) but to achieve the kind of program envisioned by Starshot we need to be ground based.

The issue of filamentation is not a critical issue for use as our aperture flux is so low. There is some heating of the atmosphere during the time the laser is on (we are about 100x normal sunlight flux) but this is not what is normally termed filamentation which occurs at lastly higher flux (power per area) levels.

The issue of mitigating the atmospheric perturbations is critical to our Starshot program and this is dealt with in a hierarchical program of “beacons” in the atmosphere and on the spacecraft to allow us to maintain phase alignment and targeting.

For technical details see section 11 in the paper “A Roadmap to Interstellar Flight”:
http://arxiv.org/abs/1604.01356

– Prof. Philip Lubin, Breakthrough Starshot

Aug 24, 2016 22:18 Alberto Rabell Posted on: Breakthrough Initiatives

Maybe shooting the laser for a longer period might help. Almost all ground based lasers wouldn't do the trick due to Earth’s rotation, but an Antarctic base would do it. I'm not an astronomer but a quick Google Sky Map search pointed out that Alpha Centauri is south, so I assume that Alpha Centauri would be high in this sky in the Antarctic. Been high in the sky would help reducing the atmosphere the laser would need to travel and it would help maintaining it focused for a longer time (during Antarctic winter).
Mount Kirkpatrick (4,528 m) would be the best choice, since it's the highest Antarctic point, thus reducing to a minimum laser absorption. Incredibly enough Mount Kirkpatrick is quite dry, again, reducing laser absorption. Of course it would be a problem to generate the required energy for the lasers in the cold dark Antarctic, but then again we would have 6 months of sun to accumulate in batteries. Probably these batteries would need to be heated since batteries tend to malfunction in this extreme weather.
Even a crazier idea would be to lay a long cable to a nuclear submarine since there is a frozen bay right next to Mount Kirkpatrick, obtaining every bit of energy required for the lasers to work.
Maybe I’m talking non-sense and Alpha Centauri is north, and the answer would be an Artic based laser. From me nonscientific point of view is a win-win-win situation.

Aug 28, 2016 20:06 Nathan Bemis Posted on: Breakthrough Initiatives

A realistic idea for a dry high altitude location is perhaps the Nevada high desert 1200m(valleys)-3000m+(ridges) within the basin is Mt. Whitney in CA reaching 4400m. Elon Musk has his gigabattery factory located near Reno Nevada which could make for a convenient partnership in terms of nearby energy production and storage, battery research and development, cost savings, accessibility for people working on the project, easy access of resources to and from the location, and perhaps provide clear enough skies. Expected cost of land should be low, plots as large as desired, and it won't be an obstruction for cities/towns. This location should satisfy all the international security, supervisory, and regulatory conditions expected.
An idea to have the nanocraft return- Is to program the smart chip to activate an eventual "U-Turn" once it has completed its objective. If it can come back, we may be able to slow it for recovery with the same array we used to send it out. If this is possible, then opportunities for more advanced equipment could be used on these missions.
To minimize risk of space debris collision- Perhaps modify the chip into the form of a thin tube (like a dart/needle) or in the very least try to align the components to form into a smart stick rather than a square chip. When the craft has reached max speed and no longer receiving propulsion have the sails dispatch unless they provide further purpose.

Sep 15, 2016 10:45 Jacopo Maroli Posted on: Centauri Dreams

This is an amazing project and I wish to contribute to solving this challenge with my humble idea:

What about building a dyson swarm? https://en.m.wikipedia.org/wiki/Dyson_sphere#Dyson_swarm
We could place many collector-probes around our beloved star and re-directing the collected energy toward those which can "see" the exploration-probes.

This would have the advantage of not moving the beamer if you need to redirect the beam, because you'll just have to reconfigure which target every collector-probe should aim.
Anyway if really needed they could be moved using dynamic solar sails.

Another advantage is that we can start with a low number of collector-modules (let's say 8 to cover every angle) and increase them over time.

We can get very close to the sun to collect energy: the Solar Probe Plus should get at about 6,000,000 km in his perihelion.
https://en.wikipedia.org/wiki/Solar_Probe_Plus

We could also output the energy from multiple probes so we don't overcharge one in particular.
If I'm right every collector-sound in the half of the sun toward the exploration-probe should be able to "beam" it.

I'd really like to hear your comments about this idea and your opinions about feasibility.

Nov 05, 2016 03:17 Breakthrough Initiatives Posted on: Breakthrough Initiatives

RE:
"Aug 24, 2016 22:18Alberto RabellPosted on: Breakthrough Initiatives
Maybe shooting the laser for a longer period might help. Almost all ground based lasers wouldn't do the trick due to Earth’s rotation, but an Antarctic base would do it. I'm not an astronomer but a quick Google Sky Map search pointed out that Alpha Centauri is south, so I assume that Alpha Centauri would be high in this sky in the Antarctic. Been high in the sky would help reducing the atmosphere the laser would need to travel and it would help maintaining it focused for a longer time (during Antarctic winter). Mount Kirkpatrick (4,528 m) would be the best choice, since it's the highest Antarctic point, thus reducing to a minimum laser absorption. Incredibly enough Mount Kirkpatrick is quite dry, again, reducing laser absorption. Of course, it would be a problem to generate the required energy for the lasers in the cold dark Antarctic, but then again we would have 6 months of sun to accumulate in batteries. Probably these batteries would need to be heated since batteries tend to malfunction in this extreme weather. Even a crazier idea would be to lay a long cable to a nuclear submarine since there is a frozen bay right next to Mount Kirkpatrick, obtaining every bit of energy required for the lasers to work. Maybe I’m talking non-sense and Alpha Centauri is north, and the answer would be an Artic based laser. From me nonscientific point of view is a win-win-win situation."

Answer:
We are considering an Antarctic Base. We are considering a base at a place called Dome C which already has some infrastructure and there is a record of the atmosphere at this location. Alpha Centauri is 60 degrees south so it is high in the sky. It turns out that the wind in the circumpolar area is very stable and dry and one does not actually have to go to very high altitudes to achieve what we need. However, the cost multiplier to place anything on the Ice in Antarctica is prohibitive. Not as bad as space but very expensive place to build anything.

- Avi Loeb, Breakthrough Starshot

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