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Survival of the fittest - follow up #3

Some time ago we introduced to you one of the problems that we face during our mission to the moon. How is it possible to keep a rover warm enough in the ice-cold lunar night, so that the damage due to the cold is kept to a minimum? You have sent us a number of interesting ideas and we promised to analyze and evaluate them.

We already discussed the idea of digging a trench the idea of digging a trench in order to reduce heat loss by radiation. Unfortunately, we saw that there are a number of unsolved problems with this idea.


"Focus a laser from earth to charge the rover battery remotely"


Suggested Solution

Powering a distant object with a laser has been achieved for small devices on earth, such as a remote-controlled helicopter. So why not use a laser on earth to charge the rover battery remotely?


Analysis

This idea seems to be one of those spectacular contraptions likely found in a James Bond movie. When looking at it more closely, one is faced with a number of tough questions.

The first question one might ask is, what is the dissipation of the laser light due to Earth's atmosphere?

Actually, that is not a very large effect. It amounts to less than a 50% energy loss, which could be tolerated. However, there are other effects that diminish the laser's power over distance. Because it would have to be very strong, the laser would also „remove“ clouds in its way, so that they wouldn't be much of an issue. It sure would be interesting to see a laser cutting through a cloud, though.

But there are other problems with the atmosphere. It's not possible to focus the laser to an accuracy better than ~2km. The reason for this is the same that also limits the resolution of earthbound telescopes; the atmosphere causes an effect called “seeing”, which broadens the beam and thus defocuses it. This is caused by turbulence in the air. In addition to that, a laser is a divergent light source, which means that its beams are not precisely parallel to begin with.

Due to this effect the energy of the laser can't be concentrated into a small point, but is spread over quite a large area. Assuming an average earth-moon distance of 380 000 km, a defocus of only one tenth of a degree turns the „point“ of the laser into an area with a diameter of 670 km. Even if the defocus was only 0.00028° (corresponding to one second of angle, the smallest aberration possible with any telescope on earth), that diameter would still be 1.89 km, spreading the beam over an area of 5.62 km². If the photovoltaic panel on the rover (the „solar cell“ responsible for collecting the laser energy and turning it into electricity) is to output a power of 100 W and has an efficiency of 30% on an area of 60x60 cm, the incident power would have to be 1400 W per m². This is roughly the same amount of energy as the moon receives from the sun. The laser would have to have a total power of 7,863,800,000 watts across the entire area, i.e. 7.86 GW. This doesn't even include atmospheric losses yet. It also depends on whether you want to use a pulsed laser, or a continuous source.

Northrop Grumman claims to have built a 100 kW CO2 laser for military applications. And that thing is already quite huge. Some other lasers have gigawatt power, but only operate in a pulsed mode. That leaves the question how efficient such a laser would be and if it can be built at all. Since the optical components of the laser have to be as good as those of the best telescopes in the world, and we need a continuous energy transfer, we have to realize that such a laser might potentially be constructed but the budget would be much greater than that of the rover, lander and rocket together.

Since it seems rather difficult to shine that laser from the surface of the earth, one might ask whether it's possible to use a lunar satellite instead.

In theory, such an approach would indeed be possible. However, such a satellite would circle the moon at a velocity of 1656 m/s at a distance of 50 km from the lunar surface. In this situation we would have a five-minute window every two hours during which to charge the rover's battery. All of these issues make this idea rather impractical.

Since it seems rather difficult to shine that laser from the surface of the earth, one might ask whether it's possible to use a lunar satellite instead.

Such a system would be very difficult to construct, especially on the earth. But it is feasible. However, when moving the laser beam around to point it at our rover, there might be some other satellites in the way that would get grilled accidentally. Well, in the same way such a device could remove space junk, which is actually a nice side effect. There are some concepts to supply energy to space elevators using lasers. For such cases this technology is actually more promising.



There is another problem: Such a laser is actually a super-weapon, and there is some understandable political resistance to that. If you aimed the laser in the wrong direction and „accidentally“ hit a satellite, you'd most probably destroy it.

Such a laser would also consume lots and lots of electricity if you have to operate it continuously for 14 days. And the rover would be confined to stay on the side of the moon facing Earth. Furthermore, since there is no spot on earth that has an uninterrupted line of sight to the moon for an entire month, several such lasers would have to be built.



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Image Credits:
  • "Laser in a test environment" © by U.S. Air Force - Public Domain
  • "Laser Ranging Facility" © by NASA - Public Domain

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