Is it time for space lasers yet? Almost.

As time passes, ideas that were once confined to the realm of science fiction become more realistic. It's true of things like using robots to explore other worlds. Space lasers are a well-used element in science fiction, and we're approaching the time when they could become a reality.

Where would we put them, and what could we use them for? In science fiction, lasers are predominantly used as powerful weapons. While some countries have investigated the idea of using lasers as space weapons, an international treaty limits their use.

A more realistic use for lasers is for deflecting incoming asteroids or as propulsion systems for spacecraft. In a new paper, a researcher examines where a giant laser array could be positioned in space to be of most use to humanity while at the same time minimizing risk.

The research is titled "Minimum Safe Distances for DE-STAR Space Lasers." The paper is available on the arXiv preprint server, and Adam Hibberd from the Initiative for Interstellar Studies in London, U.K., is the sole author.

While space lasers could also be used to utilize resources or in satellite laser ranging systems to control space traffic, Hibberd's focus is on using them to protect Earth from impacts.

DE-STAR stands for Directed Energy Systems for Targeting of Asteroids and exploRation. Of all the space laser ideas that have been discussed, DE-STAR is probably the most well-studied and developed. It would consist of a modular phased array of lasers powered by solar cells. It could heat the surface of potentially hazardous objects (PHO) to approximately 3,000 Kelvin. That's hot enough to melt all known constituents of PHOs. DE-STAR could also be used to propel spacecraft.

The idea originated in 2013 when a group of researchers published a paper titled "DE-STAR: Phased-Array Laser Technology for Planetary Defense and Other Scientific Purposes." In their paper, they outlined the idea for DE-STAR, a stand-off laser array.

In 2016, some of the same authors published another paper titled "Directed Energy Missions for Planetary Defense." It expanded on DE-STAR and added DE-STARLITE, a stand-on system that would be sent to the vicinity of an approaching object to ward it off with lasers.

In either case, the system would be based on the sun's energy. "DE-STAR is a square modular design which exploits the energy created by banks of solar cells in space to generate and amplify the power of a laser beam," Hibberd explains in his new paper.

In the literature, DE-STAR is typically referred to as DE-STAR n, where n is usually between 0 and 4 and denotes the size of the bank of lasers. The larger the array, the more powerful it is. The more powerful DE-STAR is, the more effective it will be at deflecting asteroids from greater distances.

While the merit of this idea is immediately clear, the problems follow soon after. A bank of powerful space lasers is every supervillain's dream. Its destructive power could be immense. "With a DE-STAR 4 structure (10 km × 10 km square) capable of generating a laser beam on the order of tens of gigawatts, clearly, there is the potential for such an asset to be deployed as a weapon by targeting locations on Earth," Hibberd writes.

How can this risk be mitigated so that the system can be used to protect Earth rather than as a weapon? The simple solution is to not deploy them in Earth's orbit. The lasers lose energy with range, so they could be deployed at distances where they pose no threat. "Results indicate that given they should lie 1 au from the sun, there are feasible locations for DE-STAR 0-2 arrays where there is no danger to Earth," Hibberd writes.

Of course, the more lasers there are in the array, the greater the safe minimum distance.

For DE-STAR 4 or even 5, that distance wouldn't be enough. Instead, these lasers would need to be much further away or at positions in the solar system with no direct line of sight to Earth. These systems would need to correct their positions regularly with an on-board propulsion system "or preferably using push-back from the laser itself," Hibberd explains.

The minimum safe distance also changes depending on the wavelength of the DE-STAR system. Hibberd defines minimum safe distance as a single laser with a maximum intensity on Earth's surface of 100 Wm^-2. "Or on the order 10 % of the Solar Constant at Earth (1 au from the sun)," Hibberd writes. For an infrared system, the minimum safe distance is just beyond geosynchronous Earth orbit (GEO). At the more powerful end of the scale, a UV laser would need to be beyond cis-lunar space.

There's another factor to consider. Since DE-STAR gets its energy from the sun, its power decreases the further away from the sun it is. "This reduction is a consequence of the decrease in solar flux intensity on the photovoltaic cells, where an inverse square law is followed," Hibberd explains.

For DE-STAR 1 and 2 Arrays, the minimum safe distances are not that great. Hibberd points out that for a DE-STAR 2 Array, sun/Earth Lagrange 4 and 5 points would be suitable and require no propulsion. L4 and L5 are about 400,000 km from Earth.

However, as the arrays become larger, the minimum safe distance quickly increases. Conversely, the available solar energy decreases.

A DE-STAR 3 would have to be placed somewhere beyond the asteroid belt. If it were ultraviolet, it would have to be beyond Jupiter.

A DE-STAR 4 phased array would have to be much further away. It would have to be about 30 ? 40 au away, and even further for an ultraviolet system, about 70 au from the sun.

However, there are locations where there is no direct line of sight to Earth, and they could be used as locations for powerful arrays. Hibberd explains that the Earth/Moon Lagrange 2 point and the sun/Earth Lagrange 3 point both lack direct lines of sight but, unfortunately, are unstable.

"In both cases, the instability of these points will result in the DE-STAR wandering away and potentially becoming visible from Earth, so an on-board propulsion would be needed to prevent this," Hibberd writes. It's possible that an array could be built that is physically prevented from pointing at Earth, but the author doesn't tackle that aspect of the problem.

Nobody's building a DE-STAR phased array, but that doesn't mean it's too soon to think about it. This type of technology is on the horizon, and it's difficult to predict which nation or nations might be the first to build one. Treaties are in place to prevent the weaponization of space, but not everybody signed them. Some nations are known to sign treaties and then break them, in any case. Also, an argument could be made that this isn't a weapon.

It likely won't be long before serious talk about such a system begins to surface in wider public discussions. That will surely generate a lot of political difficulty and wrangling as nations argue over what constitutes a weapon and what doesn't.

If civilization is to survive, we will eventually need a way to protect the entire globe from asteroid strikes, whether it's phased laser arrays or some other system.

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