Power beaming to accelerate a ‘lightsail’ has been pondered since the days when Robert Forward became intrigued with nascent laser technologies. The Breakthrough Starshot concept has been to use a laser array to drive a fleet of tiny payloads to a nearby star, most likely Proxima Centauri. It’s significant that a crucial early decision was to place the laser array that would drive such craft on the Earth’s surface rather than in space. You would think that a space-based installation would have powerful advantages, but two immediate issues drove the choice, the first being political.

The politics of laser beaming can be complicated. I’m reminded of the obligations involved in what is known as the Treaty on Principles Governing the Activities of States in the Exploration and Use of Outer Space, including the Moon and Other Celestial Bodies (let’s just call it the Outer Space Treaty), spurred by a paper from Adam Hibberd that has just popped up on arXiv. The treaty, which comes out of the United Nations Office for Space Affairs, emerged decades ago and has 115 signatories globally.

Here’s the bit relevant for today’s discussion, as quoted by Hibberd (Institute for Interstellar Studies, London):

States Parties to the Treaty undertake not to place in orbit around the earth any objects carrying nuclear weapons or any other kinds of weapons of mass destruction, install such weapons on celestial bodies, or station such weapons in outer space in any other manner. The moon and other celestial bodies shall be used by all States Parties to the Treaty exclusively for peaceful purposes. The establishment of military bases, installations and fortifications, the testing of any type of weapons and the conduct of military manoeuvres on celestial bodies shall be forbidden. The use of military personnel for scientific research or for any other peaceful purposes shall not be prohibited. The use of any equipment or facility necessary for peaceful exploration of the moon and other celestial bodies shall also not be prohibited.

So we’re ruling out weaponry in orbit or elsewhere in space. Would that prohibit building an enormous laser array designed for space exploration? Hibberd believes a space laser would be permitted if its intention were for space exploration or planetary defense, but you can see the problem: Power beaming at this magnitude can clearly be converted into a weapon in the wrong hands. And what a weapon. A 10 km X 10 km installation as considered in Philip Lubin’s DE-STAR 4 concept generates 70 GW beams. You can do a lot with that beyond pushing a craft to deep space or taking an Earth-threatening asteroid apart.

Build the array on Earth and the political entanglements do not vanish but perhaps become manageable as attention shifts to how to avoid accidentally hitting commercial airliners and the like, including the effects on wildlife and the environment.

Image: Pushing a lightsail with beamed energy is a feasible concept capable of being scaled for a wide variety of missions. But where do we put the beamer? Credit: Philip Lubin / UC-Santa Barbara.

The second factor in the early Starshot discussions was time. Although now slowed down as its team looks at near-term applications for the technologies thus far examined, Starshot was initially ramping up for a deployment by mid-century. That’s pretty ambitious, and we wouldn’t have a space option that could develop the beamer if that stretchiest-of-all-stretch goals actually became a prerequisite.

So if we ease the schedule and assume we have the rest of the century or more to play with, we can again examine laser facilities off-planet. Moreover, Starshot is just one beamer concept, and we can back away from its specifics to consider an overall laser infrastructure. Hibberd’s choice is the DE-STAR framework (Directed Energy Systems for Targeting of Asteroids and Exploration) developed by Philip Lubin at UC-Santa Barbara and first described in a 2012 on planetary defense. The concept has appeared in numerous papers since, especially 2016’s “A Roadmap to Interstellar Flight.”

If the development of these ideas intrigues you, let me recommend Jim Benford’s A Photon Beam Propulsion Timeline, published here in 2016, as well as Philip Lubin’s DE-STAR and Breakthrough Starshot: A Short History, also from these pages.

What Hibberd is about in his new paper is to work out how far away various categories of laser systems would have to be to ensure the safety of our planet. This leads to a sequence of calculations defining different safe distances depending on the size of the installation. The DE-STAR concept is modular, a square phased array of lasers where each upgrade indicates a power of base 10 expansion to the array in meters. In other words, while DE-STAR 0 is 1 meter to the side, DE-STAR 1 goes to 10 meters to the side, and so on. Here’s the chart Hibberd presents for the system (Table 1 in his paper).

Keep scaling up and you achieve arrays of stupendous size, and in fact an early news release from UC-Santa Barbara described a DE-STAR 6 as a propulsion system for a 10-ton interstellar craft. It’s hard to imagine the 1,000 kilometer array this would involve, although I’m sure Robert Forward would have enjoyed the idea.

So taking Lubin’s DE-STAR as the conceptual model (and sticking with the more achievable lower end of the DE-STAR scale), how can we lower the risks of this kind of array being used as a weapon? And that translates into: Where can we put an array so that even its largest iterations are too far from Earth to cause concern?

Hibberd’s calculations involve determining the minimum level of flux generated by an individual 1 meter aperture laser element (this is DE-STAR 0) – “the unphased flux of any DE-STAR n laser system” – and using as the theoretical minimum safe distance from Earth a value on the order of 10 percent of the solar constant at Earth, meaning the average electromagnetic radiation per unit area received at the surface. The solar constant value is 1361 watts per square meter (W/m²); Hibberd pares it down to a maximum allowed flux of 100 W/m² and proceeds accordingly.

Now the problems of a space-based installation become strikingly apparent, for the calculations show that DE-STAR 1 (10 m X 10 m) would need to be positioned outside cis-lunar space to ensure these standards, and even further away (beyond the Earth-Moon Lagrange 2 point) for ultraviolet wavelengths (λ ≲ 350nm). That takes us out 450,000 kilometers from Earth. However, a position at the Sun-Earth L2 Lagrange location would be safe for a DE-STAR 1 array.

The numbers add up, and we have to take account of stability. The Sun/Earth Lagrange 4 and 5 points would allow a DE-STAR 2 laser installation to remain at a fixed location without on-board propulsion. DE-STAR 3 would have to be positioned beyond the asteroid belt, or even beyond Jupiter if we take ultraviolet wavelengths into account. The enormous DE-STAR 4 level array would need to be placed as far as 70 AU away.

All this assumes we are working with an array on direct line of sight with the Earth, but this does not have to be the case. Let me quote Hibberd on this, as it’s rather interesting:

Two such locations are the Earth/Moon Lagrange 2 point (on a line from the Earth to the Moon, extending beyond the Moon by ∼ 61, 000 km) and the Sun/Earth Lagrange 3 point (at 1 au from the Sun and diametrically opposite the Earth as it orbits the Sun). 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. One solution would be to use the push-back from the lasers to provide a means of corrective propulsion. However it would appear a DE-STAR’s placement at either of these points is not an entirely satisfactory solution to the problem.

So we can operate with on-board propulsion to achieve no direct line-of-sight to Earth, but the orbital instabilities involved make this problematic. Achieving the goal of a maximum safe flux at Earth isn’t easy, and we’re forced to place even DE-STAR 2 arrays at least 1 AU from the Sun at the Sun/Earth Lagrange 4 or 5 positions to achieve stable orbits. DE-STAR 3 demands movement beyond the asteroid belt at a minimum. DE-STAR levels beyond this will require new strategies for safety.

Back to the original surmise. Even if we had the technology to build a DE-STAR array in space in the near future, safety constraints dictate that it be placed at large distances from the Earth, making it necessary to have first developed an infrastructure within the Solar System that could support a project like this. As opposed to one-off missions from Earth launching before such an infrastructure is in place, we’ll need to have the ability to move freely at distances that ensure safety, unless other means of planetary protection can be ensured. Hibberd doesn’t speculate as to what these might be, but somewhere down the line we’re going to need solutions for this conundrum.

The paper is Hibberd, “Minimum Safe Distances for DE-STAR Space Lasers,” available as a preprint. Philip Lubin’s “A Roadmap to Interstellar Flight” appeared in Journal of the British Interplanetary Society 69, 40-72 (2016). Full text.