For all their attraction as a way to leave weighty propellant behind, solar sails have a fundamental limitation. Their power source is the Sun. As you move away from the Sun, the amount of available light drops according to the inverse square law — a spacecraft that doubles its distance from the Sun encounters only a fourth of the sunlight previously available. Quadruple the distance and the sunlight drops to a sixteenth of what it was, making sail operations problematic in the outer Solar System.
And what of the stars? Solar sail specialist Greg Matloff has been juggling the numbers on interstellar travel via solar sail for decades now, and even with the best case scenario involving an extremely close solar pass, a thousand years to Centauri is about as good as it gets. And that’s quite a stretch in itself. Epsilon Eridani would actually make an easier mission as it’s much closer to the ecliptic, so you get 30 kilometers per second (Earth’s orbital velocity around the Sun) from scratch as you begin your solar approach. On the other hand, the resultant velocity (200 AU per year in one mission concept) takes 3500 years to reach Epsilon Eridani.
Laser propulsion is one way around the solar sail limitation, and as Matloff, along with co-authors Les Johnson and Giovanni Vulpetti, discuss in their new solar sail book, the method must be finely tuned for success. Earth-based lasers won’t do because of attenuation from Earth’s atmosphere, diminishing the beam’s intensity, and also causing it to diverge much more quickly. The result: Deeply compromised thrust on the sail. Earth’s rotation is also a major problem, making it impossible to keep the beam on the receding sail for extended periods.
Robert Forward pondered power stations in the inner Solar System to solve this problem, with the laser beam focused by a huge lens in the outer system for maximum effect. It’s interesting to see how that idea — created for its interstellar possibilities — has developed over the years. What Matloff, Johnson and Vulpetti talk about is a space-based laser in orbit around Jupiter. The orbital rotation problem is greatly eased because Jupiter orbits only once in twelve years, allowing ample time for beam adjustment and calibration. Not only that, but use a polar orbit and you can keep the sail under beam for a decade at a time.
And here’s where things get truly ingenious. Powering up that big laser could be handled by a tether, an idea dear to Robert Forward’s heart (he built an entire company, called Tethers Unlimited, around the concept). A long conducting wire deployed deep into Jupiter’s magnetosphere would generate a huge electrical flow. As the authors note, this is the same principle that is at work when an electrical generator produces electricity in a power plant. Wires moving through intense magnetic fields produce electricity, and Jupiter’s magnetic field is the second most powerful in the Solar System, second only to that of the Sun.
Forward envisioned the use of tethers in a much different way. Properly positioned, they could adjust spacecraft orbits and fling payloads around the Solar System without the need for rockets. But he would have loved the idea of using tethers for power generation around Jupiter, meeting the laser’s formidable needs. That would enable a beamed propulsion scenario capable of getting us into nearby interstellar space and shortening those lengthy travel times to Centauri and elsewhere.
From the book:
This is by no means the only scenario in which lasers might be used to push our sails. But it is certainly a likely one. A mission might proceed something like this: A sailcraft departs from Earth on a sunward bound trajectory. The craft falls toward the Sun and orients its sail to maximize solar thrust at perihelion, giving it an incredible boost toward the outer solar system. Sunlight continues to push on the sail until it reaches the orbit of Jupiter, at which point our tether-driven laser sends a beam of light to reflect from the sail, picking up from where the now-feeble sunlight leaves off. The laser maintains its aim point on the sail, providing continuous additional thrust, until the diffraction limit of the laser results in no net thrust being applied to the sail — somewhere in deep space…
There are, of course, alternatives to lasers when it comes to beamed propulsion. We’ll be talking soon with James Benford, the leading specialist on microwave beaming (I hope to have that interview up within the next couple of weeks), and particle beaming options using futuristic versions of a nuclear accelerator are also well worth considering. The point here is the flexibility of the sail itself. It’s a spacecraft concept that offers abundant applications right here in the Solar System, while holding out the promise of future adaptations that may well propel our first targeted star mission to its destination.