Calculating what it would take to park a solar sail at Alpha Centauri

John Timmer
Breakthrough Starshot is an initiative that hopes to send miniature spaceships on a high-speed jaunt to the Centauri system sometime within the next few decades. Doing that requires the development of a variety of untried technology—and that's before we get to the issue of sending data back. Still, even if Breakthrough Starshot never gets any hardware beyond the Solar System, it's clearly getting people to think about what it would take to get a good look at our closest stellar neighbors.
In the latest example, a German researcher named René Heller has teamed with Michael Hippke, a self-proclaimed "gentleman scientist" with 10 peer-reviewed publications. Their goal: to see if we could not just get hardware to the Centauri system, but put it into orbit for long-term observations. The answer's yes, but the trip will not be short.
Breakthrough Starshot's design is a tiny micro-spacecraft attached to a solar sail. Solar sails don't actually need the Sun; they can accelerate by using light from any source. So, Breakthrough Starshot hopes to have an array of lasers give the sail and its spacecraft a boost up to 20 percent of the speed of light. That will get the spacecraft to Proxima Centauri, the closest star. Data will get back while most of the principals involved in the work are still alive.
But that speed also means that they won't have much time to find anything out about Proxima and its orbiting exoplanet. The spacecraft "would traverse a distance equivalent to the Moon’s orbit around the Earth in just about six seconds," the researchers note, "with little time left for high-quality close-up exploration and posing huge demands to the imaging system." So, Heller and Hippke decided to look into the possibility of slowing things back down. Their results appear in The Astrophysical Journal Letters.
Two features of their plan make slowing down a solar sail possible. The first is that solar sails work just as well (or just as poorly, really) to decelerate an object as they do to accelerate it. The other is that Proxima Centauri is close to two other stars, Alpha Centauri A and B. These other stars offer the prospect of slowing spacecraft down enough to get them to gently enter orbit at Proxima Centauri. This works when all three stars are roughly in the same plane from Earth's perspective, which happens about every 80 years (with the next one occurring in 2035).
So the authors took a model that was meant to handle gravitational interactions among multiple bodies and modified it to include the radiation pressure on a solar sail from the stars themselves. They then ran multiple simulations, shooting probes with different trajectories and speeds into the system to figure out what might work.
Some limitations on the trajectories were immediately apparent. Any paths that put the spacecraft closer than three times the star's radius "lead to a physical encounter of the sail with the star." As it's notably difficult to make observations from inside a star, these trajectories were ruled out. It's also possible to balance things out so that the hardware comes to a full stop and enters a circular orbit at the first star to which it is sent. Since that's not the star with a planet known to be orbiting it, these aren't especially useful either.
But two other results are possible. One is to have the probe enter a large elliptical orbit, which would allow it to reorient and use the star's light to accelerate onwards. Others slow the craft, but allow it to proceed onward to the next star in the system, if the approach trajectory is right. Chaining two of these approaches together allows the craft to approach Proxima Centauri slowly enough to enter orbit.
That's the good news. Nearly everything else is bad. Gravity is a minor contributor to the slowdown, about three orders of magnitude less than the influence of photons on the solar sail. That means that the deceleration process is heavily dependent upon everything those photons are pushing against: the combined mass of the sail and spacecraft, as well as the area of the sail.
To have enough force to handle a spacecraft payload of just 10 grams, the sail needs an area of 100,000 square meters—a square over 300 meters a side. And that sail would have to have a mass-to-area ratio only slightly heavier than that of graphene. Of course the sail can't be graphene, since graphene is notably transparent, and solar sails need to be reflective. That incredibly light material would also somehow have to survive coming close enough to the stars it passes that the electron temperature of the plasma it encounters is over 100,000K.
But other than that, the whole idea is fine.
Well, not entirely fine. The process won't work with something going as fast as the planned Breakthrough Starshot craft. If you send it toward the Centauri system any faster than 13,800 kilometers a second, interactions with the stars won't be able to slow the craft down. Which means it would take 100 years to travel between Earth and the Centauri system. You can also tack on another 50 years for maneuvers among the three stars before the craft is placed in orbit. So we're not looking at any sort of rapid return of data, assuming we can even build electronics that will operate 150 years into the future.
So, on some level, this idea's a non-starter. But it does have some good points. For example, we won't have to build a giant phased-laser system to accelerate the craft anymore. That's because the sail would be so big that it would work as an actual solar sail. Launch it close to the Sun, and it could exit the Solar System at over 11,500 kilometers a second. And, since the acceleration is far more gradual, the strain on the onboard electronics would be far less than in Breakthrough Starshot's plan.
Another advantage of Heller and Hippke's plan is that, once at Proxima Centauri, the sail would allow the craft to move around the system. If the spacecraft finds any other planets we haven't detected, it can visit them, too. The authors suggest that a sample return would even be possible, provided people would remember to look for the hardware 300 years from now.
Perhaps more realistically, the authors point out that some extremely bright stars aren't too distant from the Sun. With more photons to work with, a craft could be sent there with a much higher initial velocity and still decelerate enough to enter orbit.
It's easy to look at all the problems identified by the authors and wonder "why bother?" But the reality is that we wouldn't know for sure that these are problems if nobody did the calculations. And identifying the problems is the first step toward getting people to start thinking about workarounds or applications in which the same technology would be useful.
The Astrophysical Journal Letters, 2017. DOI: 10.3847/2041-8213/835/2/L32  (About DOIs).
Calculating what it would take to park a solar sail at Alpha Centauri Reviewed by Bizpodia on 15:41 Rating: 5

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