Breakthrough Starshot is an initiative that hopes to send miniature starships on a fast journey to the Centauri system sometime in the next few decades. To do that will require the development of a variety of untried technology – and that’s before we get to the issue of data return. But even if Breakthrough Starshot never gets hardware outside the solar system, it’s clearly getting people thinking about what it takes to get a good look at our closest stellar neighbors.
In the last example, a German researcher named René Heller collaborates 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 is yes, but the journey will not be short.
Breakthrough Starshot’s design is a small micro spacecraft attached to a solar sail. Shade sails don’t actually need the sun; they can speed up by using light from any source. So, Breakthrough Starshot hopes to have an array of lasers that boost the sail and its spacecraft to 20 percent the speed of light. That takes the spacecraft to Proxima Centauri, the closest star. Data comes back while most of the principals involved in the work are still alive.
But that speed also means they won’t have much time to learn about Proxima and its exoplanet. The spacecraft “would cover a distance equivalent to the Moon’s orbit around the Earth in about six seconds,” the researchers note, “with little time left for high-quality close-up reconnaissance and placing enormous demands on the imaging system. ” So Heller and Hippke decided to explore the possibility of slowing things down again. Their results appear in The Astrophysical Journal Letters.
Two features of their plan allow slowing down a solar sail. The first is that solar sails work just as well (or, really, just as badly) at slowing down an object as they do at speeding it up. The other is that Proxima Centauri is close to two other stars, Alpha Centauri A and B. These other stars hold the prospect of slowing spacecraft enough to gently get them into orbit around 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 in 2035).
So the authors took a model intended to handle gravitational interactions between multiple bodies and modified it to include the radiation pressure on a solar sail from the stars themselves. They then ran multiple simulations, firing probes into the system at different trajectories and speeds to figure out what might work.
Some limitations on the routes were immediately apparent. Any paths that take the spacecraft closer than three times the star’s radius “lead to a physical encounter of the sail with the star.” Because it is extremely difficult to make observations from a star, these trajectories were excluded. It is also possible to balance things out so that the hardware comes to a complete stop and enters a circular orbit at the first star it is sent to. Since that’s not the star known to have a planet orbiting it, these aren’t particularly useful either.
But two other outcomes are possible. One is to put the probe into a large elliptical orbit, which would allow it to reorient itself and use the star’s light to accelerate further. Others slow the craft, but allow it to continue to the next star in the system, if the approach path is good. Linking two of these approaches together will allow the craft to approach Proxima Centauri slowly enough to enter orbit.
That’s the good news. Almost everything else is bad. Gravity makes a small contribution to the deceleration, about three orders of magnitude less than the influence of photons on the solar sail. That means the deceleration process is highly dependent on whatever those photons push against: the combined mass of the sail and spacecraft, as well as the sail’s surface area.
To have enough power to carry a spacecraft weighing just 10 grams, the sail would need an area of 100,000 square meters – a square of more than 300 meters per side. And that sail should have a mass-to-surface area ratio only slightly heavier than that of graphene. Of course, the sail cannot be made of graphene, since graphene is highly transparent and shade sails must be reflective. That incredibly light material should also somehow survive getting so close to the stars it passes that the electron temperature of the plasma it encounters exceeds 100,000 K.
But otherwise the whole idea is fine.
Well, not whole fine. The process won’t work with anything as fast as the planned Breakthrough Starshot craft. If you send it to the Centauri system faster than 8,000 miles per 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 another 50 years for maneuvers between the three stars before the craft is launched into orbit. So we’re not looking at a rapid return of data, assuming we can even build electronics that will work 150 years into the future.
So on some level this idea is a non-starter. But it does have some good points. For example, we no longer need to build a giant phased laser system to accelerate the craft. That’s because the sail would be so big that it would act like a real shade sail. Launch it close to the sun and it can leave the solar system at a speed of more than 11,500 kilometers per second. And since the acceleration is much more gradual, the load on the onboard electronics would be much 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 other planets we haven’t detected, it could visit those as well. The authors suggest that a sample return might even be possible, provided people remembered to look for the hardware 300 years from now.
Perhaps more realistically, the authors point out that some extremely bright stars are not too far from the sun. With more photons to work with, a craft could be sent there at a much higher initial velocity and still slow down enough to enter orbit.
It’s easy to look at all the issues 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 in getting people thinking about workarounds or applications where the same technology could be useful.
The Astrophysical Journal Letters2017. DOI: 10.3847/2041-8213/835/2/L32 (About DOIs).