One of the great problems of lightsail concepts for interstellar flight is the need to decelerate. Here I’m using lightsail as opposed to ‘solar sail’ in the emerging consensus that a solar sail is one that reflects light from our star, and is thus usable within the Solar System out to about 5 AU, where we deal with the diminishment of photon pressure with distance. Or we could use the Sun with a close solar pass to sling a solar sail outbound on an interstellar trajectory, acknowledging that once our trajectory has been altered and cruise velocity obtained, we might as well stow the now useless sail. Perhaps we could use it for shielding in the interstellar medium or some such.

A lightsail in today’s parlance defines a sail that is assumed to work with a beamed power source, as with the laser array envisioned by Breakthrough Starshot. With such an array, whether on Earth or in space, we can forgo the perihelion pass and simply bring our beam to bear on the sail, reaching much higher velocities. Of the various materials suggested for sails in recent times, graphene and aerographite have emerged as prime candidates, both under discussion at the recent Montreal symposium of the Interstellar Research Group. And that problem of deceleration remains.

Is a flyby sufficient when the target is not a nearby planet but a distant star? We accepted flybys of the gas giants as part of the Voyager package because we had never seen these worlds close up, and were rewarded with images and data that were huge steps forward in our understanding of the local planetary environment. But an interstellar flyby is challenging because at the speeds we need to reach to make the crossing in a reasonable amount of time, we would blow through our destination system in a matter of hours, and past any planet of interest in perhaps a matter of minutes.

Robert Forward’s ingenious ‘staged’ lightsail got around the problem by using an Earth-based laser to illuminate one part of the now separated sail ring, beaming that energy back to the trailing part of the sail affixed to the payload and allowing it to decelerate. Similar contortions could divide the sail again to make it possible to establish a return trajectory to Earth once exploration of the distant stellar system was complete. We can also consider using magsail concepts to decelerate, or perhaps the incident light from a bright target star could allow sufficient energy to brake against.

Image: Forward’s lightsail separating at the beginning of its deceleration phase. Laser sailing may turn out to be the best way to the stars, provided we can work out the enormous technical challenges of managing the outbound beam. Or will we master fusion first? Credit: R.L. Forward.

But time is ever a factor, because you want to reach your target quickly, while at the same time, if you approach it too fast, you’re incapable of creating the needed deceleration. Moreover, what is your target? A bright star gives you options for deceleration if you approach at high velocity that are lacking from, say, a red dwarf star like Proxima Centauri, where the closest terrestrial-class world we know is in what appears to be a habitable zone orbit. In Montreal, René Heller (Max Planck Institute for Solar System Research), a familiar name in these pages, laid out the equations for a concept he has been developing for several years, a mission that could use not only the light of Proxima itself but from Centauri A and B to create a deceleration opportunity. You can follow Heller’s presentation at Montreal here.

Remember what we’re dealing with here. We have two stars in the central binary, Centauri A (G-class) and Centauri B (K-class), with the M-class dwarf Proxima Centauri about 13000 AU distant. Centauri A and B are close – their distance as they orbit around a common barycenter varies from 35.6 AU to 11.2 AU. These are distances in Solar System range, meaning that 35.6 AU is roughly the orbit of Neptune, while 11.2 AU is close to Saturn distance. Interesting visual effects in the skies of any planet there.

Image: Orbital plot of Proxima Centauri showing its position with respect to Alpha Centauri over the coming millennia (graduations are in thousands of years). The large number of background stars is due to the fact that Proxima Cen is located very close to the plane of the Milky Way. Proxima’s orbital relation to the central stars becomes profoundly important in the calculations Heller and team make here. Credit: P. Kervella (CNRS/U. of Chile/Observatoire de Paris/LESIA), ESO/Digitized Sky Survey 2, D. De Martin/M. Zamani.

Using a target star for deceleration by braking against incident photons has been studied extensively, especially in recent years by the Breakthrough Starship team, where the question of how its tiny sailcraft could slow from 20 percent of the speed of light to allow longer time at target is obviously significant. Deceleration into a bound orbit at Proxima would be, of course, ideal but it turns out to be impossible given the faint photon pressure Proxima can produce. Investing decades of research and 20 years of travel time is hardly efficient if time in the system is measured in minutes.

In fact, to use photon pressure from Proxima Centauri, whose luminosity is 0.0017 that of the Sun, would require approaching the star so slowly to decelerate into a bound orbit that the journey would take thousands of years. Hence Heller’s notion of using the combined photon pressure and gravitational influences of Centauri A and B to work deceleration through a carefully chosen trajectory. In other words, approach A, begin deceleration, move to B and repeat, then emerge on course outbound to Proxima, where you’re now slow enough to use its own photons to enter the system and stay.

Working with Michael Hippke (Max Planck Institute for Solar System Research, Göttingen) and Pierre Kervella (CNRS/Universidad de Chile), Heller has refined the maximum speed that can be achieved on the approach into Alpha Centauri A to make all this happen: 16900 kilometers per second. If we launch in 2035, we arrive at Centauri A in 2092, with arrival at Centauri B roughly six days later and, finally, arrival at Proxima Centauri for operations there in a further 46 years. That launch time is not arbitrary. Heller chose 2035 because he needs Centauri A and B to be in precise alignment to allow the gravitational and photon braking effects to work their magic.

So we have backed away from Starshot’s goal of 20 percent of lightspeed to a more sedate 5.6 percent, but with the advantage (if we are patient enough) of putting our payload into the Proxima Centauri system for operations there rather than simply flying through it at high velocity. We also get a glimpse of the systems at both Centauri A and B. I wrote about the original Heller and Hippke paper on this back in 2017 and followed that up with Proxima Mission: Fine-Tuning the Photogravitational Assist. I return to the concept now because Heller’s presentation contrasts nicely with the Helicity fusion work we looked at in the previous post. There, the need for fusion to fly large payloads and decelerate into a target was a key driver for work on an in-space fusion engine.

Interstellar studies works, though, through multiple channels, as it must. Pursuing fusion in a flight-capable package is obviously a worthy goal, but so is exploring the beamed energy option in all its manifestations. I note that Helicity cites a travel time to Proxima Centauri in the range of 117 years, which compares with Heller and company’s now fine-tuned transit into a bound orbit at Proxima of 121 years. The difference, of course, is that Helicity can envision launching a substantially larger payload.

Clearly the pressure is on fusion to deliver, if we can make that happen. But the fact that we have gone from interstellar flight times thought to involve thousands of years to a figure of just over a century in the past few decades of research is heartening. No one said this would be easy, but I think Robert Forward would revel in the thought that we’re driving the numbers down for a variety of intriguing propulsion options.

The paper René Heller drew from in the Montreal presentation is Heller, Hippke & Kervella, “Optimized Trajectories to the Nearest Stars Using Lightweight High-velocity Photon Sails,” Astronomical Journal Vol. 154 No. 3 (29 August 2017), 115. Full text.