Proxima Centauri: The Problem of Arrival

Given his key role in the development of sail ideas for interstellar flight, Robert Forward inevitably comes up in any discussion of deep space missions. The late physicist put forward a number of sail concepts and mission ideas, including a laser-driven lightsail to Epsilon Eridani with return capability that would travel at 50 percent of the speed of light. Those were numbers that made a manned mission theoretically possible, though demanding a huge sail (1000 kilometers in diameter) and a mind-bending space-based 75,000 TW laser system.

Yesterday we looked at the critical problem of deceleration in a sail-based interstellar mission, with reference to the new paper by René Heller and Michael Hippke. I only wish Forward were here to give us his thoughts on the newly proposed ‘photogravitational assist’ method of deceleration, because for years his own method for the Epsilon Eridani mission — a ‘staged’ sail that separates, so that one sail ring reflects laser light back onto another — has been the only method I’ve seen for slowing a sailcraft down for orbital insertion at another star.

Image: Forward’s separable sail concept used for deceleration, from his paper “Roundtrip Interstellar Travel Using Laser-Pushed Lightsails,” Journal of Spacecraft and Rockets 21 (1984), pp. 187-195. In the paragraph above, I didn’t even mention the ‘paralens,’ a huge Fresnel lens made of concentric rings of lightweight, transparent material, with free space between the rings and spars to hold the vast structure together, all of this located between the orbits of Saturn and Uranus. The structure would be used to collimate the laser beam.

To my knowledge, Forward never considered the possibility of using stellar photon pressure combined with gravity assists as a means of deceleration. The method wouldn’t have occurred to him in relation to Epsilon Eridani in any case. For one thing, moving at 50 percent of c, his sailcraft would be unable to achieve the needed braking from the method, and for another, Epsilon Eridani, a single star, is the wrong kind of target for this type of maneuver. As Heller and Hippke explain, a multiple star system is the destination of choice.

This quote from the paper gets the point across. In the passage, L ☉ refers to stellar luminosity:

In multi-stellar systems, successive fly-bys at the system members can leverage the additive nature of photogravitational assists. For multiple assists to work, however, the stars need to be aligned within a few tens of degrees along the incoming sail trajectory of the sail. Such a successive braking is particularly interesting for multi-stellar system, where bright stars can be used as photon bumpers to decelerate the sail into an orbit around a low-luminosity star, such as Proxima (0.0017 L ☉ ) in the α Cen system or the white dwarf Sirius B (0.056 L ☉ ) around Sirius A.

Sirius A? Indeed. For the paper notes that other nearby stars offer more favorable conditions even than the Alpha Centauri triple system for decelerating an incoming lightsail. Sirius A is about twice the distance from the Sun as Alpha Centauri but offers an extremely bright target (25 L ☉ ) for deceleration, making the maximum injection speed into the system almost 15 percent of lightspeed. It would take something other than a solar photon sail to get the initial payload up to cruise speed for such a journey, but deceleration upon arrival is possible.

We need to learn everything we can about deceleration given the advantages of a sail that operates for years in a bound orbit within a stellar system (and even around a target planet like Proxima b) vs. a flyby mission. Early probes to nearby stars might well be flyby missions, particularly if we build the Breakthrough Starshot infrastructure, which would also be useful here in our own Solar System. But detailed follow-ups could come through decelerating lightsails in those destinations most suited for such methods. Fortunately, the nearest stars to our own form one such system.

I refer you back to yesterday’s post if you’re just coming into the discussion, but the brief summary is that the combination of the gravitational pulls of Centauri A and B along with their photon pressures is what makes deceleration of Heller and Hippke’s 316-meter sail possible. Centauri A is thus the first target, with the flyby there being manipulated through autonomous onboard technologies to maximize the braking effect before sending the sail on to Centauri B.

With the help of Centauri B, we slow from 4.6% of c to about 1280 kilometers per second, the figure that Heller and Hippke have determined would allow entry into a bound orbit around Proxima Centauri. A flight time of 46 years to Proxima ensues. At the destination, the resulting highly elliptical orbit is then circularized over time using photon pressure; we wind up with a functioning, data-returning probe in the star’s habitable zone. This obviously demands extreme and precise maneuvering but needs no onboard fuel.

Image: Artist’s concept of Proxima b orbiting Proxima Centauri. (Image: ESO./L. Calçada/Nick Resigner).

Navigation during the critical period of the photogravitational assists demands careful attention. The paper argues that multiple spacecraft may be one way to handle this. In the passage below, r min refers to the sail’s minimum distance from the star:

Regarding the nautical issues of an A-B-C trajectory, communication among sails within a fleet could support their navigation during stellar approach, as it will be challenging for an individual sail to perform parallel observations of both the approaching star and its subsequent target star or of other background stars. Course corrections will need to be calculated live on board. In particular, the location of r min will need to be determined on-the-fly as it will depend on the actual velocity and approach trajectory and, hence, on the local stellar radiation pressure and magnetic fields (Reiners & Basri 2008) along this trajectory.

I mentioned yesterday the question of what any beings on Proxima b might see if a sail like this one were headed for them. In a Frequently Asked Questions document timed for release with the paper, the authors point out that the sail would indeed be observable, appearing as a new star in Proxima b’s skies that would have the same electromagnetic spectrum as Proxima Centauri itself, although blue-shifted. There’s also this:

…any time variability of their host star’s spectrum would be delayed in that star — initially by years, later only by months, weeks, and finally just days or seconds. This new star would also become brighter as the sail approaches Proxima b, and the blue-shift would decrease until, upon the sail’s arrival at Proxima b, the blueshift would disappear and the time delay would be very short, e.g. seconds only. At some point, when the sail would reorient itself into an oblique angle to transfer into an orbit at Proxima b, this fake star would suddenly disappear for an observer on Proxima b. As the sail would orbit the planet over the next months or years, it could occasionally reappear for just a few seconds as a very bright star-like dot in the sky. In principle, if these potential inhabitants of Proxima b were able to identify the sail as being artificial, they might conceive of a way to deliberately betray their presence to the cameras aboard the sail.

Interesting fodder for science fiction! I can recall the incoming lightsail seen by characters in Niven and Pournelle’s The Mote in God’s Eye (Simon & Schuster 1993), but I’m hard pressed to think of other science fictional treatments of this scenario. Perhaps the readers can help me out. Meanwhile, have a look at the Heller and Hippke paper, whose methods offer serious hope for solving the critical question of slowing down at another star.

The paper is Heller, R., & Hippke, M. (2017), “Deceleration of high-velocity interstellar photon sails into bound orbits at α Centauri,” The Astrophysical Journal Letters, Volume 835, L32, DOI:10.3847/2041-8213/835/2/L32 (preprint).