Decelerating at Alpha Centauri

As we await results from ongoing observations of the Alpha Centauri stars, let’s summarize for a moment what we currently know. While the subject is still up for debate, a number of studies have suggested that terrestrial planets can form around either Centauri A or B, with planetary systems extending as far out as 2.5 AU. And while planets have been discovered in binary systems not dissimilar to the Centauri stars, current estimates are that Centauri B has the greater chance of having a planet within the habitable zone. A warm blue and green world with oceans and continents, not so different from Earth, perhaps, could yet be found around Centauri B.

Supposing this scenario is proven correct, Greg Matloff (CUNY) has gone to work on how we might use Centauri A, even if it turns out to be without planets, to help us explore Centauri B. He’s thinking, of course, in terms of solar sails and the need to decelerate upon arrival in the destination system. Centauri A, a G2V star, is larger and brighter than Centauri B, a K1V. And as Matloff notes in a paper authored for the International Astronautical Congress in Daejeon, Korea this past October, the high luminosity of Centauri A improves solar sail deceleration for a future interstellar mission.

This leads to some interesting scenarios. Centauri A is better at decelerating a solar sail starship than the Sun would be at accelerating it in the first place. Suppose, then, we start thinking in terms of getting the most out of both stars. From the paper:

One possibility is a two-stage starship. A solar sail could first be used to accelerate a starship leaving the solar system. Since α Centauri A could decelerate a faster craft, a second stage (fusion pulse, beamed energy or antimatter) could be used after the sail has concluded solar acceleration.

Matloff is clearly thinking here about an initial acceleration based on solar photons alone, in which case the sail has done most of its work by the time it has passed, roughly, the orbit of Jupiter. We could extend the idea even further by coupling beamed sail concepts with the two-stage approach, using laser or microwave beaming to provide enhanced acceleration for a much longer period before a second-stage using any of the technologies Matloff mentions kicks in.

A second prospect conjures up a well-known science fiction novel:

Another approach is to utilize both the α Centauri suns to decelerate a solar-sail starship. The spacecraft would first approach one of these stars, decelerate and perform a gravity-assist maneuver to approach the second star to complete the deceleration process. This is the reverse of the acceleration maneuver of the fictional residents of the α Centauri system described by Apollo 11 astronaut Buzz Aldrin and John Barnes in their novel Encounter with Tiber (Warner, NY, 1996).

Matloff examines several scenarios that put these ideas in context, including the case of a spacecraft that uses a solar sail at Centauri A to decelerate to rest relative to the star. Here he looks at various values for sail reflectivity and areal mass thickness, showing the maximum velocity (i.e., the initial spacecraft velocity at the start of deceleration) possible for each of these conditions to bring the sail safely to rest at 0.066 AU from the star. Even in the best case scenario, we are talking about velocities less than .004c, or roughly 1150 kilometers per second. Quite a step up from current technologies (such as New Horizon’s current heliocentric velocity of 16.49 kilometers per second), but a long 1100 year haul to Centauri space.

The paper is Matloff, “Solar Photon Sail Deceleration at Alpha Centauri A.” Many thanks to Dr. Matloff for passing along a copy of this paper.