Decelerating at Alpha Centauri

by Paul Gilster on December 29, 2009

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.

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John Hunt December 29, 2009 at 13:44

> …but a long 1100 year haul to Centauri space.

Because of the later/faster problem, it’s too long a haul if it is a probe and we are wanting science return. In that case, travel time should be less than 250 years or so. If we are sending living adult colonists as insurance to ensure the survival of humanity, then 1,100 years makes guaranteeing life support a real problem. But if we are sending frozen embryos then 1,100 years is pretty good.

Could this be combined with magnetic sailing? We could launch when the solar wind is gusting up to 800 km/sec.

Often we think about the use of superconducting magnetic loops to decelerate against the interstellar medium. But, using both stars of the Alpha Centauri system to decelerate provides another valid option.

Even if there aren’t rocky planets around any of the A.C. stars, would we none-the-less presume that there are asteroids there? If so, then there are resources from which a rotating, space-based habitat could be constructed. Perhaps we could limit the distance of our first target for colonization to 4.3 light-years regardless of whether there is a blue-green planet with an atmosphere there.

Carl December 29, 2009 at 13:45

An informal set of calculations I performed about a year ago gave me an idea of the scale faced here. By converting light-hours to British units, a rough model was laid on a warehouse floor. A piece of tape with two dots represented the sun and Pluto, with five light-hours separation represented as .06″ or 1/16th inch. 37 feet from this pair is our first (red) dot, Proxima Centauri. This was figured from 4.22 ly = 36992.52 light hours. The pair AB was 38.3 feet from our sun, or 38′,4″. Setting down the tapes with the dots, and walking slowly between the systems they represent, is a humbling exercise. What a gulf!

nate December 31, 2009 at 2:45

what about He3 fusion? how much fuel would be required for a trip at constant acceleration / deceleration? solar sails sound so primitive when compared to nuclear / antimatter technology. kinda like asking the navy to trade nuclear powered ships for sail boats.

Adam January 1, 2010 at 6:07

Hi All

Solar-sails have a certain elegance, so that the long trip times are almost forgiveable. A two-stage mixed system does have a lot of appeal – if we could photo-levitate a large mirror-focus system to boost the sail for longer, then the first stage boost can be up to ~5% of c. A fusion rocket after that can bring up the cruising speed to the usual 10% c. Decceleration is more problematic, but perhaps a magnetic-sail can get us down to ~1%, then a solar-sail brake to in-system speeds.

Sounds all very complicated, but then interstellar travel isn’t an easy endeavour undertaken lightly.

AlfaCentavra January 2, 2010 at 10:27

For deceleration the electric sail can be used .

http://en.wikipedia.org/wiki/Electric_sail

Jitindra Khan June 5, 2010 at 10:25

You can use the reverse of how you accelerate from this system (outbound, on way out from our Sol system)… that’s if there are planets there in Alpha Centauri multiple stars. A good example is there on the Centauri ark page:

http://firstarktoalphacentauri.wikia.com/wiki/Centauri_Princess

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