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The "Big One of 1972"

DESTINATION: ALPHA CENTAURI
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Image graciously provided by Todd Henry
Director, RECONS- Research Consortium on Nearby Stars
Professor of Astronomy Georgia State University, Atlanta, GA

WHY GO TO ALPHA CENTAURI?: Alpha Centauri is 4.35 light years away, this would make it about 4.35 years travel time if the vessel were traveling at the speed of light.  You will find a table of travel times at differing POTSOL, to cover one parsec or 3.26 light years, on the section where Dr. Hasslein answers your questions.  Alpha Centauri is about 1.33 parsecs away, multiply all times listed on the table by this.

Alpha Centauri lies only 4.35 light-years from our Sun, but it is in reality a triple star system. The two brightest components of the trinary star system are referred to as Alpha Centauri "A" and Alpha Centauri "B".  These two stars form a binary union within the group as they orbit each other, an orbit which takes 80 full years to complete and has a mean separation of 23 astronomical units (1 astronomical unit = 1 AU = the distance that we measure as existing between the Sun and the Earth) between the two stars.  The third star of this trinary system is labeled Alpha Centauri C.  "C" lies 13,000 AU from both A and B (a distance roughly equivalent to 400 times the distance between our Sun and the eighth planet in our system, Neptune).  The distance between "A" and "B" and their relationship with "C" is so far that it is not known whether Alpha Centauri "C" is really bound to the twin movements and orbits of "A" and "B", or if "C" will simply leave the union of its own accord in the next several million years.

Alpha Centauri "C" lies measurably closer to us than either "A" or "B", being located at a distance of only 4.22 light-years away.   Alpha Centauri "C" is the nearest individual star to our Sun.   Because of this proximity, and its proximity to both Alpha Centauri "A" and "B",  Alpha Centauri "C" is also called Proxima (Centauri).

Alpha Centauri "A" is a yellow star with a spectral type of G2, matching almost exactly the characteristics, including temperature and color, as our own Sun.  Alpha Centauri "B" is an orange star with a spectral type of K1, somewhat cooler than our own Sun and Alpha Centauri "A" in both color and temperature. Whereas Alpha Centauri "A" and "B" are stars with very similar characteristics to our own Sun, and therefore high percentages for the existence of life supporting planets, Proxima Centauri is a dim red dwarf with a spectral type of M5.   Proxima Centauri is much fainter, cooler, smaller and fainter in appearance than our Sun, so much so that astronomers did not discover the existence of Proxima Centauri until 1915.

Alpha Centauri is a very interesting star system because it may offer ideal life conditions similar to that which occur in our own solar system.  A star must pass five tests before scientists can consider it to be a possible place for life, as we know it, terrestrial life, to exist.  Most stars in our Milky Way galaxy fail these simple tests, however, in the case of Alpha Centauri, we see that Alpha Centauri "A" passes all five tests, Alpha Centauri "B" passes all but one, with Proxima Centauri failing to be capable of supporting any life as we know it.

The first test of any known or newly discovered star is the main sequence test, which is administered to be sure of the star's maturity and stability.  A test for the star's maturity and stability means that the star has to lie along the main sequence. Main-sequence stars fuse hydrogen into helium at their cores, generating both heat and light as emitted energy.  The natural abundance of hydrogen in stars means that most stars stay on the main sequence a very, very long time, giving life an ample chance to exist. The Sun as well as all three components of the Alpha Centauri trinary star system pass this test.

The second test, or spectral type test, is to determine if a star's spectral type is compatible with life as we know it.  This test in turn determines how much energy the star emits in both light and heat forms, two very important criteria to judge when trying to determine if a star can support life or not.  Spectral types of "O", "B", "A", and early "F" classifications are considered to be 'hot' stars, with very short (comparatively) life times.  These stars usually burn hotter and quicker, meaning that they burn out and die quickly, within a cosmic time frame, and thus have a lower chance to support the time frames required for life to both develop and evolve.  In direct contrast, spectral type "M" and "K" are considered to be 'cooler' stars.  These types of cooler stars may not emit enough light and heat as energy to sustain a viable life system.  The spectral type that we are looking for are rated as "G".  Our own Sun is a yellow G-type star.  These stars are within the realm for producing the correct amount of heat and light energy to maintain both a long solar life and the right amount of energy to support life over that lifetime.  Late F stars (cooling) and young K spectral type (hot) stars can also support life, but for far less time frames than the more stable G-type spectral class stars.  Applying this test to Alpha Centauri "A" finds that our neighbor star is in the exact same class as our Sun.   Alpha Centauri "B" is a K1 star, hotter and brighter than most other K spectral class stars, therefore it may pass this test or it may not.   The tiny red dwarf Proxima Centauri fails this test completely.

For the third test, a system must demonstrate stable energy emission conditions.  A star's brightness may not vary in such a wide range that the star would alternately burn and freeze any life that does manage to develop around it. The special case of Alpha Centauri "A" and Alpha Centauri "B" forming a larger binary duo brings up another issue in that scientists must determine exactly how much light would be received by a planet orbiting one of the stars as the other star revolved around the primary star?  During their 80-year orbit, the separation between Alpha Centauri "A" and Alpha Centauri "B" changes from a meager 11 AU to a massive 35 AU.  When viewed from the surface of a planet of one of the stars, the brightness of the other star increases as the stars approach and naturally decreases as the stars recede. Fortunately, the variation, as far as scientists can currently determine, is too small to matter.  Given this result of the test, Alpha Centauri "A" and Alpha Centauri "B" both pass the third test. Proxima, again, fails this test as well.  Proxima Centauri, like most red dwarfs, is what is termed a 'flare star' in that it is prone to outbursts that cause its emitted light to double or triple in the space of only a few minutes.

The fourth test is used to determine a stars vitality and age.  Stars that are very young may still be evolving on their own, and thus are not good prospects for colonization while older stars may be cooling, dying, and thus, obviously are also not good candidates for colonization.  Several billion years seems to be a good estimate of the median desired for life to exist in our battery of tests.  Our own Sun is roughly 4.6 billion years old, so on Earth life had enough time to develop and evolve into a advanced civilization with plenty of time yet to evolve or move beyond our solar system.   In order to pass the fourth test, a given star must be old enough to give life a chance not only to exist, but also to evolve.  Alpha Centauri "A" and Alpha Centauri "B", as best as we can tell, have an age of between 5 and 6 billion years old each, which is even older than our own Sun!  Therefore both main stars in the system pass the fourth test.  Proxima, to the best of our knowledge, may be only a billion years or so old.  Being such a 'young' star, Proxima Centauri fails the fourth test as well.

The fifth and final test involves the elements found in the makeup of the stars themselves: do the stars have enough heavy elements - elements such as nitrogen, iron, oxygen, and carbon - to support biological life as we know it?  Scientists refer to elements that are heavier than helium as 'metals'.  Using this criteria, and looking at our own Sun, we find that it is composed of primarily hydrogen and helium.   However, about 2 percent of the Sun's weight is considered to be metals.   Although 2 percent may not sound a lot, it is enough to build rocky planets within the system and to give rise to life as we know it.  Using this criteria to judge the three stars that make up the system of Alpha Centauri, we find that both "A" and "B" pass the fifth test as they are metal-rich stars, gifted with an abundance of the elements which would both promote and sustain life.  Proxima again fails this test.

To summarize what we know, we have prepared this chart for you to reference.

Now to the most important question... Will we find at Alpha Centauri teeming with warm, rocky planets like Earth?  Planets that are full of liquid water and perhaps life of their own?  We don't really know yet whether Alpha Centauri even has planets or not, but our first manned flight to Alpha Centauri will determine this, as well as what types of planets are present, and if any are capable of supporting life as we know it.  What we do understand is that in a binary star system that the planets must not be too far removed from a particular 'parent' star, otherwise their orbits become unstable.  Computer modeling suggests that if the distance of the orbit of the planet exceeds about one fifth of the closest approach of the two stars then the second member of the binary star catastrophically disturbs the orbit of the planet.  Our understanding of the nature of the orbits of the binary Alpha Centauri "A" and Alpha Centauri "B" point to the fact that their closest approach towards each other is 11 AU which means that the theoretical limit for planetary orbits is somewhere in the region of two astronomical units.

Comparing these extrapolated models with our own known solar system, we see that both Alpha Centauri "A" and Alpha Centauri "B" might hold four inner planets, in much the same way that our own Sun holds Mercury (0.4 AU), Venus (0.7 AU), Earth (1 AU) and Mars (1.5 AU). Therefore, Alpha Centauri "A" and Alpha Centauri "B" might have one or two planets in the life zone where liquid water is possible and life not only can exist, but actually may already exist and flourish!

One may conclude that Alpha Centauri is a most promising star system regarding terrestrial planets and possible life therefore it would be the first destination for interstellar flights by any human nation or power capable of launching such flights. It is with this criteria, that we at ANSA have judged Alpha Centauri to be the target destination for our first interstellar flight, a flight which will be launched in 1972.

In closing, we here at the space flight command center leave you with one final reference chart showing our criteria, how we have judged our mission parameters, and why we consider Alpha Centauri to be a viable target for our mission beyond our own solar system.  Any further questions about this project should be directed to Dr. Hasslein.