Preliminary Report on the Alpha Centauri System

The "star" Alpha Centauri, as it appears in Earth’s sky, is actually a grouping of three stars: Alpha Centauri A, ("Alpha Prime") a G2V class main sequence yellow star slightly larger and older than our Sun; Alpha Centauri B ("Hercules"), a K5V orange star which orbits the primary with an aphelion of 35 astronomical units and a perihelion 11 A.U.; and Proxima Centauri, an M5 red dwarf a full light year distant. It is not so much a trinary system as a binary with a third star (Proxima) at a distance.

At perihelion, or nearest approach to Alpha Prime, Hercules’ elliptical orbit brings it to about the distance of Saturn from our Sun. The orbital dynamics of a binary star system drastically limit the number of stable orbits available for planet formation. Thus we did not expect to find, and in fact did not find, planets further away from the primary than about 1.5 astronomical units (Earth is one astronomical unit away from our Sun).

Two planets make up Alpha Prime’s inner system: Eurytion, a small Mercury-like world, and Chiron. A large Jovian planet, Prometheus, orbits close in to Hercules.

Our target destination, Chiron, or "Planet", is a terrestrial world somewhat larger than our Earth. Under a gravitational force of 12.85 m/s2 (compared to Earth’s 9.81), colonists will feel 1.31 times heavier than on Earth, which will be uncomfortable at first but should not cause drastic long-term health problems. Chiron is somewhat farther from Alpha Prime than Earth is from our Sun (1.07 A.U.), but the higher luminosity of Alpha Prime more than offsets this. Moreover, since Alpha Prime is somewhat larger than the Sun, its size in Chiron’s sky will be indistinguishable from our Sun.

Chiron’s higher gravity results in a thick, dense atmosphere, so we can expect to find larger and more numerous flying creatures than on Earth. The massive preponderance of nitrogen in the atmosphere (over 90%, compared with Earth’s 78%) has several significant implications. First, the dangers of nitrogen narcosis with prevent humans and terrestrial animals from breathing the unfiltered air. Secondly, the low oxygen partial pressure will tend to suppress fires.

Chiron’s two moons, tiny inner Pholus and the somewhat larger Nessus, are each much smaller than Earth’s moon, but they orbit much closer in, so the overall tidal effect is comparable to Earth’s.

Chironian Calendar

Chiron’s year is slightly longer than Earth’s, but it rotates considerable faster, finishing a day in slightly under 18 hours. Since there is no further point in retaining hour, minute, and second time measurements calibrated for Earth time, the following calendar will be used on Planet:

The hour, the minute, and the second will each be shortened by approximately 2.68%, in order to accomodate a Chironian day of precisely 18 hours.

The 532 days in a year shall be divided into 53 ten day long "decurns", which corresponds both to the approximate length of Earth’s week and the approximate length of Chiron’s lunar month (measuring by the cycles of the larger moon, Nessus). The two "leftover" days shall be used as special holidays. The first day of the year shall be "Planetfall Day", on which we celebrate our arrival on Planet. The middle (27th) decurn of the year shall also be a time of celebration, and shall end on "Earth Day", the second extra day, on which we remember Earth, our lost home planet.

                        PLANETS OF ALPHA CENTAURI A
                        ===========================

Solar constants                     Sun         Alpha A     Ratio

  Mass                  kg          1.99E+30    2.15E+30    1.08
  Luminosity            W           3.89E+26    5.63E+26    1.45
  Radius                m           6.96E+08    7.59E+08    1.09

Planetary constants                 Earth       Planet/     Ratio
                                                Chiron

  Mass                  kg          5.98E+24    1.10E+25    1.84
  Equat. radius         m           6.38E+06    7.54E+06    1.18
  Dist. from star       m           1.50E+11    1.60E+11    1.07
  Axial tilt            degrees     23.45       2.00        0.09
  Surface area          m2          5.10E+14    7.18E+14    1.41
  Standard gravity      m s-2       9.81        12.85       1.31
  Escape velocity       m s-1       11184       13947       1.25
  Density               kg m-3      5519        6150        1.11
  Size of sun           degrees     0.27        0.27        1.02
  Year                  our days    365.3       388.6       1.06
  Year                  local days  365.3       532.0       1.46
  Day                   hours       24.00       17.53       0.73
  Mountain height       m           10626       8112        0.76
  Horizon distance      m           5051        5493        1.09
  Ocean tide (sun)      m           0.12        0.12        0.94
  Ocean tide (moon 1)   m           0.27        0.18        0.67
  Ocean tide (moon 2)   m                       0.11
  Ocean tide (both)     m           0.39        0.41        1.05

Atmosphere

  Total pressure        Pa          101325      176020      1.74
  Nitrogen              Pa          79125       160000      2.02
  Oxygen                Pa          21228       15000       0.71
  Argon                 Pa          942         1000        1.06
  Carbon dioxide        Pa          30          20          0.67

  Nitrogen                          78.09%      90.90%      1.16
  Oxygen                            20.95%      8.52%       0.41
  Argon                             0.93%       0.57%       0.61
  Carbon dioxide                    0.03%       0.01%       0.4

  Surface density       kg m-3      1.22        2.06        1.68
  "Flammability"        mmol K J-1  7.17        2.87        0.40

  Effective temp.       K           253         261         1.03
  Greenhouse effect     K           +36         +32         0.90
  Surface temp.         K           288         293         1.01
  Surface temp.         C           15.4        19.7        1.28
  Solar constant        W m-2       1383        1750        1.27

Outer Moon (Nessus)                 The Moon    Nessus

  Mass                  kg          7.35E+22    6.50E+21    0.09
  Radius                m           1.74E+06    8.00E+05    0.46
  Dist. from planet     m           3.84E+08    2.00E+08    0.52
  Surface area          m2          3.80E+13    8.04E+12    0.21
  Mean gravity          m s-2       1.62        0.68        0.42
  Density               kg m-3      3342        3031        0.91
  Synodic month         our days    29.5        7.7         0.26
  Synodic month         local days  29.5        10.6        0.36
  Syn. months/yr                    12.4        50.2        4.05
  Angular radius        degrees     0.26        0.23        0.88

Inner Moon (Pholus)                 The Moon    Pholus

  Mass                  kg          7.35E+22    5.20E+20    0.01
  Radius                m           1.74E+06    3.50E+05    0.20
  Dist. from planet     m           3.84E+08    1.00E+08    0.26
  Surface area          m2          3.80E+13    1.54E+12    0.04
  Mean gravity          m s-2       1.62        0.28        0.17
  Density               kg m-3      3342        2895        0.87
  Synodic month         our days    29.5        2.7         0.09
  Synodic month         local days  29.5        3.7         0.13
  Syn. months/yr                    12.39       143.76      11.60
  Angular radius        degrees     0.26        0.20        0.77

Eurytion (Mercurian planet)         Earth       Eurytion    Ratio

  Mass                  kg          5.98E+24    5.16E+23    0.09
  Equat. radius         m           6.38E+06    2.82E+06    0.44
  Dist. from star       m           1.50E+11    7.06E+10    0.47
  Surface area          m2          5.10E+14    9.98E+13    0.20
  Standard gravity      m s-2       9.81        4.33        0.44
  Escape velocity       m s-1       11184       4942        0.44
  Density               kg m-3      5519        5503        1.00
  Size of sun           degrees     0.27        0.62        2.31
  Year                  our days    365.3       113.8       0.31
  Year                  local days  365.3       SYNCHRONOUS
  Horizon distance      m           5051        3357        0.66
  Surf. temperature     K           288         438         1.52
  Surf. temperature     C           15.4        165.3       10.71
  Solar constant        W m-2       1383        8999        6.51