Marooned on Eden

by Robert L. Forward

  One leg of the SLAM is part of the "Jacob's Ladder," while another leg acts as the lowering rail for the Surface Excursion Module. The wings of the Surface Excursion Module are chopped off in mid-span just after the VTOL fans. The remainder of each wing is stacked as interleaved sections on either side of the tail section of the Surface Excursion Module. Once the Surface Excursion Module has its wings attached, it is a completely independent vehicle with its own propulsion and life support system.

  Surface Excursion Module

  The Surface Excursion Module (SEM) is a specially designed aerospace vehicle capable of flying as a plane in a planetary atmosphere or as a rocket for short hops through empty space. The crew has given the name Dragonfly to the SEM because of its long wings, eyelike scanner ports at the front, and its ability to hover. An exterior view of the SEM is shown in Figure 4.

  For flying long distances in any type of planetary atmosphere, including those which do not have oxygen in them, propulsion for the SEM comes from the heating of the atmosphere with a nuclear reactor powering a jet-bypass turbine. For short hops outside the atmosphere, the engine draws upon a tank of monopropellant, which not only provides reaction mass for the nuclear reactor to work on, but also makes its own contribution to the rocket plenum pressure and temperature.

  Unfortunately, the SEM IV aerospace plane was damaged and sank under 200 meters (600 feet) of water during the rocket engine burnthrough and subsequent crash of the SLAM IV lander during an attempted landing on Zuni. Fortunately, the entire crew of ten humans and three flouwen buds managed to survive the crash and are still on Zuni, but without the flying ability of the SEM, the exploration range of the humans is limited to a single small island on the large moon.

  Figure 4—Exterior View of Surface Excursion Module (SEM)

  Christmas Bush

  The hands and eyes of the near-human computers that ran the various vehicles on the expedition are embodied in a repair and maintenance motile used by the computer, popularly called the "Christmas Bush" because of the twinkling laser lights on the bushy multibranched structure. The bushlike design for the robot has a parallel in the development of life forms on Earth. The first form of life on Earth was a worm. The stick-like shape was poorly adapted for manipulation or even locomotion. These stick-like animals then grew smaller sticks, called legs, and the animals could walk, although they were still poor at manipulation. Then the smaller sticks grew yet smaller sticks, and hands with manipulating fingers evolved.

  The Christmas Bush is a manifold extension of this concept. The motile has a six-"armed" main body that repeatedly hexfurcates into copies one-third the size of itself, finally ending up with millions of near-microscopic cilia. Each subsegment has a small amount of intelligence, but is mostly motor and communication system. The segments communicate with each other and transmit power down through the structure by means of light-emitting and light-collecting semiconductor diodes. Blue laser beams are used to closely monitor any human beings near the motile, while red and yellow beams are used monitor the rest of the room. The green beams are used to transmit power and information from one portion of the Christmas Bush to another, giving the metallic surface of the multibranched structure a deep green internal glow. It is the colored red, yellow, and blue lasers sparkling from the various branches of the greenly glowing Christmas Bush that give the motile the appearance of a Christmas tree. The central computer in the spacecraft is the primary controller of the motile, communicating with the various portions of the Christmas Bush through color-coded laser beams. It takes a great deal of computational power to operate the many limbs of the Christmas Bush, but built-in "reflexes" at the various levels of segmentation lessen the load on the main computer.

  Figure 5—The Christmas Bush

  The Christmas Bush shown in Figure 5 is in its "one gee" form. Three of the "trunks" form "legs," one the "head," and two the "arms." The head portions are "bushed" out to give the detector diodes in the subbranches a three-dimensional view of the space around it. One arm ends with six "hands," demonstrating the manipulating capability of the Christmas Bush and its subportions. The other arm is in its maximally collapsed form. The six "limbs," being one-third the diameter of the trunk, can fit into a circle with the same diameter as the trunk, while the thirty-six "branches," being one-ninth the diameter of the trunk, also fit into the same circle. This is true all the way down to the sixty million cilia at the lowest level. The "hands" of the Christmas Bush have capabilities that go way beyond those of the human hand. The Christmas Bush can stick a "hand" inside a delicate piece of equipment, and using its lasers as a light source and its detectors as eyes, rearrange the parts inside for a near instantaneous repair. The Christmas Bush also has the ability to detach portions of itself to make smaller motiles. These can walk up the walls and along the ceilings using their tiny cilia holding onto microscopic cracks in the surface. The smaller twigs on the Christmas Bush are capable of very rapid motion. In free fall, these rapidly beating twigs allow the motile to propel itself through the air. The speed of motion of the smaller cilia is rapid enough that the motiles can generate sound and thus can talk directly with the humans.

  Each member of the crew has a small subtree or "imp" that stays constantly with him or her. The imp usually rides on the shoulder of the human where it can "whisper" in the human's ear, some of the women use the brightly colored laser-illuminated imp as a decorative ornament. In addition to the imp's primary purpose of providing a continuous personal communication link between the crew member and the central computer, it also acts as a health monitor and personal servant for the human. The imps go with the humans inside their spacesuit, and more than one human life was saved by an imp detecting and repairing a suit failure or patching a leak. The imps can also exit the spacesuit, if desired, by worming their way out through the air supply valves.

  SECTION 2

  BARNARD SYSTEM ASTRONOMICAL DATA

  Prepared by:

  Linda Regan—Astrophysics

  Thomas St. Thomas, Captain, GUSAF—Astrodynamics

  Barnard Planetary System

  As shown in Figure 6, the Barnard planetary system consists of the red dwarf star Barnard, the huge gas giant planet Gargantua and its large retinue of moons, and an unusual co-rotating double planet Rocheworld. Gargantua is in a standard near-circular planetary orbit around Barnard, while Rocheworld is in a highly elliptical orbit that takes it in very close to Barnard once every orbit, and very close to Gargantua once every three orbits. During its close passage, Rocheworld comes within six gigameters of Gargantua, just outside the orbit of Zeus, the outermost moon of Gargantua. It has been suggested that one lobe of Rocheworld was once an outer large moon of Gargantua, while the other lobe was stray planetoid that interacted with the outer Gargantuan moon to form Rocheworld in its present orbit. Further information about Barnard, Rocheworld, and Gargantua and its moons follows:

  Figure 6—Barnard Planetary System

  Barnard

  Barnard is a red dwarf star that is the second closest star to the solar system after the three-star Alpha Centauri system. Barnard was known only by the star catalog number of +4o 3561 until 1916, when the American astronomer Edward E. Barnard measured its proper motion and found it was moving at the high rate of 10.3 seconds of arc per year, or more than half the diameter of the Moon in a century. Parallax measurements soon revealed that the star was the second closest star system. Barnard's Star (or Barnard as it is called now) can be found in the southern skies of Earth, but it is so dim it requires a telescope to see it. The data concerning Barnard follows:

  BARNARD DATA

  Distance from Earth = 5.6x1016 m (5.9 lightyears)

  Type = M5 Dwarf

  Mass = 3.0x1029 kg (15% solar mass)

  Radius = 8.4 x 107 m = 84 Mm (12% solar radius)

  Density = 121 g/cc (86 times solar density)

  Effective Temperature = 3330 K (58% solartemperature)

  Luminosity = 0.05% solar (visual); 0.37% solar(
thermal)

  The illumination from Barnard is not only weak because of the small size of the star, but reddish because of the low temperature. The illumination from the star is not much different in intensity and color than that from a fireplace of glowing coals at midnight. Fortunately, the human eye adjusts to accommodate for both the intensity and color of the local illumination source, and unless there is artificial white-light illumination to provide contrast, most colors (except for dark blue—which looks black) look quite normal under the weak, red light from the star.

  Note the high density of the star compared to our Sun. This is typical of a red dwarf star. Because of this high density, the star Barnard is actually slightly smaller in diameter than the gas giant planet Gargantua, even though the star is forty times more massive than the planet.

  Rocheworld

  The unique co-rotating dumbbell-shaped double planet Rocheworld consists of two planetoids that whirl about each other with a rotation period of six hours. As shown in Figure 7, the two planetoids or "lobes" of Rocheworld are so close together that they are almost touching, but their spin speed is high enough that they maintain a separation of about 80 kilometers. If each were not distorted by the other's gravity, the two planets would have been spheres about the size of our Moon. Because their gravitational tides act upon one another, the two bodies have been stretched out until they are elongated egg-shapes, roughly 3500 kilometers in the long dimension and 3000 kilometers in cross section.

  Although the two planetoids do not touch each other, they do share a common atmosphere. The resulting figure-eight configuration is called a Roche-lobe pattern after E.A. Roche, a French mathematician of the later 1880s, who calculated the effects of gravity tides on stars, planets, and moons. The word "roche" also means "rock" in French, so the dry rocky lobe of the pair of planetoids has been given the name Roche, while the lobe nearly completely covered with water was named Eau, after the French word for "water." The pertinent astronomical information concerning Rocheworld follows:

  ROCHEWORLD DATA

  Type: Co-rotating double planet

  Diameters: Eau Lobe: 2900x3410 km

  Roche Lobe: 3000x3560 km

  Separation: Centers of Mass: 4000 km

  Inner Surfaces: 80 km (nominal)

  Co-rotation Period = 6 h

  Orbital Semimajor Axis = 18 Gm

  Orbital Period = 962.4 h

  = 160 rotations(exactly)

  = 40.1 Earth days

  Axial Tilt = 0o

  One of the unexpected findings of the mission was the resonance between the Rocheworld "day," the Rocheworld "year," and the Gargantuan "year." The period of the Rocheworld day is just a little over 6 hours, or 1/4th of an Earth day, while the period of the Rocheworld "year" is a little over 40 Earth days, and the orbital period of Gargantua is a little over 120 days. Accurate measurements of the periods have shown that there are exactly 160 rotations of Rocheworld about its common center to one rotation of Rocheworld in its elliptical orbit around Barnard, while there are exactly 480 rotations of Rocheworld, or three orbits of Rocheworld around Barnard, to one orbit of Gargantua around Barnard.

  Figure 7—Rocheworld

  Orbits such as that of Rocheworld are usually not stable. The three-to-one resonance condition between the Rocheworld orbit and the Gargantuan orbit usually results in an oscillation in the orbit of the smaller body that builds up in amplitude until the smaller body is thrown into a different orbit or a collision occurs. Due to Rocheworld's close approach to Barnard, however, the tides from Barnard cause a significant amount of dissipation, which stabilizes the orbit. This also supplies a great deal of heating, which keeps Rocheworld warmer than it would normally be if the heating were due to radiation from the star alone. Early in the expedition, both Rocheworld and Gargantua were "tagged" with artificial satellites carrying accurate clocks, and the planets have been tracked nearly continuously since then. The data record collected extends for almost four years. The 480:160:1 resonance between the periods of Gargantua's orbit, Rocheworld's orbit, and Rocheworld's rotation, is now known to be exact to 15 places.

  Rocheworld was explored extensively in landings made during Phase I and Phase II of the mission, and more detailed information about the double-planet, and its interesting astrodynamics, can be found in the Phase I and Phase II reports.

  Gargantua

  Gargantua is a huge gas giant like Jupiter, but four times more massive. Since the parent star, Barnard, has a mass of only fifteen percent of that of our Sun, this means that the planet Gargantua is one-fortieth the mass of its star. If Gargantua had been slightly more massive, it would have turned into a star itself, and the Barnard system would have been a binary star system. Gargantua seems to have swept up into itself most of the original stellar nebula that was not used in making the star, for there are no other large planets in the system. The pertinent astronomical information about Gargantua follows:

  GARGANTUA DATA

  Mass = 7.6x1027 kg (4 times Jupiter mass)

  Radius = 9.8x107 m = 98 Mm

  Density = 1.92 g/cc

  Orbital Radius = 3.8x1010 m = 38 Gm

  Orbital Period = 120.4 Earth days (3 times Rocheworld period)

  Rotation Period = 162 h

  Axial Tilt = 8o

  The radius of Gargantua's orbit is less than that of Mercury. This closeness to Barnard helps compensates for the low luminosity of the star, leading to moderate temperatures on Gargantua and its moons.

  Gargantuan Moon System

  There are nine major moons in the Gargantuan moon system. Their orbital and physical properties are listed in the following table. The five smaller moons are rocky, airless bodies, while the four larger moons have atmospheres and show distinctive colorings. All the moons are tidally locked to their primary.

  Figure 8 presents a comparison of the orbits of the four large moons in the Gargantuan system with the orbits of the four large moons in the Jovian system. The Gargantuan system is seen to be quite similar to the Jovian system, although a little more compact.

  Jupiter Io Europa Ganymede Callisto

  71 420 670 1070 1880 Mm

  ( )—————o———o———o———————o—

  ( )———o——o———o———————o————

  98 330 530 730 1650 Mm

  Gargantua Zulu Zuni Zouave Zapotec

  Figure 8—Comparison of Gargantuan and

  Jovian Moon Systems

  Conjunctions

  The three inner large moons, Zouave, Zuni, and Zulu, can exert significant tidal effects on each other. This happens during a conjunction, when the distance between the two moons is a minimum. After a conjunction has once occurred, it will reoccur when after a certain time period, the inner moon (which always revolves faster than the outer moon) has rotated exactly one revolution more than the outer moon. The joint conjunction periods for the three innermost large moons of Gargantua are:

  Zulu/Zouave21.1 h

  Zulu/Zuni28.9 h

  Zuni/Zouave78.4 h

  Triple conjunctions, when all three moons are nearly in alignment, are much rarer. The triple conjunction period is about 549 hours (about 23 Earth days). This triple conjunction occurs every 26 conjunctions of Zulu with Zuni, 19 conjunctions of Zulu with Zouave, and 7 conjunctions of Zuni with Zouave.

  Quadruple conjunctions, when all three moons and Barnard are nearly in alignment, are even rarer, occurring every third triple conjunction. The quadruple conjunction period is about 1646.8 hours. This is about 68 Earth days, 55.5 Zuni days, 111.5 Zulu days and 33.5 Zouave days.

  Intermoon Tides

  Since the tidal force exerted by one moon on another goes as the inverse cube of the separation distance, the tides will be short and strong during conjunction, but negligible otherwise. This is a different situation than the tides on Earth, where the distance from the Earth to the Moon and the Sun stays nearly constant with time. Because the distance from the Earth to the tide-making body stays roughly
constant, the dual oceanic tidal bulges (one bulge toward the body making the tide and a matching bulge in the opposite direction) from the tidal effects of the Moon and Sun stay roughly constant in height. The lunar tide turns out to be roughly twice the height of the solar tide because the closer proximity of the Moon more than makes up for its smaller mass. The approximately twice daily variations in tides that are observed on the Earth comes from the Earth rotating its continents around underneath the two oceanic bulges one each day. The seasonal variations of spring tides and neap tides occurs because the Sun and Moon tidal bulges move with respect to each other from new moon to full moon, and season to season, sometimes reinforcing each other and sometimes partially canceling each other.