Near Galactic Space
Figure 1.3. Sun to near-Galactic space in three segments.
gas planets without a rocky core, but could have cores of liquefied or frozen gases. Within this band are the gas giants of Jupiter (11.1 times the diameter of Earth), and Saturn (9.5 times the diameter of Earth). Uranus and Neptune are 4 and 3.9 times the diameter of Earth respectively. Jupiter is so massive that it is almost a sun. The radiation associated with Jupiter is very intense and without significant shielding would be lethal to any human or electronics in the vicinity. The second zone extends to the boundary of our Solar System and the galactic medium, the Heliopause. The third zone spans the distance from the Heliopause to the vicinity of Alpha Centauri. In this zone you can see the Jovian planets and the terrestrial planets compressed into two narrow bands. That is, the size of our Solar System (100 AU) compared to the distance to our nearest star (149,318 AU) is very small indeed. The near galactic space contains a spherical shell about 140,000 AU thick that contains icy and rocky objects of differing sizes. Because the objects appear dark they are very difficult to resolve in visible light. It is from this shell of objects that most long-term comets (such as Halley's) appear to originate. The volume of space encompassed by our Solar System traveling through the galactic medium is called the Heliosphere. Note that between the Heliopause boundary that defines the volume of space encompassed by our Solar System traveling through the galactic medium, and the nearest star, space is essentially devoid of any substantial objects. Even the Oort Cloud begins at a distance some 100 times greater then the Heliopause. If we look at distances measured in light travel time these dimensions are reaffirmed. The outermost planet Pluto is 38.9 AU distant from the Sun. Even with these figures in mind, it is still difficult to visualize the size of our local space. That is important because it is the size of space that determines the character of the propulsion system needed.
The Sun is a logical reference point for visualizing size and distance. One approach to permit visualization of our Solar System is to scale down the system to comprehensible object sizes and distances. To do that, visualize the Sun not as a sphere 856,116 statute miles (1,377,800km) in diameter, but as a 400mm diameter (14.75 inch) soccer ball. Doing so means the diameter of the Earth (7927 miles or 12,757 km) is about the diameter of a pea some 43 meters from the soccer ball. Table 1.2 gives the diameter (mm) and distances (m or km) of the objects listed, from our Sun to our nearest galaxy.
In this analogy, Pluto is about one-half the diameter of the Earth, and on this scale is at 1.7 kilometers from the soccer ball. To illustrate now the snail's pace of our travels, traveling to Pluto directly, e.g., without gravity assists from the massive planets, with our current chemical and future nuclear-electric or nuclear-thermal propulsion systems, would take 19 years, at the blinding speed of 220 mm per day on this scale. We truly move at a snail's pace in the dimensions of our Solar System! If we are to move faster, it is propulsion that will enable that greater speed. Over 19 years the true average speed to Pluto using conventional propulsion mentioned, is 32,326 ft/s (9.853 km/s). Of course that is an average, i.e., if the spacecraft flew along a radial path from Earth, through the Sun and on to Pluto as if they were all aligned. That is not the case, and the actual path is actually a curve longer than a radius, so the actual speed should be faster. If we wanted the spacecraft to reach Pluto in one year, its average speed would have to be 19 times faster, or 614,100 ft/s (187.2 km/s). To obtain the incremental speed, the specific impulse of the propulsion system (the performance index defined in the Introduction) would have to be not the 300 s of current chemical boosters, or the 3000 s (2,942 m/s) of electric thrusters, but 5,509 s (54,025 m/s). This number is well beyond our current capability.
In one popular space travel television show it is merely specifying the warp speed and pronouncing, "engage" that (within several minutes or hours) transport the crew of the Enterprise to their destination. In reality nothing could be further from reality, as we know it today. The Heliopause (the boundary between our Solar System and the oncoming galactic space medium our Solar System travels through space in the
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