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The Traveler's Guide to the Solar SystemPut Your Space Shorts on and Take a Cruise on an Intergalactic Getaway
By Giles Sparrow
HarperCollins Publishers, Inc.Copyright © 2007 Giles Sparrow
All right reserved.
As with any long journey, a trip across the Solar System will be a lot easier if you have some idea of where you're going and what you're likely to see on the way, and indeed if you know how to drive in the first place. This chapter gives you an introduction to our celestial neighborhood and should tell you just enough about planetary motion and spacecraft navigation to avoid embarrassing blunders. It's also a good place to explain a little about spacesuits, which you'll be relying on a great deal, and to offer some general health advice.
Solar System basics
A vacation away from Earth isn't like your average package tour. While plenty of people visit Venice, the pyramids, and the ruins of Atlantis each year with barely the faintest idea of where they are on the map, space travel is more of a challenge. Assuming you don't want to limit your explorations to a rigorously regimented, scheduled lunar daytrip (and then why would you be reading this book?) you'll need some idea of Solar System geography.
Earth is the third of nine planets (some say eight, some say ten, but nine is kind of traditional), orbiting our local star, the Sun. The four worlds closest to the Sun—Mercury, Venus, Earth and Mars—arecomparatively small and rocky (Earth is the biggest), and are collectively termed "terrestrial planets". Worlds five to eight are giants in comparison: Jupiter is the largest and innermost of these, followed in size and distance by Saturn. Uranus and Neptune are further still from the Sun, and smaller than the inner giants. Although all four outer worlds are enormous, they are made from much lighter elements than the terrestrial planets-hence the terms "gas giant" and "ice giant".
Many of these eight planets are orbited by natural satellites or moons (Mercury and Venus are the only exceptions). Between and beyond the major worlds are countless smaller objects, sometimes lumped together as "minor planets". The majority are found in two bands—rocky asteroids concentrated between Mars and Jupiter, and frozen ice dwarfs beyond the orbit of Neptune. Pluto, the traditional ninth planet, is actually a large ice dwarf, but it's generally treated as a planet as well (for more on this, see Chapter 11).
Apart from the Sun, everything else in the Solar System is a moving target—and that includes Earth. All the planets move around the Sun according to a set of laws discovered by German astronomer Johannes Kepler in the early i6oos (see "Planetary motions," opposite). The same laws also apply to moons in orbit around their planets.
Most of the objects in the Solar System have orbits in or around a relatively flat plane, so that if you looked at them all side-on, they'd nearly line up. Most planets and asteroids are also in almost-circular orbits, and spin more-or-less "upright" relative to their orbits (see p.206 for some explanations about all this).
For want of a better definition, the actual "flat plane of the Solar System" is taken as the plane across the Earth's orbit. The technical term for this plane is the "ecliptic" and any star atlas will show you how it runs across the sky, marking out the path that the Sun appears to take against the background stars. (Just don't go looking for a convenient dashed line on the sky itself.) Of the other planets, the most significant "inclinations" to this orbit are Mercury, tilted at 7°, and Pluto, tilted at 17°. So, tiny and distant Pluto aside, all the planets generally loiter close to the ecliptic and, seen from Earth, only ever appear against a narrow band of stars (the constellations of the zodiac).
Planning your flight
Although each of the chapters on specific destinations opens with some detailed travel tips, it's a good idea to understand the basics of interplanetary navigation too. There are some cowboy operators out there, just waiting to sell you an "off-peak" bargain that will turn into the vacation from hell, and you'll save yourself a lot of time and trouble if you can see through any dodgy offers straight off.
As we said before, every destination in the Solar System, the Sun aside, is a moving target. Because every world goes around the Sun at a different speed, the distances between them can change enormously depending on where they sit on their respective orbits. The best example is Venus, which can get to within 26 million miles (42 million km) of Earth at its closest approach, but is up to i6o million miles (257 million km) away when we're on opposite sides of the Sun. The fairly elliptical orbits of some planets add another complication—for example Mars's "close approaches", when Earth and Mars line up on the same side of the Sun, can vary between million miles (6 million km) and 62 million miles (98 million km)
Earth moves round its orbit at about 19 miles per second (30 km/s). It's faster-moving than anything further out, so once you've broken out of Earth orbit, you'll already have the edge on speed over the outer planets. But of course you'll want to get to your destination as quickly as you can, and any extra speed you can pick up will help you to do that. Details of your various propulsion options are available in the "Traveler's Reference" section (pp.204-220), but there are a number of general points to bear in mind.
First off, it's generally more efficient to accelerate for a longer time with less thrust, rather than piling on all your speed in one burst from an old-fashioned chemical rocket engine (there are health benefits in this too, as we'll see).
Second, don't forget that you can get a significant speed boost from the gravity of other planets you pass on the way to your destination (see "Gravitational slingshots", opposite). Some seemingly bizarre detours can actually shorten your journey time by months or even years.
Excerpted from The Traveler's Guide to the Solar System by Giles Sparrow Copyright © 2007 by Giles Sparrow. Excerpted by permission.
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