Diameter: 10 to 1,000 kilometers Distance From Sun: average of 2.8 AU Gravity: Minimal
Year: average 4.7 Earth years Orbital Velocity: 17.7 kilometers/sec Atmosphere: None
The asteroids — rubble left over from the for-mation of the Solar System — are most common in a belt between the orbits of Mars and Jupiter.
Although there are thousands of asteroids in the belt, there is a lot of empty space there, too; the average separation between asteroids is a million kilometers.
Individual asteroids are irregular chunks of rock, ranging from a few kilometers across to giants like Ceres, half the size of Earth’s Moon. Their sur-faces are probably powdered regolith, shattered by frequent impacts. The composition of individual asteroids varies quite a bit, with some rich in metals, some in carbon, some in ice, and some in nothing but rock.
In fiction, the Belt has often been depicted as a futuristic version of the Gold Rush, with lone asteroid prospectors steer-ing their jury-rigged ships from rock to rock, search-ing for asteroids rich in valuable radioactives or Helium-3. Often the inde-pendent-minded Belters come into conflict with Earth’s stuffy bureaucrats and greedy corporations.
Life in permanent zero gravity sometimes makes the Belters physically dif-ferent from Earthlings, unable to walk upright even in Mars’s light gravity.
The asteroids also present a potential threat to Humans, since their orbits
sometimes bring them uncomfortably near Earth.
It’s likely that an impact with one millions of years ago led to the extinction of the dinosaurs, and an impact today could easily wipe out Humanity. Many stories center around threats by megalomaniacal villains to draw an asteroid down to Earth, or the efforts of a bold team of adventurers to stop an asteroid from hitting the planet.
Jupiter
Diameter: 140,800 kilometers Distance From Sun: 5.2 AU Gravity: 2.6 G (at cloud tops) Year: 11.9 Earth years Moons: 16 known
Orbital Velocity: 13 kilometers/sec Atmosphere: Dense
Jupiter is the biggest of the planets, with a mass 300 times that of Earth. It’s about twice as big as all the other objects in the Solar System combined (excepting the Sun, of course). Jupiter itself is no place for Humans — the gravity at the cloud tops is almost 3 G, and there’s no identifiable solid surface.
The atmosphere is mostly hydrogen and helium, though clouds of ammonia and methane give the planet its swirling yellow and scarlet appearance.
The most notable feature of Jupiter is the Great Red Spot, a storm covering an area about twice the size of Earth. Its origins are unknown, though it’s apparently been in existence for at least 300 years (probably much longer), and becomes redder during periods of heightened solar activity.
Carl Sagan once speculated about the possibil-ity of life in Jupiter’s titanic atmosphere, envisioning huge balloon creatures drifting among the clouds.
While current theories suggest Jovian life is unlikely, nobody has actually gone there to check. However, at least one of Jupiter’s moons, Europa, is currently on the hot list of places where Humans might find alien life.
EUROPA
Diameter: 3,120 kilometers
Distance From Jupiter: 671,000 kilometers Gravity: 0.13 G
Month: 3.55 days (82 hours)
Orbital Velocity: 13.74 kilometers/sec Atmosphere: Trace
Europa is a large moon, comparable in size to Earth’s Moon, composed largely of ices with a solid core. It has no atmosphere to speak of, and its surface is dirty water ice. What makes Europa interesting is that it has almost no craters, which suggests some process constantly resurfaces the crust. Current theories suggest that beneath the icy surface lies a deep ocean of liquid water, kept warm by tidal heating from Jupiter. If that is the case, then Europa’s ocean may be home to some form of life.
Adventures on Europa can be exploration missions in search of native life, who’s-hunting-who battles with dangerous Europan predators under the ice, or covert operations to find hidden pirate or rebel bases in the black ocean.
GANYMEDE
Diameter: 5,280 kilometers
Distance From Jupiter: 1,070,000 kilometers Gravity: 0.14 G
Month: 7.16 days
Orbital Velocity: 10.88 kilometers/sec Atmosphere: Trace
Ganymede is the biggest moon in the Solar System — bigger than the planet Mercury, in fact.
It has a metallic core with a crust and mantle of ice, and appears to have active geology driven by tidal forces from Jupiter and the other moons. It was once considered a possible site for terraforming, but now that its composition is known, raising the tempera-ture to habitable levels would create an ocean miles deep. Ganymede has its own magnetic field, shield-ing it somewhat from Jupiter’s radiation belts.
INSIDE JUPITER
Astronauts venturing into the atmosphere of Jupiter are in for an exciting time. The winds are fierce: 360 kilometers per hour in the upper atmosphere, and 500 kph or more deeper down. Storms like the Great Red Spot can last for centuries.
The pressure increases with depth, starting at about .001 atmosphere at the cloud tops and doubling for every 20 kilometers down. Tem-perature is 0 Centigrade at the cloud tops, going up by 1 degree per kilometer of depth. This means explorers could fly through the upper 200 kilometers with reasonable life support systems, but below that the heat and pressure begin to damage, and will eventually destroy, spacecraft.
JUPITER’S RADIATION BELTS
Like everything else about Jupiter, its mag-netic field and radiation belts are huge and pow-erful. The belts extend out from the planet to a distance of half a mil-lion to a milmil-lion kilo-meters, and are at least ten times as intense as Earth’s Van Allen belts.
The inner moons, Io, Europa, and Ganymede, are in the thick of the radiation. Explorers visiting the inner moons of Jupiter need Life Sup-port: High Radiation, or considerable amounts of armor. The icy crust of Europa would offer adequate protection, so radiation suits aren’t necessary down in the subsurface ocean.
Saturn
Diameter: 120,300 kilometers Distance From Sun: 9.5 AU Gravity: 1.2 G (at cloud tops) Year: 29.5 Earth years
Moons: 18 known (probably more) Orbital Velocity: 9.6 kilometers/sec Atmosphere: Dense
Saturn is much like Jupiter, but is less than a third as massive (only 95 times the mass of the Earth), less dense, and less active. Like Jupiter, it has an atmosphere of hydrogen and helium, with clouds of methane and ammonia giving it a pale yellow color from space. It is best known for its spectacular ring system, a belt of ice and rock par-ticles extending from a few thousand kilometers above the surface to about 70,000 kilometers up.
Saturn has a large family of moons, ranging from tiny asteroidal chunks like Pan or Atlas, to good-sized objects like Tethys and Iapetus, to the aptly-named Titan.
The astounding beauty of Saturn and its rings make it a natural tourist attraction for a spacefar-ing future society (as in Arthur C. Clarke’s short story “Saturn Rising”). There might be orbiting resorts or hotels on the moons. Tycoons could build luxurious private stations just to admire the view. The rings could also hide fugitives or space pirates.
TITAN
Diameter: 5,150 kilometers
Distance From Saturn: 1.2 million kilometers Gravity: 0.14 G
Month: 16 days.
Orbital Velocity: 5.58 kilometers/sec
Atmosphere: Standard (atmospheric pressure approximately 1.5)
The biggest moon of Saturn, Titan is a remark-able place. Its surface is hidden by a dense atmo-sphere of nitrogen and methane, with a surface pressure half again as great as Earth’s. Current theories suggest Titan’s surface may have oceans of liquid methane dotted with islands or continents of dirty ice. The tempera-ture on Titan is a chilly 94 degrees Kelvin (-194 Centigrade). It is thought that Titan is very rich in organic chemicals, making it potentially very valuable to future space colonists.
Titan could possibly be home to some form of exotic life, based on ammonia or methane (see page 88). As a colony world it might be the Persian Gulf of the outer system, with valuable carbon for sale.
Uranus
Diameter: 56,300 kilometers Distance From Sun: 19 AU Gravity: 0.8 G (at cloud tops) Year: 84 Earth years
Moons: 21 (possibly more).
Orbital Velocity: 6.8 kilometers/sec Atmosphere: Dense
Uranus is very similar to Neptune, a small gas giant with an atmosphere of hydrogen, colored
blue-green by methane clouds. It is notable because of its extreme axial tilt — 97 degrees. It rolls about the outer Solar System on its side, surrounded by a dark ring of dust and pebbles. Uranus has a large family of moons, mostly small objects 100 kilometers across or smaller. The five biggest are Titania (1,578 kilometers), Oberon (1,522 kilo-meters), Umbriel (1,170 kilokilo-meters), Ariel (1,158 kilometers), and Miranda (see below). The moons of Uranus, unlike those of other planets in the Solar System, have names taken from the plays of Shake-speare instead of classical mythology.
Conditions within Uranus and Neptune are considerably less violent and dangerous than in Jupiter or Saturn. The gravity is bearable, and the winds aren’t as fierce. Explorers in high-tech bal-loon-spaceships might venture beneath the clouds, and thus encounter airborne life.
MIRANDA
Diameter: 470 kilometers
Distance From Uranus: 129,780 kilometers Gravity: .008 G
Month: 1.41 days
Orbital Velocity: 6.68 kilometers/sec Atmosphere: None
Miranda is not the biggest moon of Uranus, but it is a remarkable object. Photographs from the Voyager probes show Miranda’s surface as a chaotic jigsaw-puzzle of different terrains, and the current theory is that the moon was literally smashed apart by some immense impact long ago, and the jum-bled fragments gradually fell together again. This suggests the interior of Miranda may be a maze of faults and crevices, possibly extending all the way to the core. Who knows what adventurers might find in such deep places?
Neptune
Diameter: 49,900 kilometers Distance From Sun: 30 AU Gravity: 1.2 G (at cloud tops) Year: 164.8 Earth years Moons: 8 known
Orbital Velocity: 5 kilometers/sec Atmosphere: Dense
Neptune’s day is 16 hours long, which makes it slightly flattened at the poles. Like Jupiter and Saturn, it emits more heat than it receives from the Sun. Its atmosphere is mostly hydrogen and helium, with clouds of methane. All of Neptune’s moons except Triton are small objects, no more than 400 kilometers across. The planet also has faint dusty rings.
Like Uranus, Neptune qualifies as a “bearable”
gas giant. As the outermost major planet, it might serve as the launching station for interstellar mis-sions, especially for slower-than-light ships using huge nuclear rocket motors. Neptune’s atmosphere would provide fuel.
TRITON
Diameter: 2,700 kilometers
Distance From Neptune: 354,800 kilometers Gravity: 0.076 G
Month: -5.88 days
Orbital Velocity: -4.39 kilometers/sec Atmosphere: None
Triton’s orbit is retrograde, going the opposite direction from most moons in the Solar System (hence the “negative” numbers for its month and orbital velocity). It is also highly tilted, 157 degrees from the plane of Neptune’s equator. These suggest it was captured, rather than forming along with Neptune itself. Its most likely origin is the Kuiper Belt, but GMs can have Triton be an interstellar interloper, perhaps with artifacts or even survivors in stasis. Triton is composed of rock and ice, with a fairly smooth icy surface, though there are patches of old cratered terrain. It has a very thin atmo-sphere of methane and nitrogen.
Pluto
Diameter: 2,300 kilometers Distance From Sun: 39 AU Gravity: .06 G
Year: 247.7 Earth years
Orbital Velocity: 4.7 kilometers/sec Atmosphere: None
Pluto is the outermost planet, and is the excep-tion to almost every rule. It is tiny, less than half as massive as Mars. It has a giant moon, Charon, which is nearly as big as Pluto itself. Its orbit is tilted and very eccentric, at times taking it within the orbit of Neptune (the two planets orbit in a resonant relationship so there is no danger of a col-lision). A growing number of astronomers prefer to classify Pluto as the biggest of the Kuiper Belt
objects rather than a true planet, but in the popular mind it’s unlikely to be kicked out of the planet club. Pluto has a trace atmosphere of nitrogen with tiny amounts of methane.
CHARON
Diameter: 1,200 kilometers
Distance From Pluto: 20,000 kilometers Gravity: .015 G
Month: 6.4 days
Orbital Velocity: 0.23 kilometers/sec Atmosphere: None
Pluto and Charon essentially compose a
“double planet.” Pluto is the larger of the pair, with a mass 5 times that of its big moon. Both Charon and Pluto are tidally locked, keeping the same face towards each other. The two worlds are composed almost entirely of ices — mostly nitrogen, methane, carbon monoxide, and water in solid form.