The Sun
The largest, and possibly most important, object in the solar system is the Sun.  An immense fusion reactor, it is the center of the solar system both gravitationally and in terms of power, which it radiates as light.  On its surface, the sun has a temperature of about 6000 Kelvin (or 10,000 degrees Farenheit), which corresponds to an object that is glowing bright yellow (hence its color).  In the interior, the temperature rises as high as 15,000,000 Kelvin at the core, where fusion is actively taking place.  Although it has lived half of its life already, and will end by destroying the Earth and all of the other inner planets, we have something like 4.5 billion years before we have to worry.

Mercury
Mercury, closest to the sun, is small and airless, pocked by craters.  At a glance, it is difficult to tell Mercury apart from the Moon.  Mercury has the greatest extremes of temperature of any planet, with extremely high daytime temperatures and extremely cold night temperatures on the dark side.  This is exacerbated by the fact that it is gravitationally locked with the sun, meaning that the same side always faces the Sun, while the dark side is never touched by the Sun's rays (which may make the night side habitable).

Venus
Next outwards is Venus, a planet with strong similarities to Earth.  Both planets have a similar mass and composition, however, the proximity of Venus to the Sun has caused a runaway greenhouse effect.  Enormously high temperatures combined with an atmosphere with large quantities of sulfuric acid make this planet totally uninhabitable.  Of note, it also has an unusual rotation pattern, where it rotates very slowly in the opposite direction from the rest of the planets (retrograde rotation), leading to a day that is nearly the same length as Venus's year.

Earth/Moon
The third planet, Earth, is already well known to anyone reading this, so I'll skip to the Moon.  The only extraterrestrial body we've ever landed men on, the surface of the moon is the best known of any extraterrestial body.  Due to the effect of the Earth upon its orbit, the moon is locked into a rotation where the same side always faces Earth.  Thus, you can always see the same features on its face, and it was not until Russian probes in the 1960s that men ever saw the back side of the Moon (to which no manned flight was ever made).  Possessing no atmosphere, the Moon suffers from radiation and meteor impacts, which make life impossible without heavy shielding and extensive life-support systems.  While the discovery of water ice in small quantities in craters on the Moon has recently attracted interest, it has long been known to have large quantities of oxygen, iron, titanium and aluminum within a few feet of the surface.  Unfortunately, however, the limited scope of missions to the surface (manned and unmanned), has not yet allowed us to discover what, if any, resources may be buried at even moderate depths.

Mars
Further yet is Mars.  After the Moon, this is the most explored body in the solar system, and the most likely target for a future manned mission.  Significantly smaller than Earth, its gravity is too weak to hold onto lighter elements that would make up a significant atmosphere.  It does have a tenuous atmosphere with about 1% the density of Earth's, but this is too little to breathe,  is made up mostly of carbon dioxide (95%, and 1% is the toxic level for humans) and has no appreciable oxygen.  Water is also scarce, and indeed cannot exist in liquid form on the surface, due to the low pressure.  There is strong evidence in the form of erosion patterns that Mars at one time (about 3 billion years ago), must have had a much thicker atmosphere and liquid water on the surface, leading many to believe that there might still be large quantities of water below the surface and that life may have arisen on Mars, just as it did on Earth in the same period.  Having all of the requirements of life, if only in very small quantities, Mars is a very attractive option for a long-term colony, especially if water can indeed be found in copious quantities.  Some theorists even advocate various plans which would Terraform Mars to make it very Earth-like.  While that sounds like a nice idea, there are a number of technical problems, including the fact that Mars simply does not have the gravity to hold onto an atmosphere, even if we can create it.  Also, it troubles me that we would blindly set about destroying the current state of the planet in a way that we would never do on Earth for fear of destroying a single species or landmark.  The moons of Mars, Phobos and Deimos, are often also considered for future colonies.  Due to their tiny size, they have almost no appreciable gravity, making them ideal staging platforms for missions to Mars or further out into the solar system.

Asteroid Belt
The asteroid belt is usually listed next, and I will adhere to tradition.  However, it is very important to note that while a great many of the asteroids are neatly orbiting where they are drawn between Mars and Jupiter, a great many are elsewhere in the solar system, those inside Earth's orbit, those that cross it, the entire stretch between Mars and Jupiter, and many more out beyond.  In fact, the most important ones, both in terms of possible resources and in danger to Earth, are the 2000 or so that periodically cross our orbit.  In general, the asteroids were formed by unknown and likely varied processes very different than those which formed the planets.  In size, most are quite small, with the total mass likely to be less than that of the Moon.  They are, however, possibly the best source for raw materials.  In the case of the planets, their molten pasts allowed most of the heavy elements to separate out and sink towards the center, thus making them unobtainable to us by any technology yet discovered.  For asteroids, this process never occurred.  Thus, it is quite possible to find very rich ores of most metals, while at the same time offering a decent quantity of the lighter elements that make up living matter.

Outer System (Jupiter-Oort Cloud)

Last Updated 2/26/05