Geography of the Solar System

The Solar System - the sun and all things bound to it - is our neighbourhood in space. Previously the object of only astronomical study, it is now accessible to human endeavours, at least in its nearest parts. Since this is a Geography course, we will think of it in geographical terms: places, resources, hazards, etc. We will only briefly discuss the Solar System in astronomical terms.


The central mass which holds the Solar System together and provides us with warmth and light. The sun, a fairly average star, produces power by converting hydrogen to helium. It will do so for several billion years more, gradually becoming warmer. Some projections suggest it will become too warm for comfortable life on Earth in a billion years or so. The sun ejects sub-atomic particles which stream out through the solar system, the 'solar wind'. These produce the aurora (northern lights) on Earth. Larger solar flares disrupt communications and power grids, damage satellites, and could kill unprotected astronauts. The sun is thus both a natural resource and a potential hazard.
Sun images
Solar flares and hazards


The sun is orbited by nine planets. Or is it eight? The definition of 'planet' was traditional until recently. The word had no exact meaning but implied that the object orbited a star and was quite large, but not a star itself. In 2006 astronomers tried to create a new definition, but there is widespread dissatisfaction with the result. The reason: an object larger than Pluto was discovered. Is it a new planet? If we can add to the list of planets when new objects are found, which new things are planets? We might eventually have 50 planets. Is that OK? If these new objects are not going to be planets, why is Pluto a planet?

Planets include mid-sized rocky worlds (Mercury, Venus, Earth, Mars) and giant gas planets (Jupiter, Saturn, Uranus, Neptune). Smaller mixed ice/rock worlds like Pluto are now called "dwarf planets". Every planet is unique, presenting different versions of geology and/or meteorology. Looking at them all enriches the theoretical backgrounds of all the physical sciences - the study is called comparative planetology. Planets are now being discovered around other stars.
Views of the Solar System
Planetary Photojournal
Extra-solar planets

Moons (satellites)

A satellite (= a moon) orbits a planet rather than a star. Satellites are found in all sizes up to 5000 km across, which is bigger than Pluto and about the size of Mercury. The only difference between small planets and large moons is where they happen to orbit. Satellites may be rocky (moons of Earth and Mars) or icy (moons of Saturn) or a mixture of the two. Some are geologically active, with volcanoes and fractured surfaces. Europa (moon of Jupiter) seems to have a liquid ocean under its smooth icy surface. Titan (moon of Saturn) has a thick hazy atmosphere. Enceladus (moon of Saturn) emits jets of water vapour from warm vents at its south pole. Triton (moon of Neptune) has an atmosphere and geyser-like eruptions from its surface. All four gas giant planets have rings made of billions of small particles. Each particle is a tiny moon, and there is no exact dividing line between a moon and a ring particle, no lower size limit to help define a moon.
Views of the Solar System (look at individual objects...)
Planetary Photojournal (look at individual objects...)


Asteroids are also called 'minor planets' - they are just small planets orbiting the sun. Most orbit in a broad belt between Mars and Jupiter, but others are found between the planets or crossing planetary orbits. Some come close to Earth - the NEOs (Near-Earth Asteroids). An asteroid whose orbit crosses a planet's orbit will eventually hit it or be deflected. The collision forms a crater - that's why many worlds are covered with craters. Deflected asteroids either fall into the sun, escape from the solar system, or end up in another planet-crossing orbit which just delays their fate. Although Earth has never experienced a serious asteroid impact during the human historical period, impacts are inevitable - we live in a shooting gallery. We are just beginning to plan how to prevent such collisions in the future. Asteroids are not just hazards - they also may contain valuable resources for future space developments.
Near-Earth Asteroids
NEAR mission website
images from NEAR mission
Earth impact craters


Comets are seen in the night sky as a smeared or fuzzy light among the stars. We are seeing a cloud of gas and dust being blown off a small icy world (the nucleus of the comet) as its ice evaporates. These small icy worlds are in effect just ice-rich asteroids - there is no sharp division between comets and asteroids, as used to be thought. Comets can hit a planet to make a crater, and also provide water, carbon compounds or other resources to future space travellers.
Stardust comet nucleus images
Deep Impact comet nucleus images

Kuiper Belt and Oort Cloud

The Kuiper Belt is a zone outside the orbits of the main planets where numerous ice-rich asteroids or comets orbit. They are probably left over from the formation of the larger planets. Pluto was considered the largest of the 'Kuiper-Belt Objects' (KBOs), but other KBOs as big as Pluto or bigger are being found now. The Oort Cloud is a huge spherical cloud of comets surrounding the sun, extending out a good way towards nearby stars. The inner parts of the Oort Cloud merge with the Kuiper Belt.
Oort Cloud (bottom of page)


Gravity determines orbits. An orbit is the path one object follows in space as a result of its current velocity and the gravity it experiences. Orbits (if not disturbed by other objects) are ellipses. An object moves faster when experiencing a stronger gravitational attraction (for instance when closer to the object it is orbiting). A typical satellite in Low Earth Orbit (say 250 km high) takes about 90 minutes to circle the Earth, whereas the Moon, 400,000 km out, takes a month. It takes a lot of energy to get off Earth's surface and into orbit, and a lot of energy to change the plane of an orbit. We can't just push the Hubble Space Telescope into the same orbit as the Space Station because they are 'weightless' - a big rocket would be needed because of the very high speeds involved. The most efficient way to travel from one orbit to another (e.g. Earth to Mars) is via a 'transfer orbit', an ellipse whose ends are tangent to the other two orbits. These paths are long and slow but require less fuel that a direct route.
How orbits work


Low Earth Orbit - within a few hundred km of Earth's surface. This is where the Space Shuttle and Space Station orbit, together with satellites that need to be close to Earth, such as remote sensing satellites. An orbit over the equator always flies over the same areas, but a polar orbit can fly over any point as the planet rotates under it, much more useful for remote sensing. These orbits are not high enough to avoid our atmosphere altogether, so they eventually lose energy to friction (drag) and drop down, burning up as they re-enter the atmosphere. The Space Station has to be raised from time to time to prevent this. LEO is getting crowded - collisions are a potential hazard, and Shuttle flights are planned to avoid known 'space junk'.
Low Earth Orbit
Debris hazard in orbit

Geostationary Orbit

As orbits get higher the time they take gets longer, so at some height an orbit must take 24 hours. An object in an equatorial 24 hour orbit stays over the same place all the time, making antenna pointing easy. This is where most TV and other communication satellites are, so satellite dishes don't have to move to track them. This special orbit is called Geostationary Orbit (GEO). GEO is getting a bit crowded too! Some equatorial nations have laid claims to the parts of GEO above their territories. International agreements govern the allocation of spaces within this special orbit.
Clarke's idea.
GEO (note this page uses different terms)


Lagrange Points

Lagrange points are places where objects seem to orbit 'in formation', staying in the same place relative to another orbiting object. For instance, imagine an object orbiting Earth as far away as the Moon but opposite the Moon in the sky. It stays in the same place relative to the Moon. So does an object orbiting the Moon, but far enough out that it takes a month to orbit the Moon. It and the Moon move around Earth together, 'in formation'. These points are called Lagrange points after the person who first descibed them. Stable points require one big object to orbit around, one medium object orbiting the first, and a small object at the Lagrange Point. Some are fairly stable, others need small rocket burns from time to time to keep them stable. Some asteroids travel in Lagrange-type paths relative to Jupiter - they are called Trojan asteroids. Points like these can be used to park spacecraft for research (Genesis mission, future James Webb Telescope). They have also been proposed as space station sites (the famous L5 plan for orbiting colonies). Lagrange points exist for any pair of larger objects (e.g. Earth and Sun; Earth and Moon, Jupiter and Ganymede).
Lagrange points
space colonization

Van Allen belts

LEO is getting crowded, but we can't just go higher to put people in a safer environment. The first discovery of any scientific satellite was that Earth is surrounded by dangerous radiation belts, named the Van Allen belts after the person whose instrument found them. All astronauts orbit safely below them, and the Apollo astronauts passed through them quickly enough that they were not affected. The inner belt extends from about 600 to 12000 km high, the outer belt runs out to about 50000 km. The belts are held in place by Earth's magnetic field. Because the field is not symmetrical, the inner belt dips down especially low over the South Atlantic Ocean, an area called the South Atlantic Anomaly. This is especially dangerous to electronic equipment, and some satellites have been damaged as they pass through it. So - space is NOT the same everywhere...
Van Allen belts
South Atlantic Anomaly

The Moon

Earth's natural satellite. The Moon is a rocky world only 400 000 km away, an obvious early target for spacecraft and astronauts. It is very unearthlike - no air, no water, no active volcanoes. As such it preserves its ancient geological record much better than Earth. In effect it is a geological museum, telling us what our world was like before billions of years of erosion and continental movements erased the record of our earlier years. The moon is scientifically valuable for that reason, and may also hold resources we can use in future space activities. Ice may exist in permanently shadowed polar craters. The moon rotates in the same time it takes to orbit Earth, so one side always faces us.
Browse these lunar atlases!
Lunar missions
map the Moon


The only planet likely to be hospitable enough for people to visit. Mars is smaller than Earth, cold and with a very thin atmosphere. It is intermediate in geological development between Earth and the Moon. It has had water on its surface in the past, though now all water is frozen. Or is it? In 2006 evidence was announced for brief flows of muddy water even today. Life? - we don't know but it may be possible. Mars is a focus for much of the scientific exploration of the solar system in coming decades.
Mars rover updates
Mars exploration
Current water activity?
map Mars