Earth and Moon photographed by the Galileo spacecraft as it passed by on its way to Jupiter in December 1992.
Lecture 5
Earth and Moon photographed by the Galileo spacecraft as it passed by on its way to Jupiter in December 1992.
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 activities, mainly with robots but even with people 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.
Sun
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 nuclear fusion, 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
Planets
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 worlds like Pluto, usually mixtures of ice and rock about 1000 to 2000 km across, are now called "dwarf planets". Even smaller things are called asteroids or comets. Every one of these worlds 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, well over a thousand already and many are very different from our own planets.
Solar system guide
Another solar system guide
Views of the Solar System
Planetary Geomorphology monthly images
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 what they orbit around, a star or a planet. 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. Others have hardly changed since they formed, and are just covered with craters. Io (a moon of Jupiter) has dozens of active volcanoes. Europa (a moon of Jupiter) seems to have a liquid ocean under its smooth icy surface. Titan (a moon of Saturn) has a thick hazy atmosphere, and lakes of liquid methane at its poles. Enceladus (moon of Saturn) emits jets of water vapour from warm vents at its south pole. Triton (a moon of Neptune) has an atmosphere and gas-jets erupting 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 - link to moons from their planets
Planetary Photojournal (look at individual objects...)
Asteroids
Asteroids are also called 'minor planets' - they are just very small planets orbiting the sun, hundreds of thousands of them. Most of them 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 Objects). An asteroid whose orbit crosses a planet's orbit will eventually hit it or be deflected by the planet's gravity. If it hits another world 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. See links to asteroid pictures in Lecture 6.
Solar system guide - link to the asteroid and comet pages
A bit more information.
Near-Earth Asteroids
images from NEAR mission
Earth impact craters
resources
Comets
Comets are seen in the night sky as a smeared or fuzzy patch of 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 (sublimes). The ice is not just frozen water, it includes many other frozen substances such as carbon dioxide or methane. These small icy worlds formed far from the Sun where low temperatures allowed ices to accumulate, and we usually see them when they are deflected into the inner solar system and the warmer temperatures release the gas. Comets are in effect just ice-rich asteroids - there is no sharp division between comets and asteroids, as people used to think. Comets can hit a planet to make a crater, and also provide water, carbon compounds or other resources to future space travellers. See links to comet nucleus pictures in Lecture 6.
Comets
Comet gallery
Stardust comet nucleus images
Deep Impact comet nucleus images
Rosetta comet mission
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 in the same plane as the planets. 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. Some people have suggested there might be big planets far out in or beyond the Kuiper Belt, but nothing has been seen despite years of searching. The Oort Cloud is a huge spherical cloud of comets surrounding the sun, extending out a good way towards nearby stars. It was formed mainly from comets closer to the Sun which were deflected into large orbits by random planetary encounters. The inner parts of the Oort Cloud merge with the Kuiper Belt. It is assumed that manyy stars have an Oort cloud and that some comets may slip from one star's cloud to anothers when stars pass close to each other.
KBOs
Comets from other Oort clouds?
Oort Cloud
Orbits
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 are ellipses unless they are disturbed by the gravity of another object or by using a rocket. 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 300 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 (orientation) 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 and the different orientations of the orbits. 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
Different kinds of orbits
LEO
Low Earth Orbit - within a few hundred km of Earth's surface. This is where the Space Station orbits, together with human crews and any 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, mostly burning up as they re-enter the atmosphere (but big pieces can reach the ground). The Space Station has to be raised from time to time to prevent its orbit decaying. LEO is getting crowded - collisions are a potential hazard, and the Space Station and other objects have to be moved occasionally to avoid known 'space junk'.
High and Low Earth Orbit
Debris hazard in orbit
Chinese anti-satellite test - debris in orbit
Space junk recovered on the ground
Space junk in Kazakhstan
Geostationary Orbit
As orbits get higher the time they take gets longer, so at some height an orbit must take 24 hours, and a satellite there would appear to hover over the point underneath it on Earth. An object in a circular equatorial 24 hour orbit stays over the same place all the time, making antenna pointing very 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 also getting crowded. 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. A similar concept is the Geosynchronous orbit - it also takes 24 hours but it's not above the equator, or not circular. In that case, the satellite seems to wander over the sky in a looping pattern instead of appearing fixed. There are other variations on this, between LEO and GEO, such as the Molniya orbits (see link). When a satellite stops working it is supposed to be removed from its place in GEO so it is not a hazard to other satellites.
Clarke's idea.
Geostationary satellites.
Geostationary satellites.
Geostationary orbit
Molniya orbits.
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, and for any system of two objects (one orbiting the other) there are five Lagrange points. Stable points require one big object to orbit around, one medium object orbiting the first, and a small object at one of the Lagrange points. 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 and some other planets - 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). These sets of five Lagrange points exist for any pair of larger objects (e.g. Earth and Sun; Earth and Moon; Jupiter and Ganymede). At Saturn, several of its small moons are in the Lagrange points of larger moons.
Lagrange points
Lagrange points
Objects at Lagrange points
space colonization
Van Allen radiation 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. Astronauts usually 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 can be 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
Earth and Moon photographed by the Galileo spacecraft as it passed by on its way to Jupiter in December 1992.