Obviously, when considering our life on earth, the most important body to consider is the Sun. Our Sun, as a main sequence star, will burn steadily for another 5 billion years or so. That's not to say it will emit the exact same amount of energy over all this time. Over a long timescale (billions of years) the Sun will gradually increase its brightness, maybe by a factor of 2. (That is, 5 billion years from now, it will be twice as bright as it is now.) But that won't concern us --- what does concern us is smaller fluctuations that happen on a more rapid timescale.

The outer part of the Sun is very turbulent. Moreover, the Sun rotates in a very interesting manner. Near the equator, the Sun rotates once every 25 days; however, near the pole, the rotation rate is once every 30 days. This differential rotation has an interesting consequence. Imagine the Sun as a magnet, with a north pole and a south pole. Now imagine that there are magnetic field lines that start at the north pole, extend over the surface of the Sun, and end at the south pole. Because the Sun rotates differentially, these magnetic field lines (which are coupled to the material in the Sun) get stretched, and contorted tremendously. Eventually, there is a ``pop'' where the lines break, and then reconnect. This produces energy. (Note: if my explanation is a bit vague, it's probably because astronomers are themselves confused about exactly how this process works.)

As a result of the interaction between its magnetic field and differential rotation, the surface of the Sun occasionally enters a ``stormy season'' with alot of high energy events. When these periods of high solar activity occur, we see sunspots which are areas of the Sun that are slightly cooler than the background, but with alot of magnetic energy, solar prominences, where protons and alpha particles (i.e., hydrogen and helium nuclei) are lifted far from the Sun, and solar flares, which are intense explosions that heat sections of the Sun to over a million degrees.

Do any of these events affect the earth? Yes, they do. These magnetic events throw off protons and alpha particles that leave the Sun completely. Under normal circumstances, the Sun is always losing some mass, in its solar wind. Normally, this mass-loss is very low (maybe 0.0000001 earth-masses a year), but during the Sun's stormy season, the solar wind is increased dramatically. When these particles come near the Earth, they are forced to follow the Earth's magnetic field lines. They are then funneled to the place where these field lines intersect the atmosphere, i.e., near the Earth's north and south pole. The particles hit nitrogen and oxygen atoms near the top of the earth's atmosphere, and eject their electrons. These free electrons then collide with other atoms, pushing their electrons up into higher energy states. When the electrons fall back down, they emit light via emission lines. (The process is virtually identical to that which lights this room.) When this happens, we see an aurora. Aurorae don't affect the earth directly (other than make it very pretty), but the extra ionization that occurs in the upper atmosphere can disrupt communications on earth which rely on satellites or signals that bounce off the ionosphere.

(Note that without the atmosphere and the earth's magnetic field, these solar wind particles would quickly kill us. The Moon has neither an atmosphere nor a magnetic field, so to survive the particles ejected from the Sun during a solar flare, people on the Moon would have to be buried deep (many feet) below the Moon's surface. Similarly, astronauts in space can survive one of these events if they are in low earth orbit, below the magnetic field lines of the earth. However, once outside these radiation belts, there is no protection.)

All those explosions and mass ejections from the Sun have another effect: during periods of increased solar activity, the earth receives slightly more energy. The planet is therefore warmed a bit more than normal. Observations indicate that solar activity follows a cycle of 11 years, that is, every 11 years there is a year where sunspots, solar flares, prominences, etc., are common. Examination of the Carbon-14 in tree-rings on earth (which indicate how well the tree grew that year) shows that when solar activity is high, tree growth (on average) is good.

(On the other hand, people have looked for sunspots every since the time of Galileo. In the 1600's, there weren't many! During this period, the sun was abnormally quiet. This Maunder minimum in the sunspot cycle corresponds to a time where the winters in Europe and Asia were abnormally cold -- a mini ice-age.) Similarly long "quiet sun" periods (and "active sun" periods exist in records from the middle ages (when they kept track of aurorae).

Even small changes in the energy output of the Sun can change the climate on the Earth. But other objects in space can have an affect as well. As popularized by a couple of recent movies, let's consider comets.

Comets are essentially dirtly iceballs. They were probably formed when the solar system was formed, but due to gravitational interactions between them and other planets (principally Jupiter), virtually all these objects have been ejected from the inner solar system and now reside in the Oort Cloud, many thousands of astronomical units from the Sun. These objects have highly eccentric orbits. That is, they have orbits that are almost purely radial: although their closest approach to the Sun may be less than 1 A.U., their furthest distance is in the Oort Cloud. From Kepler's 3rd law, the period of a typical comet with a semi-major axis of 100,000 A.U. is over 30 million years!

Most of the life of a comet is spent moving very slowly in the regions far beyond Pluto. But occasionally, one will enter the solar system. According to Kepler's 2nd law, the comet will pick up speed. Hence, if it comes into the inner part of the solar system, it won't stay here for long. Also, since the comet is essentially made of ice, the heat from the Sun will evaporate particles off the comet. Once in space, these particles will be blown away from the comet, by radiation pressure from the Sun's light, and by the solar wind. Thus, the comet will have a tail, extending out away from the Sun.

As mentioned above, the period of most comets is many millions of years, and some (many?) will make only 1 approach to the Sun ever. So how can bodies like Halley's comet be observed many times over the history of human civilization? Most likely, these short period comets started out like any other comet, but during their approach to the Sun, their orbit was perturbed by another gravitational body -- probably Jupiter. As a result, these periodic comets have lost some energy, and are now in smaller orbits. (Halley's comet now only goes out to the space between Uranus and Neptune, and it's period is only 76 years.) There aren't many comets (or asteroids, for that matter), that have orbits like Halley's comet. Bodies like Halley's comets have their orbits continually distorted and, over time are usually ejected from the Solar System. (Either that, or they crash into something.) In addition, comets that hang around in the inner part of the solar system too long get evaporated.

Some comets come close to crossing the Earth's path (but thankfully not when the Earth is there). Although a direct collision is improbable, the earth often moves through the small particles left behind by the comet, i.e., the centimeter sized (and smaller) pieces of the tail. These tiny ice crystals hit the earth's atmosphere and high speed (after all, the earth is moving around the Sun at 30 km/s, or 70,000 miles per hour) and glow brightly as they evaporate. We see this as a meteor shower. Since the earth repeats its journal around the Sun every year, these meteor showers are periodic. Some of the best meteor showers are listed below.

(Meteor shows take their names from where the meteors appear to be coming from. Just like parallel railroad tracks appear to meet at a common point in the distance, the meteor streaks all appear to come from a single location. The name of the meteor shower is the constellation that this location is in.)

It is possible that the earth will intersect the orbit of a comet (or an asteroid) precisely. In this case, there will be a collision. Depending on the size of the body colliding and where it hits, the body may make a hole is someone's house (a fist size meteorite), destroy everything over a 30 km radius (as a 60-m comet did in Tunguska, Siberia in 1908), or throw enough dust and dirt into the atmosphere to block out the Sun for over 50 years (as a 10-km size body would). The dinosaurs were probably killed by such a body, which hit in the Yucatan pennisula of Mexico 65 million years ago. (The evidence is that the dinosaur killer was an asteroid, since all over the earth, a thin layer of iridium, a common element found in these hunks of rock/iron, can be found at the proper geological age.) In fact, over 70% of all the species of life on earth became extinct due to that impact.

The end of the Cretaceous period is not the only time in earth's history when mass extinctions occurred. Paleontologists have identified many such times where large numbers of species have died out suddenly. Presumably, each is associated with some sort of impact.