The Solar System

Our Solar System is basically a star with a very small amount of space crud orbiting around it. The Sun holds 99.9% of all the mass in the system. The Solar System is very large and mostly empty space. The planets are VERY small compared to the distance between them. As an example, consider that the Moon is about 30 Earth diameters away, but the nearest-approaching planet, namely Venus, gets only as close as about 3375 Earth diameters. Mars approaches only as close as 4312 Earth diameters. The Apollo crews reached the Moon in 3 days. Getting to Mars takes over 6 months.

The ancient observers knew the planets only through Saturn; the outer planets are too faint to see. Actually, Uranus CAN just barely be seen with the naked eye; you just need a good star chart, the exact position of Uranus, and some practice in order to find it.

The planets can be assembled into three groups - terrestrial, jovian, and Pluto. The little outer planet doesn't fit in either of the other groups. Look at Table 4.1 - it lists a lot of planetary properties. You might try the orbit radii (semi-major axis) and periods in Kepler's Third Law (see sec. 1.4). Table 4.2 lists comparisons between the two groups. Note that the terrestrial planets are warmer than the Jovians as an obvious result of being closer to the Sun.

Pluto is the obvious oddball. It is the smallest planet; at 1,500 miles diameter it's about 3/4 the size of our Moon. Its orbit is the most inclined of the planets and it is farthest from the Sun. Its density is greater than the Jovians but well less than the terrestrials. It does, however, have a moon of its own: Charon. We now have the fabulous images and other data from New Horizons, the spacecraft that flew by Pluto in the summer of 2015.

In 2006 the International Astronomical Union (IAU) demoted Pluto to the status of "dwarf planet." It annoyed a lot of Pluto fans, but Pluto IS smaller than our Moon.


The next group of Solar System objects consists of asteroids and meteoroids. Our book contains, for the first time I have found it, some definition of the size break; under 100 meters is a meteoroid and over it is an asteroid. A large number of these orbit between Mars and jupiter, but don't imagine that they are as thick as a swarm of gnats. A number of planet-exploring spacecraft have flown right through the asteroid belt safely.

The largest asteroid is Ceres, discovered in 1800; it is about 580 miles in diameter. Pallas is next, found about a year later. In third place is Vesta at about half the diameter of Ceres. On some occasions, one of the large asteroids will get bright enough (just barely) for naked-eye visibility. The NEAR (Near Earth Asteroid Rendezvous) spacecraft orbited the asteroid Eros for a year, then landed! The landing wasn't in the original plan, but after NEAR successfully completed its entire planned mission, the mission team decided to try a landing. Even though NEAR was not designed for landing and had no landing gear, the landing was completely successful.

Your book has pictures of four asteroids - Eros, Gaspra, Ida, and Mathilde. Each of these pictures came from a deep-space probe (either Galileo or NEAR). Asteroids show the same kind of cratering that appears on every solid surface in the Solar System.

Some more asteroid pictures.

Thre are very few large asteroids and LOTS of small ones. Remember that anything larger than 100 meters is considered an asteroid. Any large ones whose orbits passed close enough to allow them to hit Earth have already done so. Earth was fiercely bombarded in the early years of the Solar System. What's left is lots of small ones whose orbits may be perturbed by Jupiter so that they have a small chance of hitting a planet. The Solar System is very large and Earth is a very small target, so major impacts are now extremely rare.


Meteoroids are small versions of asteroids; they are small pieces of solid material orbiting the Sun. They are less than 100 meters across (anything larger is an asteroid). Meteoroids range from there down to dust particles. The "shooting star" you see in the night sky could be the size of a pea.

The meteor you see is a meteoroid being heated by collision with Earth's atmosphere. Air SEEMS quite thin and soft - until you try to move through it at 7 to 15 miles per second. At speeds like this it's almost like hitting a wall. Collisions with air molecules heat the meteoroid. The air ahead of the fragment is heated by compression and can't get out of the way. The energy of the fragment's motion is converted into heat and heats the little piece. Small pieces vaporize.

So - do these pieces ever survive the atmosphere and hit the ground? Yes. When that happens, we call the piece that lands a meteorite. The survival of a piece is determined by its size and what it is made of. First - it must be made of something tough enough to stand the forces that result from hitting the atmosphere. Second - it must be large enough that it cannot be heated all the way though in the short time it takes to pass through the atmosphere. Imagine a rock 100 meters across. In the few seconds that it takes to traverse the atmosphere the heating will only penetrate a very short distance. The outer layer of the object will get singed and burned, but the vast bulk of it will not get heated; heat just won't flow through 50 meters of rock in 10 seconds.

When it comes to meteors, there are three words to learn.

They look similar but have very specific meanings. A "meteor" is that bright streak that is often called a "shooting star." A meteoroid is a small piece of solar-system stuff orbiting the Sun. A meteorite is that meteoroid of stuff AFTER it has plunged through our atmosphere making that bright streak across the sky (the meteor). Got It???

Speaking of meteorites, there are two general classes of them - stony and iron. Stony types account for about 93% of all falls and irons make the other 7%.

There are two ways to find a meteorite.

The first one is called a "find" and the second is called a "fall." The term "fall" implies that the meteorite is recovered soon after it was observed falling. Funny thing - finds are about 80% irons while falls are about 95% stones. So 95% of what falls is stone but 80% of what gets found is iron! This is actually not mysterious. For a meteorite to get found, it must look different from the rocks in the area. An iron meteorite is almost guaranteed to do that. A stone meteorite can look a lot like any other rock, and someone not familiar with the appearance of meteorites will likely miss it.

Asteroids and Earth

Most asteroids remain between Mars and Jupiter, but the gravity of one of those planets can perturb an orbit and send an asteroid in closer to the Sun. Some of these cross Earth's orbit and get called Earth-crossing asteroids. These objects have the potential to hit Earth, although we are a small target.

Be sure to read Discovery 4-1 beginning on page 110.

When you look at the asteroid pictures in your book (taken by spacecraft) along with pictures of the terrestrial planets, along with a lot of moons of various planets, you see craters. It is obvious that everything got bombarded very heavily at one time in the past. Strangely, the Earth is not covered with craters like everything else. The reason for this is that Earth has surface conditions that do not exist anywhere else in the Solar System. We have an active atmosphere that produces rain, a lot of surface water, and continents that move. All of these things produces influences that erode and cover up craters. Well over 100 craters have been found on Earth. Some are visible on the surface while many more are covered completely.

Craters come in quite a range of condition.

The list should grow as more craters are discovered.