PART I: INTRODUCTION

Our solar system is composed of 9 planets which orbit the sun. All the planets have different sizes, densities, and compositions, and some have satellites orbiting them like our Moon orbits the Earth. We can compare properties such as these for the different planets, and deduce how the solar system formed, look for patterns in its structure, and contrast conditions on other planets to those on Earth.

Information we want to know about planets:

• radius: What is the distance from the center of the planet to the surface?

• mass: How much does the planet weigh?

• average density: How much does a piece of the planet of a certain volume weigh?

• composition: What is the planet made of? (ice, dirt, rock, gas, or anything else)

• surface temperature: How hot or cold would it be if we were on the planet?

• orbital distance: How far is the planet (or satellite) from the body it's orbiting?

• orbital period: How long does it take a planet to go around its orbit once?

• rotational period: How long does it take a planet to spin once on its axis, like a top?

Measuring these values:

• For the Earth:

  Q: How would you measure the radius of the Earth? What are some problems with this?

  A: One way to measure the Earth's radius would be to take a huge tape measure and stretch it all the way from the North Pole to the South Pole. Since the Earth is basically a sphere, we can determine its radius this way. (distance from pole to pole = pi*r)

  Problems: This assumes the Earth is a perfect sphere. Also, where could you find a big enough tape measure?

  Q: How would you measure the density of the Earth? What are some problems with this?

  A: We can dig up pieces of the Earth from all over the world, measure their volumes and how much they weigh, and try to figure out the average density of the Earth.

  Problems: We can't dig all the way to the center of the Earth, so how can we determine what the density there is? We have to guess.

  Q: How would you measure the average temperature of the Earth? Problems?

  A: We can put people with thermometers all over the world, measure the temperature at each location and try to compute an average temperature.

  Problems: The temperature changes daily (day and night), seasonally (winter and summer), and with location (it's a lot hotter in Arizona than at the North Pole!).

  Q: How would you measure the orbital period and the rotational period of the Earth? Problems?

  A: We can watch the stars as we go around the sun, and measure how long it takes the Earth to go around the sun once (1 year), as well as how long it takes the Earth to turn once on its axis (1 day).

  Problems: How accurate is this?

• For other planets:

  Q: How would you measure values like these for the other planets? What are some problems with each method?

  A1: Look at the planets from Earth

  Problems: They're far away, so we can't get very accurate measurements.

  A2: Send robot space probes to the planets

  Problems: We have to program the computers on the probes far ahead of time, they can only make limited decisions on their own, and the planets are so far away that it takes a long time for the probe to send data back to humans on Earth so that they can tell it what to do next.

  A3: Send people to the planets in spaceships

  Problems: It takes a long time to get to the other planets, and humans might be in danger. Also, the spaceship would need to hold enough food and other supplies to get to the planet, let the people make their measurements, and get them back home safely.

  Q: Which of the methods discussed above are best for which planets? Which have we done already? Which do you think we should do more of, for which planets?

  A1: Look at planets from Earth: This is best for nearby planets (Mercury, Venus, Mars) or very large planets (Jupiter).

  A2: Robot space probes: These have been sent to fly by all the planets except Pluto, to orbit Venus, the Moon, Mars, and Jupiter, and to land on Venus, the Moon, Mars, and Jupiter (atmosphere). Future missions are planned to orbit the Moon and Saturn, and to land on Mars and Saturn's moon Titan.

  A3: People: The Moon is the only place that humans have been sent, though there are tentative plans to send people to Mars sometime early in the next century.

It is relatively easy to measure the physical properties of the Earth, since, after all, we live here. It's much harder to measure them for the other planets, though, since the only other world people have traveled to so far is the Moon. We have sent computerized spacecraft to all the planets except Pluto, however, and while computers can't do everything that humans can do, they don't mind being cooped up in a tiny spaceship for the months or years it takes to get to another planet. They don't need to eat, either! Robot spacecraft can also survive in the hostile conditions on other planets -- we take for granted the fact that the Earth has air we can breathe, food we can eat, and temperatures that don't let us burn up or freeze to death. Conditions on other planets aren't nearly as nice. Computerized spacecraft can land on other planets and not have to worry as much about the temperature or the fact that there's no air to breathe. Probably, people will travel to other planets someday, but until then, we will continue sending robot probes to gather information and help future astronauts know what to expect.

Although our resources for studying the other planets are limited compared to those available for the Earth, we have refined our measurements over the years and currently have fairly good data for most of the solar system. By comparing information about the other planets to the Earth, which we know the most about, we can try to discover what the other planets are made of (so far, we only have pieces of the Moon and Mars to measure in labs on Earth), how they formed, and what their history has been like compared to that of the Earth. We can also look at pictures of the surfaces of other planets, and try to determine what the geology and weather are like -- so far we've seen exciting things like volcanoes on Jupiter's moon Io and the Great Red Spot, a giant storm in Jupiter's atmosphere. The science of looking at other planets and using this information to learn more about them, and about Earth, is called comparative planetology, and it helps us find out how the Earth formed, what it was like long ago, and what it might be like in the future.