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Thomas J. Wdowiak is an Athena scientist who takes time out of his busy schedule to show kids how to do fun science experiments at home. When he isn't teaching at the University of Alabama at Birmingham or training at the Jet Propulsion Lab in California, he writes for the local newspaper in Birmingham in the "Just for Kids" section. He goes by "Tommy Test Tubes," a nickname given to him when he was a kid. Now "Tommy Test Tubes" contributes his experience and knowledge to the Athena web page with a new column. If you have a question for Tom, click here.

How far is Mars from Earth?

First the short answer. Mars has an average distance from the Sun that is about 1.5 times that of the Earth's average distance from the Sun. This means that when Earth and Mars are closest to each other, the distance would be about 1.5 - 1.0 =0.5 times the average distance of Earth from the Sun. When Earth and Mars are on opposite sides of the Sun, the distance between would be about 1.5 + 1.0 = 2.5 times the average distance of the Earth from the Sun. Astronomers, and later space exploration scientists, have been working at measuring the average Earth-Sun distance, called the Astronomical Unit, for several hundred years and arrived at a value of about 149,598,000 kilometers (since a spacecraft crashed into Mars because someone didn't use units of measure like meters, kilograms, etc... I steadfastly refuse to state such distances in miles).

If this is all you wanted to know that's fine. If you want to continue the story, read on!

When you're thinking about taking a trip, one of the first things that comes to mind is "how far away is it?" Until space flight began over forty years ago this was a relatively simple question to ask and get an answer for. You get out a map and, using the distance scale generally provided, measured how far apart places are on the surface of the Earth. Even if you had to use a globe and stretch a piece of string between two locations, it still was an easy thing to do. This was because of three reasons: the first being that people, over hundreds of years of history, had traveled to enough places on Earth to make the map; the second being that you only have to deal with the two dimensions of the Earth's surface; and finally, places on Earth don't change position much in less than millions of years.

For the Solar System it's a whole different kind of thing. Robotic spacecraft and the astronauts going to the moon have made it possible to construct a very accurate "map" of the Solar System, but it's in three dimensions, and every "place" is moving all the time!

The continuous motion is what keeps everything; Earth, Mars, etc... from falling into the Sun. If an object is placed anywhere in the Solar System such that at first it is not moving, it will begin to "fall" toward the Sun, eventually hitting it. In fact about every two weeks a comet (link to: http://seds.lpl.arizona.edu/billa/tnp/comets.html) does exactly that, something that is very difficult to observe because of the brightness of the Sun.

If instead of just letting go, you also give a good enough sidewise push, the object would miss the Sun, curving around it, and come back to where you started again. That's how an orbit works.

When the sidewise motion is substantial, the orbit never comes near the Sun, which is the situation for the Earth, Mars, and most other things in the Solar System. In fact that's what makes the Solar System a "permanent" thing that, after looking at it long enough, becomes predictable. Don't forget that because this takes place in three dimensions, the individual orbits are tilted individually, although not that much (Pluto has the greatest "tilt" which astronomers call inclination).

Another thing to remember is that, in order to miss the Sun when falling toward it, the "sidewise" motion has to be a sufficient amount. Since an object falling toward the Sun with enough motion to miss it goes back out to the distance from which you start it at, the orbit is not a circle! When the astronomer Johannes Kepler discovered this about 400 years ago, the story is that he really got upset with this complication. He came to terms with it and convinced himself (and everyone else after) that the shape of an orbit is the shape of the geometric closed curve called an "ellipse".

Now an ellipse, instead of having a single point from which it is drawn, as a circle does, has two points called "foci". Kepler proved that the Sun is at one of these, the same one, for every orbit of every body in the Solar System. There is nothing, I repeat nothing, at the other (each is called a focus). This means that the Earth, Mars, or any other Solar System body is sometimes at one point closest to the Sun (this point is called perihelion), and sometimes at the opposite side of the orbit, farthest from the Sun (this point is called aphelion).

Kepler realized that the best way to deal with a closest distance to the Sun and a farthest point was to use the average distance. You can look up the average distance for many bodies in the Solar System in Astronomy books, where it is given in units like meters or kilometers, but is actually easier than that. Since the time of Kepler, astronomers have used the average distance of the Earth from the Sun as the standard. That distance is 1 Astronomical Unit, or 1 AU for short. Also, the time it takes the Earth to travel once around its orbit, 365.25 days or one year, is used as the standard for time. Astronomers call it the orbital period.

Kepler found that if you took the orbital period of a planet in years and squared it, multiplied it by itself, and cubed the average distance from the Sun (multiplied it by itself two times), both answers were the same! Issac Newton later explained why, and the reason was because this is how gravity behaves. Since it is easier to measure how long it takes a body like Mars to make one orbit than it is to directly measure the distance from the Sun (you have to be a really good surveyor) astronomers could use what Kepler figured out to calculate (by squaring and getting the cube root some way) the average distance of a planet form the Sun.

It takes 687 days for Mars to orbit the Sun, which is 1.88 Earth years. 1.88 x 1.88 = 3.5378 which has a cube root of 1.5237 meaning that on average Mars is 1.5237 times the Earth's average distance from the Sun.

If Earth and Mars are closest to each other on the same side of the Sun, the distance between them would be about 1.5 - 1.0 = 0.5 times the average distance of the Earth from the Sun. If the worlds are on opposite sides, the separation would be about 1.5 + 1.0 = 2.5 times the average distance of the Earth from the Sun.

Because the orbits are elliptical, oriented in different directions, and are tilted from each other, these numbers are only approximate. Astronomers and the people who do space navigation have expert knowledge of where the Earth, Mars, and other Solar System bodies are right down to the second. This is necessary to carry out a space mission. In the United States the U.S. Naval Observatory in Washington DC is the arm of the government holding this responsibility.

Finally, if you want to know the numbers in kilometers (or even miles) you multiply the distance in Astronomical Units (AU) times the average of the Earth's orbit, which is about 149,597,900 kilometers. If you really feel that you must use miles, I'll let you figure out how to do that.

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Tommy Test Tubes!
Tommy Test Tubes!