29:52 Exploration of the Solar System

            Lecture 1                                                                                                         Overview of the Solar System

 

Incipitur: Introduction and Niceties

·                     Purpose and goal of class; concentration on solar system astronomy. A case where one can develop and introductory science class almost totally with "hot off the press material".   A perfect example is the Mars Exploration Rover spacecraft that have been reporting back from the surface of Mars since last January,  or the Cassini spacecraft which arrived at Saturn this summer. 

·                     Course will also cover some very fundamental material that was known to the ancients. Will give you the pleasure of identifying the objects we discuss in the night sky, and seeing for yourself some of the fundamental things like the orbital motion of the planets (we can do this in the course of the semester). 

·                     This semester we will have a chance to see Saturn in the evening sky later on.  Jupiter and Venus will remain morning objects (get up before dawn!).  The best planets to observe in the evening sky will be Uranus and Neptune, but you will need binoculars and star charts to observe them.  An apology to my students from Modern Astronomy from last semester. 

            Topic 1: The Size and Geography of the Solar System, its Relation to the Stellar Universe

            Let's pick some numbers out of the Appendices and talk about them.

(1) Radius of Earth = 6378 kilometers. (Basic unit of distance in the course will be kilometer; more fundamentally the meter.1 kilometer = 0.6214 miles.  Imagine drawing a circle that represents the diameter of our home planet, the Earth. 

            Let's go out in Outer Space

(2) Typical orbit of space shuttle is 250 - 400 kilometers above sea level. A long way to fall but not very far from Earth.

(3) Distance to Moon (nearest major astronomical object) = 384,000 kilometers. I should be picky and say that this is the average distance between the Earth and Moon. For those of you who have had some astronomy, I can be even more exact and say this is the semimajor axis.  The Moon is the furthest human beings have gone out into space, and that is likely to remain so for the foreseeable future.

(4) The next jump is a big one, to the Sun, which is the dominant object in the sky.

           

The average distance of the Sun is 149.6 million kilometers (93 million miles). In terms of scientific notation this is 1.496 X 10**11 meters. This distance is so important in solar system astronomy that is has a special name, the astronomical units or au. The average distances of the planets in the inner solar system from the Sun are as follows:

·                     Mercury 0.387 au

·                     Venus 0.723 au

·                     Earth 1.0000000 au

·                     Mars 1.524 au

 

(5) The outer solar system contains big, massive, gaseous, weird planets.

Ø       There is  a feature on the Cassini spacecraft web page that can be used to show the positions of the outer planets at the present time. .  http://saturn.jpl.nasa.gov/index.cfm

Ø        

You can see how much further Jupiter is from the Sun than the Earth; it is the closest of the outer or Jovian planets. The distances of the Jovian planets are as follows

·                     Jupiter  5.20  au

·                     Saturn 9.54 au

·                     Uranus 19.19 au

·                     Neptune 30.06 au

A reality check: Uranus is 2.85 billion kilometers from Earth (1.77 billion miles) and Neptune is 4.35 billion kilometers (2.70 billion miles).

 

> Comments on where the planets are right now:   Venus is the very bright object in the eastern sky just before dawn.  On August 31,  get up before dawn.  The bright object very close to Venus will be the planet Saturn.  The other naked eye planets will be too close to the Sun to be visible in the first part of the semester. 

(6) Neptune is the furthest out of the major planets, but objects are further out, including ones you could stand on. An important class is the Kuiper Belt objects, which are frozen worlds (including the planet Pluto) that extend outwards tens of astronomical units. It is believed they are the most primitive objects in the solar system.

 

Last Spring,  what may be a new type of object was discovered.  This is the object Sedna.  We will discuss it further later in the semester. 

            Even further out is the Oort Cloud a frozen locker for comets, which extends out to tens of thousands of astronomical units.

(7) Finally we have the starry sky. When we get to the stars, we need a whole new unit of distance.

           

We use the parsec, which is 206,265 au (3.26 light years). The typical spacing between stars is a couple of parsecs.

But now let's go back to the solar system