Exploration of the Solar System
Week 12, Topic 19
Let’s start by pulling together all the objects we have discussed in the class this far. We can place them in different categories. You should visually plot them up on a map of the solar system.
A few lectures ago I discussed meteors (shooting stars) that produce meteorites. These come from the asteroid belt. There is another type of meteor that is probably better known, and these are associated with meteor showers.
Meteor showers are times of the year when a higher than normal number (sometimes a much higher than normal) of meteors are seen, that come from the same general region on the sky. These are pieces of matter that flake off comets. As comets outgas, they blow pieces of ice and carbonaceous material out into space, that follows the comet.
Meteor showers correspond to comets whose orbits intersect the orbit of Earth. They occur annually because this is the time of year when the Earth passes through the comet debris path. When you look at the radiant of the meteor shower, you are looking back along the path of the comet in space.
Look at Table 15.1 on p343 of your book. It gives a list of the prominent meteor showers. Notice especially the Eta Aquarids, which reach maximum on a few days around May 4. These are pieces off the famous Comet Halley. In addition, next week the famous Leonid meteor shower reaches a maximum. It will peak after midnight on the night of November 16-17 (Tuesday-Wednesday). Look for very fast meteors coming out of the east, radiating from the constellation of Leo.
A few lectures ago, I discussed the fact that we now have spacecraft far beyond the most distant major planets, and beyond the Kuiper Belt. These are the Voyager 1 and Voyager 2 spacecraft.
It is believed that these spacecraft are close to the Heliopause, which is the boundary between the part of space where the gas was part of the Sun, and the interstellar medium, which is the extended atmosphere of the Milky Way galaxy.
A artist’s conception of a Voyager spacecraft in deep space is shown below.
One of the instruments carried on the Voyagers was built here at the University of Iowa. It was the Very Low Frequency radio receiver built by the group of Professor Donald Gurnett. The antennas for this experiment are the two slender wires seen in the picture above.
One of the exciting discoveries from Voyager has been made by Dr. William Kurth and Dr. Gurnett of Iowa. They looked at the spectrum of radio emission observed over many years. Their data are shown in the figure below.
The coordinate on the x axis is time, running from 1982 to nearly the present time. The second x axis is the distance of the Voyager spacecraft from the Sun (remember, we are 1 on this scale.
The y axis is the frequency of the radio waves, just like you select on your AM of FM radio. The frequency range measured by this instrument is from 1 kiloHertz (kHz) to 4 kHz. By comparison, your AM radio tunes from about 600 kHz to 600 kHz. Finally, the color of the display indicates the intensity of the radio emission. Dark blue indicates just background noise, bright red means a strong signal. This diagram shows three “events” during the 20 years of observation, when the instrument started receiving strong emission in the frequency range of 2 – 4 kHz. It is believed that these are times when the heliopause started “transmitting”.
Astronomers would dearly love to send a spacecraft out through the heliopause, and into the interstellar medium. That won’t be possible with the Voyagers; they are moving too slowly and are near the end of their power. What is planned however, (although not yet approved for launch) is the Interstellar Medium Mission which is a spacecraft designed to leave the solar system at high speed, reach a heliocentric distance of 200 – 500 astronomical units in 20 years, make extensive measurements of the interstellar medium, then head out to deep space, never to return. Stay tuned to developments via the NASA websites.
It is inconceivable that one could teach a course on the solar system, and not talk about the Sun. That will be clear when I start giving some of the stats below. As will also be clear, the properties of the Sun “pull together” a lot of our understanding of the solar system.
In the title of this lecture I have given the astronomical name for stars like the Sun. I think it is as important to recognize as the term “Homo Sapiens” for us. The term also hints that there are many others like the dear old Sun, that gives us a great insight in and of itself.
Let’s take a look at it. This is how it appeared earlier this week, on November 3.
You can see the famous sunspots, of which I will say more later.
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You can get similarly good views of the Sun through relatively small telescopes. We have a very good solar telescope on the roof of our building, Van Allen Hall at the University of Iowa. A web site with many views of the Sun is the home page of the SOHO spacecraft at:
http://sohowww.nascom.nasa.gov
The structure of this lecture will be in the form of a number of (perhaps irritating) questions. This is intended to get you thinking right at the start of this section. I will then proceed to answer these questions in the course of the lecture. Here goes.
Before contemplating the
consequences of this, you should ask yourself how we know this? Next, how does its chemical composition
compare with that of the planets. An additional interesting aspect of the
Sun is that we can get direct samples of it in the solar wind.
The direct way of measuring the composition of the Sun came from the Genesis spacecraft, which was on station for two years deep in space, collecting samples of the solar wind. The solar wind is so tenuous however, that even after scooping it up for two years, the total amount of material collected was less than a thousandth of a gram (a milligram). By contrast, the recommended portion of breakfast cereal is 35 – 50 grams.
The picture below shows the Genesis spacecraft with the “wafers” or collectors exposed to the solar wind.
Genesis returned to Earth in September. It was supposed to have softly parachuted to the surface and been picked up by a helicopter. Instead, this is what it looked like on its return.
The canisters containing the ultra-pure wafers broke open. However, my bet is that scientists will still be able to salvage most of the information they were looking for. It will just take more work.
One of the most fascinating facts about the Sun is that it is a controlled hydrogen bomb going off. I won’t talk about that much more; it is discussed at length in our other astronomy course, 29:50, Stars, Galaxies, and the Universe. It is relevant for a topic we’ll come to later, which is how long the Sun can shine. That will be important for our discussion of the future of the solar system.