When trying to decide which planet to observe, it is important to keep in mind the limitations of HST and of observations in general. What one can actually observe is often quite remote from the question one wants to answer. As an example, let me describe my motivation: individuals and classes might want to make a similar hierarchy for their planet.
Astronomers don't really look directly through the HST at a planet, star or galaxy. Instead, they use four major scientific instruments that are attached to the HST. Two of these are cameras, and two are instruments known as spectrographs.
The cameras take PICTURES like the ones you see in books or on the LFHST or SCScI web pages. The spectrographs split the light of planets, stars and galaxies into a spectrum of colors (like a prism does with sunlight) and then make a one-dimensional image of this spectrum.
The cameras and spectrographs don't use film but instead take pictures electronically rather like your home camcorder. But the equipment in the HST's cameras are much more sensitiuve to light than your home equipment because most objects in space are very faint. Also, pictures are recorded in only black and white but using computers on earth, astronomers can combine two or more black and white pictures taken through different colored filters to make a picture in color. (We'll explain more of how this is done in a later posting).
One of the HST's cameras is known as the Wide-Field/Planetary Camera (WFPC pronounced "wiff pick" for short). It records its images on four devices known as CCD's (Charge Coupled Devices) instead of film. The four CCD's are arranged to make a bigger square than could be done with just one CCD alone (like combining four squares that are next to each other on a checker board).
The other HST camera is known as the Faint Object Camera (FOC). It works more like an old fashioned TV camera but the point is that in both cameras the light coming into the HST from the planet, star or galaxy gets converted into electricity and this electrical signal is then beamed down to earth. The brighter the object at a particular point in the picture, the stronger will be the signal at that point. Then back on earth, the signals can be converted back into a picture of what the HST is looking at. (We'll will explain this in more detail later and you will be able to do activities from your LFHST Teacher's Guide to better understand how this all works.)
Again, the same kind of process takes place in your home camcorder. And just like with your home camcorder, the cameras on HST can be used over and over again to take pictures of different things. This is another advantage of electronic cameras over film cameras because with film cameras a piece of film can only hold one image and then a new piece of film must be put into the camera if another picture is to be taken.
Get up close to your TV set and you will see that what looks like a complete picture from a distance of several feet is really made up of a whole bunch of little dots. The same is true of the pictures that come down from the HST. Each little dot is known as a "picture element" or "pixel" for short. The bigger an object appears in the HST's cameras, the more pixels will make up its image and the more detail we will see. If an image has more detail, astronomers say that it has greater "spatial resolution". Also, because different types are technology are used in the two HST camers, the WFPC has larger pixels than the FOC and so, in general, can see less detail. But the WFPC allows us to see a larger area of the sky. And there are other differences. The FOC is generally able to see farther into the ultraviolet part of the spectrum than the WFPC but the WFPC can see farther into the red portion of the spectrum (and so, for example, can see those methane cloud features that Rita Beebe talked about a few days ago).
Now let's turn to the other two instruments -- the spectrographs. The spectrographs also act like electronic cameras but again they create images of the SPECTRA of planets, stars and galaxies NOT pictures of the planets, stars and galaxies themselves. Spectra, however are very important to astronomers because an analysis of an object's spectrum can tell us an extraordinary amount about the object including things like what it is made of and its temperature. The spectrographs also really focus in on a very small portion of the sky so, in the case of large planets like Jupiter, you don't take a spectrum of the whole planet but instead only a selected small portion. This, however, would allow you to concentrate on, for example, Jupiter's Great Red Spot or the rings of Uranus and not the planet itself if that's what you wanted to do.
The two HST spectrographs are known as the Faint Object Spectrograph (FOS) and the Goddard High Resolution Spectrometer (GHRS). (So many words and letters to remember!) Again, both of these instruments spread out the light of planets, stars and galaxies into a spectrum of colors. The GHRS, however, spreads the light out more (kind of like spreading out a dab of peanut butter more over a wider piece of bread). Because it spreads out the spectrum more, it allows astronomers to see more detail in the spectrum. That's why we call it a HIGH RESOLUTION Spectrometer. But because it does spread the spectrum out more, the spectrum is that much fainter (like the peanut better is that much thinner). For this reason, we cannot typically use the GHRS on really faint objects. Fortunately, planets are very bright. There is another difference, too. The GHRS works better in the far ultraviolet than the FOS (so if, for some reason you need to see an object's far ultraviolet spectrum, you'll have to use the GHRS).
So as you consider which planet you want to observe with the HST, there are many things to consider. Just what specific question or questions you think you want to answer about which planet will, in turn, point you toward using one instrument on the HST vs another. And the capabilities and limitations of the different instruments will, in turn, give you an idea of which questions you can hope to answer and which you cannot. There will always be tradeoffs. Welcome to doing science!
As a general rule, I have found that the more you understand something, the more you may find data of greater detail (higher spectral resolution) to be of value. It's like with anything else -- first you want to look at the overall picture, then you want to zero in for a closer look at specific details. That's what you do when looking at a picture in a magazine and that's what we do in studying a planet. Then you can take a close up picture of that special feature or you can zero in and take a spectrum. And the same is true for spectra. First, you might want to take a look at the whole spectrum not very spread out and then zero in for a closer look (a higher resolution spectrum) of a particular part of the spectrum.
Finally, let's talk a little about how much of the sky can be seen with the HST at one time. Astronomers measure the width of most objects in the sky in a unit called an arc second. A full circle has 360 degrees in it. So the sky, from say the eastern horizon to a point straight overhead (the zenith) to the western horizon contains a half circle's whole of degrees or 180 degrees. Each degree can be broken down into 60 minutes of arc (this has nothing to do with time, we just use the same words). And each minute of arc can, in turn, be divided into 60 seconds of arc. By way of an example, the moon is about 30 minutes of arc across in the sky (or, in other words, about 30 x 60 = 1800 seconds of arc across).
Now, to put things into perspective, the instrument with the widest field of view on the HST is the WFPC and, at any one time, it can only see a little square in the sky about 160 seconds of arc on a side -- far, far less than the diameter of the full moon. Remember also that this image is actually made up of little pixels. Given the size of the individual pixels, we can further calculate that the WFPC can see details on an object that are roughly one tenth of one second of arc across (which can also be written 0.1"). Put a fancier way, we would say that the spatial resolution of the WFPC is about 0.1". For different objects at different distances from earth, this corresponds to different actual sized objects in miles or kilometers. (Part of this camera, however, can actually see even clearer. In a small square only 32 seconds of arc on a side, the camera can actually see details down to a resolution of only 0.043'".
By comparison, the FOC see an area in the sky about 7 seconds of arc on a side (also written 7'). The spectrographs see much, much smaller sections of the sky but remember, we don't use them to take "pretty picture" but rather to zero in on stars and parts of planets and galaxies to get their spectra.
Note also that the pixels of the planetary camera of the WFPC are 0.046 arcseconds (0.046") on a side-- slightly larger than the point-spread function for the HST. Thus you can use some mathematical jiggery-pokery to improve the spatial resolution of well-exposed WFPC images. This aspect of data reduction is somewhat controversial, however, and some of the planetary advocates may want to comment...
Some planetary facts: At the time of observation (3/14/96),
Io will pass in front of Jupiter early in the morning (about midnight EST).
Triton will be 15" from Neptune (near closest approach)-- you'll get both in one PC image, although Triton will only be 3-4 pixels across.
Charon will be near closest approach to Pluto, although it will be only 1 PC pixel, maybe 4 FOC pixels, across.