- What kind of orbit was FUSE be placed into?
FUSE is in a nearly circular orbit roughly 760 km (475 miles) above the earth's surface. The orbit is inclined 25 degrees with respect to the equator and it will take FUSE about 100 minutes to go around once. - Where is FUSE right now?
We calculate FUSE's position in orbit using ground tracking data provided by NORAD (which actually tracks all kinds of stuff up there!). Each week we get a new set of "orbital elements" that allow us to predict FUSE's position and calculate when we will be able to contact the satellite from the available ground stations.
You can see where FUSE is right now, too! Check out the Heavens Above Satellite Predictions page, which will calculate FUSE's current position in its track around the earth and show you a graphic of it's position. - How do you communicate with FUSE in orbit?
An interesting question. Most of the time, we don't! We can only "talk" to FUSE when it is within range of a ground-station. Our primary ground station antenna is located at the University of Puerto Rico, Mayaguez, and we can see FUSE from this site about 7 times a day for about 12 minutes at a time. During those time periods, FUSE downlinks its stored scientific and engineering data, and new commands are uplinked telling the satellite what to do for the next time period. We may also chat with out satellite a couple of times a day from a ground station in Hawaii, if desired, but this costs the project additional money.
(Check out the FUSE Photo File for images of our antenna in Puerto Rico!) - Does FUSE take pictures?
Well, yes and no. For its primary scientific objectives, FUSE does not take pictures, but rather analyzes far-ultraviolet light by spreading it into a spectrum, much the way a prism disperses white (optical) light into a rainbow. This technique of light analysis, known as spectroscopy, allows all kinds of information to be derived about the objects being observed, such as temperatures, chemical compositions, the velocity of the object with respect to the earth, and more.
FUSE does include an electronic camera that takes optical light pictures in the direction where the telescope is pointing. These images are primarily used for identifying guide stars and verifying where the telescope is pointing at a given time, and otherwise look much the same as a picture from a ground-based telescope. See Mission Status Report 11, which links to the "first light" image with this camera. - What is ultraviolet light?
Ultraviolet and optical light are really very similar; they are both part of a much broader range of light radiation known as the electromagnetic spectrum. It's just that our eyes are not sensitive to ultraviolet light. UV light is just out the "blue end" of the optical light spectrum, and FUSE observes the universe at "far-UV" wavelengths, which are quite a ways out of the optical range. - Why did FUSE have to be launched into space?
Basically because to observe objects in the UV, we have to get telescopes above the earth's atmosphere. The atmosphere filters out all but a tiny amount of UV light, and it's a good thing it does, or we would get "fried"! But there are many facinating things to be learned about the universe by observing UV light, and so we put telescopes in space. - But doesn't the Hubble Space Telescope also observe UV light?
It sure does! (You're right on top of things, aren't you?) Hubble observes optical, UV, and a little bit of near-infrared light (out the "red end" of the optical spectrum). But it would be silly to put up another telescope to does what Hubble does. FUSE complements Hubble, because it observes farther into the ultraviolet (to shorter wavelengths of light) than Hubble can observe. FUSE provides a unique capability to astronomers, a new tool for astronomers to use in learning about the universe. - So how does FUSE "look" at UV light that other telescopes can't see?
As we move to shorter and shorter wavelengths of light, the light becomes harder and harder to reflect from a surface (like a mirror, for instance); that is, the light wants to pass through objects instead of bouncing off. (At wavelengths shorter than ultraviolet light comes X-rays, and you know X-rays like to go "through" things!) This is bad news for a telescope, which has to try and reflect this light to a focus. The "magic" of FUSE lies largely in the special materials that are used as coatings on all of its reflecting surfaces. These materials, known as "silicon carbide" and "lithium fluoride" (over aluminum), are the best materials known for reflecting far-UV light, and they are what make FUSE so special. - How does FUSE find the stars and galaxies it is supposed to point at?
A timeline of observations is scheduled on the ground and uplinked to the satellite. However, actually pointing the telescope in the right direction and locking onto the objects of interest is something we have taught FUSE to do for itself! It takes coordination between the computer on the "spacecraft" and the computer in the telescope itself, as well as information provided by the Fine Error Sensor (FES) guide camera.
Coarse pointing, to about 2 degrees accuracy, can be done by the spacecraft itself using its gyroscopes and devices called magnetometers. But 2 degrees is still 4 times the size of the full moon, a pretty big area on the sky! The FES camera takes pictures, and by identifying the stars we see, we can find EXACTLY where we are pointed! Once we know where we are, we can tell FUSE to move to a new position and find another set of expected stars, including the object we want it to observe in ultraviolet light. And so it goes.Now the clever reader will notice that, although this sounds pretty simple, it really means a lot of work! Every time we point the satellite to a new place on the sky, we have to uplink a table that includes the pattern of stars FUSE should find at that position. If FUSE does not find the expected pattern of stars, it will conclude it is "lost in space" (so to speak), and will just lock onto whatever stars it can find and hold itself there until ground controllers can figure out where we are pointed and what might have gone wrong. We certainly don't want to do that very often, and so great care is taken in selecting the guide star patterns for each pointing.
For more information on how astronomers learn about the universe, visit the site
What are Those
Squiggly Lines? Using Light to Learn About the Universe.
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