From the Classroom to Outer Space
II. The FUSE Satellite - Observing from Space
Fact Sheet: Space Based Astronomy
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Exploring Our Universe:
From the Classroom to Outer Space II. The FUSE Satellite - Observing from Space Fact Sheet: Space Based Astronomy |
Figure 1. FUSE in preparation
The FUSE satellite consists of two primary sections, the spacecraft and the science instrument. The spacecraft contains all of the elements necessary for powering and pointing the satellite: the attitude control system, the solar panels, communications electronics, and antennas. The science instrument collects the light of distant objects and contains the equipment necessary to disperse and record the light: the telescope mirrors, the spectrometer (and its electronic detectors), and an electronic guide camera called the Fine Error Sensor (or FES). The spacecraft and the science instrument each have their own computers, which together coordinate the activities of the satellite.
FUSE's spectrometer resolves light into very fine details. This will allow astronomers to examine carefully the spectral lines that give information about the physical properties of celestial objects. (Since astronomers can't go to the stars and galaxies they wish to study, they must instead examine the light they emit.) The resolution of the instruments on FUSE is one part in 24,000! This would be like identifying the edge of one particular sheet of paper in a stack ten feet high!
Ultraviolet light is particularly difficult to analyze. 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, silicon carbide (SiC) and
lithium fluoride over aluminum (LiF - Al), are the best materials known
for reflecting far ultraviolet light, and therefore are used for the
FUSE mirrors and gratings.
Figure 2. The path of light through the FUSE instrument.
When FUSE is pointed at a light source, four mirrors reflect ultraviolet light from the source onto a set of diffraction gratings. (The sketch above represents only half of the instrument.) To get another view of the way light passes through the FUSE instrument see the animation at http://fuse.pha.jhu.edu/educ/animation/ANIMATION2%5e3.gif. The diffraction gratings disperse the ultraviolet light into a spectrum. This spectrum is focused at two electronic detectors, which record the amount of light, or intensity, received at different wavelengths. Astronomers will make graphs comparing the intensity of ultraviolet light at different wavelengths. In Activity #4 you will trace the path of light through the FUSE science instrument in order to exercise your understanding of the physics of light, optics and trigonometry.
The diffraction gratings are the basic elements of the science instrument. FUSE's high resolution gratings are approximately a foot square, and contain some 5300 - 5800 lines per millimeter etched onto its surface. (The exact number changes as a function of position across each grating, and they are slightly curved, so we can't strictly call them parallel!) These etchings are what disperse far ultraviolet light into a spectrum for analysis, and provide the high spectral resolution of the spectrograph. It is interesting to note that if the lines on a FUSE grating were all placed end to end, the resulting line would be over 300 miles long!
Figure 3. One of four FUSE high resolution diffraction
gratings photographed in the laboratory before being mounted on the telescope.
The word diffraction refers to the behavior of light (or any wave) when it interacts with objects that have dimensions comparable to its wavelength. For example, when light passes through a very narrow slit, it spreads out. Moreover, if you look at the light coming through by putting a light sensitive screen behind the slit, you see a series of dark and light bands. If the light contains more than one wavelength, the light bands will show each wavelength at a slightly different position. This effect is enhanced by using many slits instead of one. If the material into which the slits are cut is a reflecting material, the reflected light will be spread in the same way. In activity #4, you will apply your knowledge of triangle trigonometry to derive an equation that predicts the angle of reflection for different wavelengths.
Most of the electromagnetic radiation in space never reaches us on Earth. A good thing too- if we were constantly bombarded by high energy gamma rays, X-rays and ultraviolet light, life on Earth would be impossible! We are shielded from most electromagnetic radiation by the Earth's atmosphere. The atmosphere allows visible light to get through easily. The main reason our eyes are sensitive to visible light is because that's what gets through. If the atmosphere let X-ray light pass through, then we would maybe have X-ray vision! In addition to visible light, the atmosphere is also transparent to radio waves and a small portion of ultraviolet light. This small amount of ultraviolet light that reaches us causes the suntans and sunburns that some people experience.
FUSE is designed to study light in the far ultraviolet region of the spectrum because light at these wavelengths provides astronomers with unique data about the universe. These wavelengths of light (from 90 to 120 nanometers) are nearly in the X-ray range of the spectrum. Light from the far ultraviolet region does not penetrate the Earth's atmosphere. To be able to study this light, FUSE instruments must be placed above most of the atmosphere.
Actually, there is a thin residual atmosphere at the altitude of FUSE's
orbit which has two important effects on the satellite. Its drag
on the satellite will increase with time and eventually cause FUSE to drop
toward Earth. As the satellite moves through the thicker atmosphere,
friction will cause FUSE to heat up and meet its fiery end.
In Activity #2, you will use a mathematical model to predict the time of
FUSE's demise. You will also apply the concepts of frequency and period
and make use of Kepler's Law of Periods which gives the relationship
between a satellites radius and its period.
One critical aspect of operating a satellite is keeping accurate track of the satellite's position. This is important for two main reasons. One is that mission planners must know where the satellite is so they can point FUSE at the particular celestial objects to be studied. The other important reason is to know when people on Earth will be able to make communication links with the satellite, so they can give it instructions and download data from the instruments.
All the commands for the satellite's computers to execute are prepared at the Satellite Control Center, located at the Johns Hopkins University in Baltimore, Maryland. Then they are sent by data link to the ground station in Puerto Rico, and from there uplinked to the satellite. Astronomers and engineers work together to coordinate scheduling of the observations FUSE will make and send the instructions up to the satellite for execution.
Figure 4. The FUSE Satellite Control Center
Actually, most of the time it is not possible to communicate with the
satellite and it must operate on its own. We can only "talk" to FUSE
when it is within range of a ground station. The primary ground station
antenna is located at the University of Puerto Rico, Mayaguez, and the
team 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. In Activity #3, you will calculate
the length of time the satellite is in contact with the ground station
at Mayaguez by applying concepts from solid geometry and using the definition
of angular speed. As a bonus, you will also be given instructions to use
the same commercial software used at the control center.
The FUSE Orbit
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 100 minutes to go around once. Why does FUSE, or any satellite, stay in orbit? What determines a satellite's speed? In Activity #1, you will find that, to good approximation, you can predict how FUSE will move using Newton's Second Law of Motion (the relationship between force and acceleration), Newton's Universal Law of Gravity( the description of the force of attraction between any two masses), and Kepler's Law of Periods (the relationship between a satellite's period and radius).
Imagine Earth the size of a soccer ball, surrounded by its atmosphere and lying within the orbits of many man-made satellites. In your mental picture, how thick is the atmosphere and how high are the satellites? As part of activity #3, you will draw a two dimensional scale diagram of Earth and the position of FUSE in its orbit and gain an improved understanding of what a thin covering the atmosphere is for Earth.
Where FUSE is right now? 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.
The Mission Operations Team calculates FUSE's position in orbit using ground
tracking data provided by NORAD ( which actually tracks all kinds of stuff
up there!). Activity #3 will show you how to get a new set of "orbital
elements" that can be used to predict FUSE's position.
How does FUSE point at stars and galaxies in space?
Pointing the telescope in the right direction and locking onto the astronomical (or celestial) objects of interest is something FUSE has been designed 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. The FES camera takes pictures, and by identifying the stars seen, the on-board computer can find EXACTLY where the instrument is pointed! Once its position is determined, FUSE can aim in a new direction and find another set of expected stars, including the object scientists want it to observe in ultraviolet light.
Knowing where FUSE should point is only part of the challenge. To actually turn the telescope requires the ability to make precise rotations about three axes in space. But the FUSE spacecraft does not include any jets or rockets to turn its telescopes around this sky. Instead it uses basic physics: the law of conservation of angular momentum. A physicist uses the word "conservation" to mean something does not change. "Angular momentum" refers to a quantity that can be calculated for a rotating object that is related to its distribution of mass about its axis of rotation and its rate of rotation. Conservation of angular momentum can explain why a skater spins faster when he pulls his arms in toward his body and , in Activity #5 it is used to explore one of the clever methods used to turn FUSE.
Making an object turn in space is not the same as making an object turn on Earth. Think of a car on a highway making a turn as the road curves. The forces that cause the car to change direction are a result of contact forces with the surface of the road. These forces exist because gravity pulls the car toward the center of Earth and the road surface, in turn, pushes up on the car's tires. In space FUSE is in a state of "free fall", so there are no external contact forces to act on the satellite. Therefore to change FUSE's orientation, we must use devices that are completely different from anything used on Earth.
Course adjustments in orientation are made by long coils of current
carrying wire, called "torque bars", that interact with Earth's magnetic
field. Fine adjustments are made with devices called "reaction wheels",
also called "momentum wheels". These are basically spinning disks.
When the spin rate is changed, FUSE turns. To understand why, go
to Activity #5. Completing this exercise will develop your understanding
of the physics concepts of torque (the rotational analog of force)
and conservation of angular momentum.
