A new age in astronomy will dawn with the launch of NASA's Hubble Space Telescope in 1990. Not since Galileo aimed the first crude telescope at the heavens almost 400 years ago has one astronomical device possessed so much potential for uncovering new knowledge about our universe and the laws that govern it.

NASA's Hubble Space Telescope is a large, high-quality optical telescope which will study the cosmos from low-Earth orbit for 15 years or more. It is the product of years of work by scientists and engineers from various NASA centers, private companies, universities, and the international scientific community. The European Space Agency, NASA's international partner, contributed both flight hardware and science personnel.

The observatory is named in honor of Edwin P. Hubble, a noted American astronomer whose observations of other galaxies in the 1920s led to his conclusion that the universe is expanding. One of the most important results the space telescope will obtain is the value of the Hubble constant, which describes the rate of expansion and age of the universe.

The Hubble Space Telescope is a reflecting telescope of much the same design as those invented by Cassegrain and Gregory in the 17th century. What makes this telescope unique is the quality of its mirrors, the sensitivity of its instruments, the precision of its pointing, and above all its position in space.

An Unsurpassed Vantage Point

The most powerful ground-based telescopes are limited because they must view the universe through the hazy veil of Earth's atmosphere. The visible light rays that penetrate our cloak of air are only a small portion of the radiation given off by celestial bodies. Other forms of light are mostly absorbed by the atmosphere, thus the majority of radiation sent out from stars never reaches the Earth's surface. The Hubble Space Telescope will study not only visible light, but also ultraviolet light (like the "black lights" that make fluorescent paints glow) and infrared rays (the kind used to make heat-sensitive photographs in the dark). Much of the information we can uncover from studying stars is found in the infrared and ultraviolet portions of the spectrum.

Even the visible-light images that reach the Earth are distorted as they pass through the atmosphere. The fact that stars seem to twinkle illustrates the turbulence created by colliding masses of hot and cold air. Looking at stars from the Earth is a lot like looking at birds flying overhead from the bottom of a swimming pool.

From its vantage point above the distorting atmosphere, the Hubble Space Telescope will be able to produce a range and quality of images impossible for its ground-based counterparts.

How the Hubble Space Telescope Came to Be

Long before mankind had the ability to go into space, astronomers dreamed of placing a telescope above Earth's obscuring atmosphere. An observatory in space was proposed in 1923 by the German scientist Hermann Oberth, whose work inspired rocket pioneer Dr. Wernher von Braun's interest in space travel. Scientific instruments installed on early rockets, balloons, and satellites in the late 1940s through the early 1960s produced enough exciting scientific revelations to hint at how much remained to be discovered.

In 1962, just four years after NASA was established, a National Academy of Sciences study group recommended the development of a large space telescope as a long-range goal of the fledgling space program. The recommendation was repeated by similar groups in 1965 and 1969.

The first two successful NASA satellites designed for observing the stars were launched in 1968 and in 1972. These Orbiting Astronomical Observatories produced a wealth of information, and support for an even larger, more powerful optical space telescope grew. The approval of the Space Shuttle, with its capacity for manned delivery and servicing of large payloads, made the space telescope concept feasible. NASA selected a team of scientists in 1973 to establish the basic design of such a telescope and its instruments. An expanded group of 60 scientists from 38 institutions was formed to refine those recommendations in 1977. That same year Congress authorized funding for the project.

NASA assigned responsibility for design, development, and construction of the space telescope to the Marshall Space Flight Center in Huntsville, Alabama. Goddard Space Flight Center in Greenbelt, Maryland, was chosen to lead in the development of the scientific instruments and the ground control center. The European Space Agency got involved with the project in 1975.

Marshall selected two primary contractors to build the Hubble Space Telescope. Perkin-Elmer Corporation in Danbury, Connecticut, was chosen to develop the optical system and guidance sensors. Lockheed Missiles and Space Company of Sunnyvale, California, was selected to produce the protective outer shroud and the support systems for the telescope, as well as to assemble the finished product.

The European Space Agency agreed to furnish the solar arrays and one of the scientific instruments. Goddard scientists were selected to develop one instrument, and three of the others became the responsibility of scientists at major universities.

The Goddard Space Flight Center normally exercises mission control of unmanned satellites in Earth orbit. Because the Hubble Space Telescope is so unique and complex, two new facilities were established under the direction of Goddard, dedicated exclusively to scientific and engineering operation of the telescope.

The Space Telescope Science Institute, on the campus of Johns Hopkins University in Baltimore, Maryland, was dedicated in 1983. It will perform the science planning for the telescope. Scientists there will select observing proposals from various astronomers, coordinate research, and generate the telescope's observing agenda. They will also archive and distribute results of the investigations. The institute is operated by the Association of Universities for Research in Astronomy and directed by Goddard.

The Space Telescope Operations Control Center, located at Goddard, was established in 1985 as the ground control facility for the telescope. The observing agenda from the Science Institute will be translated into computer commands by the control center and relayed to the telescope. In turn, observation data will be received at the center and translated into a format usable by the Science Institute. The control center also will monitor the health and safety of the satellite.

Construction and assembly of the space telescope was a painstaking process which spanned almost a decade. The precision-ground mirror was completed in 1981, and the optical assembly was delivered for integration into the satellite in 1984. The science instruments were delivered for testing at NASA in 1983. Assembly of the entire spacecraft was completed in 1985.

Launch of the Hubble Space Telescope was originally scheduled for 1986. It was delayed during the Space Shuttle redesign which followed the Challenger accident. Engineers used the interim period to subject the telescope to intensive testing and evaluation, assuring the greatest possible reliability.

The telescope was shipped from Lockheed in California to the Kennedy Space Center in Florida in October 1989. There, it is being readied for launch aboard the STS-31 mission of the Space Shuttle Discovery, scheduled for 1990.

"A Discovery Machine"

The Hubble Space Telescope will examine the size and origin of the universe, help determine how stars and galaxies are formed, and could provide clues to the existence of planets that orbit other stars in the same way the Earth revolves around the sun. It will zoom in on details of planets in our own solar system to show their weather patterns and surface markings, and it will provide the first images showing features of Pluto, our outermost planet. Astronomers have studied the Milky Way galaxy, the group of stars in which our solar system is located, for many years. But the Hubble Space Telescope will give them a dramatically improved view of our galaxy. In addition, it will allow an examination of some of the 100 billion galaxies yet to be explored in detail.

Though stars viewed with the naked eye appear to be similar points of light that vary only in their brightness, many different kinds of stars and star groupings exist in our universe. Some are in relatively early stages of development, while others are very old. They vary widely in temperature, size, density, and type of radiation they give off. By comparing stars of all ages and types, astronomers will learn more about how stars evolve. As a result of their discoveries, they may begin to piece together the history of the universe and even glimpse its ultimate fate.

Different instruments in the space telescope will reveal different information about the light sources being studied. Used together, they will be able to determine the visual appearance, internal structure, size, brightness, chemical composition, age, and distance from the Earth of these objects.

Some stars are clustered so close together, and others are so far away from us, that astronomers are not able to study them individually. The Hubble Space Telescope's instruments are precise enough to separate many of these blurred images into individual stars. This "separation ability" is referred to by scientists as resolution. The Hubble telescope can achieve a higher spatial resolution than any astronomical telescope ever made.

The space telescope is expected to improve our view of objects at the very edges of the observable universe. Light from distant stars takes billions of years to reach the Earth. When we view those stars, we are actually seeing conditions as they were when the light left its source billions of years ago. It is in effect like looking back in time. Observing light from far-away galaxies will allow scientists to study celestial events which took place about 14 billion years ago when the universe was young. Their understanding of the origins of our own solar system could be dramatically increased.

Quasars are mysterious objects that were discovered about 25 years ago. These star-like points of light emit a phenomenal amount of radiation for their size, more than even major galaxies. Thought to be located in distant parts of the universe, they have been too far away to study in detail. With the great sensitivity of the Hubble Space Telescope's instruments, astronomers may finally be able to determine what powers these cosmic dynamos.

With this new, powerful telescope, we will be able to measure more accurately distances to stars and galaxies beyond the Milky Way. The precision tracking instruments will point the telescope toward reference stars to measure their exact positions. Current estimates of distance are just that: educated guesses. Stars may really be twice as far away as we think they are, or just half as far away. These measurements to reference stars will provide astronomers with cosmic yardsticks for use in creating more accurate distance maps of the universe.

The most exciting aspect of the Hubble Space Telescope is that we really do not know what it will discover. Just as Galileo was surprised to find moons orbiting Jupiter and what proved to be rings around Saturn, there are doubtless unexpected cosmic phenomena that still lie beyond our view. The Hubble Space Telescope will allow astronomers to detect sources 25 times fainter than with ground-based observatories. They will be able to explore the entire volume of the universe in 10 times finer detail than ever before. There is no way to anticipate fully what this "discovery machine" may reveal.

Telescope Size and Elements

The Hubble Space Telescope weighs approximately 25,000 pounds, is 43 feet long, and 14 feet in diameter at its widest point. Roughly the size of a railroad tank car, it looks more like two huge cylinders joined together and wrapped in aluminum foil. Wing-like solar arrays extend horizontally from each side of these cylinders, and dish-shaped antennas stretch out on rods above and below the body of the telescope.

Many of the telescope's components are of modular design so they may be removed and replaced in orbit by astronauts. Though other spacecraft have received emergency repairs from Shuttle crews, the Hubble Space Telescope is the first specifically designed for in-orbit servicing.

The Hubble Space Telescope is made up of three major elements: the support systems module, the optical telescope assembly, and the scientific instruments.

Support Systems Module

The support systems module consists of the exterior structure of the Hubble Space Telescope and the various systems that make it possible for the optical telescope assembly and the scientific instruments to do their job.

The foil-like material with which the telescope is wrapped is actually multi-layer insulation, part of the telescope's thermal control system. The metallic silver surface reflects much of the direct sunlight which strikes the telescope to keep it from overheating. Tiny heaters are attached to many telescope components to warm them during the "eclipse" phase of orbit, when the Earth is between the telescope and the sun.

Electrical power for the Hubble Space Telescope is collected from the sun by the European Space Agency's solar arrays. These two "wings" contain 48,000 solar cells. They convert the sun's energy to electricity during the portion of orbit that it is exposed to sunlight. The power is stored in six batteries to support the telescope during eclipse.

The space telescope is rotated into the proper orientation, then pointed to the star it is to view and locked in place, by the pointing control system. This system is made up of a complex series of gyroscopes, star trackers, momentum wheels and electromagnets. The gyroscopes and momentum wheels are used to produce a course pointing toward the star. That pointing is fine-tuned by star trackers called fine guidance sensors. These sensors can locate and lock on to a position in the sky to within 0.01 arc second. An arc second is an extremely tiny angle. The area all around an object is a circle, and in geometry all circles are divided into 360 degrees. An arc minute is 1/60th of a degree. An arc second is 1/60th of an arc minute--less than one millionth of the circle. The space telescope's pointing system can lock on to an angle 1/100th of that! In spite of the fact that the telescope is moving around the Earth in orbit at about 17,000 miles per hour, it can hold that pointing without varying more than 0.007 arc second for as long as 24 hours.

Also included in the support systems module are the computer which controls the overall spacecraft; high-gain antennas which receive ground commands and transmit data back to Earth; the electrical power system; the structure of the telescope itself and its mechanical parts; and the safing system, designed to take over control of the telescope to protect it from damage in case of serious computer problems or loss of communication with ground controllers.

Optical Telescope Assembly

The optical telescope assembly contains the two mirrors which collect and focus light from the celestial objects being studied. The 94-inch primary mirror is located near the center of the Hubble Space Telescope. Made of precision-ground glass with an aluminum reflecting surface, it is the smoothest large mirror ever made. To reduce weight, the front and back plates are fused to a honeycomb core. The 12-inch secondary mirror is located 16 feet in front of the primary mirror. It is set far enough inside the open end of the telescope to assure that stray light does not interfere with the image being studied. In addition, three black cylinders called baffles surround the path of light to block out unwanted rays.

The two mirrors must remain in precise alignment for the images they collect to be in focus. But the space environment is a hostile one. The space telescope will experience wide variations in temperature as it passes from the sun to shade portions of its orbit. Expansion and contraction from the temperature extremes could easily cause the mirrors to go out of focus. Therefore, the mirrors are made of a special kind of glass formulated to resist that expansion and contraction. The telescope's insulation blankets and solar-powered heaters will maintain them at 70 degrees Fahrenheit. In addition, the mirrors are held a precise distance from one another by an extremely strong but lightweight truss structure. The truss is made from graphite epoxy, a material also chosen for its resistance to expansion and contraction in temperature extremes.

During observations, light from a celestial source travels through the tube of the telescope to the large primary mirror. It is then reflected from the primary mirror back to the secondary mirror. From there, the beam narrows and intensifies, then passes through a two-foot hole in the center of the primary mirror to a focal plane where the scientific instruments are located.

Scientific Instruments

The Hubble Space Telescope's scientific instruments are the Wide Field/Planetary Camera, the Faint Object Camera, the Goddard High Resolution Spectrograph, the Faint Object Spectrograph, and the High Speed Photometer. The fine guidance system, in addition to being used for pointing, also performs scientific measurements and is sometimes called the sixth scientific instrument. Mounted on a focal plane almost five feet behind the primary mirror, these scientific instruments will furnish astronomers with a wide range of information about the stars and galaxies they study. Each instrument is contained in a separate module and operates on only 110 to 150 watts of power.

There are two cameras among the scientific instruments. They are electronic cameras that record their images in a manner similar to television cameras. These pictures are recorded on electronic detectors and transmitted to computers at the Space Telescope Operations Control Center as "digitized images." Each picture the camera takes is divided into hundreds of thousands of tiny squares called picture elements or pixels. Each square has a number in the computer memory. In addition, a different number is assigned to many intensities of light--black, white, and a hundred or so shades of gray n between. When an image is viewed, the computer assigns an intensity number (the shade of gray) to each pixel (its position in the photograph) and sends the number code to ground controllers. There, other computers "color in" each pixel at the correct intensity to complete the picture. It is like a very sophisticated version of a child's paint-by- number art. To make color pictures, different filters are used which screen out all light except that of a particular color. When images using several filters are combined, color pictures are produced.

The Wide Field/Planetary Camera will be used to investigate the age of the universe and search for new planetary systems around young stars. It can compare near and far galaxies and observe comets such as Halley's comet, which we previously could only view every 75 years. As its name implies, the Wide Field/Planetary Camera can be used in two different ways. In its wide-field mode, its field of view will allow it to take pictures of dozens or even hundreds of distant galaxies at once. In the planetary mode, it will provide close-ups of all the planets in our solar system except Mercury, which is too close to the sun for safe pointing. The Wide Field/Planetary Camera can observe larger areas of the sky and more different forms of light (from far ultraviolet to near infrared) than any of the other science instruments. It will also produce a greater volume of information for analysis than any of the others.

Though its field of view is greater than that of any other Hubble instrument, the "wide field" in this camera's name may be a little misleading. Typical wide-field cameras at ground observatories have a field of view of around 5 degrees. This camera's is only 2.67 arc minutes. It would take a montage of about 100 "wide-field" images to get a picture of the full moon. However, the narrower field of view allows much better resolution of far-away objects.

Although it will focus on an even smaller area than its wide-field counterpart, the Faint Object Camera will extend the reach of the Hubble Space Telescope to its greatest possible distance and produce its sharpest images. It will be able to photograph stars five times farther away than is possible with telescopes located on the ground. Many stars and galaxies, now barely perceptible, will appear as blazing sources of light to the Faint Object Camera. The camera will intensify images to a brightness 100,000 times greater than they were when received by the telescope. Then a television camera will scan the intensified images and store them in the camera's memory for transmission to the ground.

The Faint Object Camera will be used to help determine the distance scale of the universe, peer into the centers of globular star clusters, photograph phenomena so faint they cannot be detected from the ground, and study binary stars (two stars so close together they appear to be one). It is part of the European Space Agency's contribution to the Hubble Space Telescope program.

Two spectrographs are also included in the Hubble Space Telescope's group of scientific instruments. A spectrograph does not take a photograph of the image it sees. Rather, one could say it takes its chemical "fingerprint." A spectrograph separates the radiation received from an object according to wavelengths, much as a prism splits visible light into colors. Every chemical element produces its own individual pattern on a spectrogram. So when the "fingerprint" of a certain element shows up on the spectrum, scientists know that element is present in the object being viewed. Scientists use spectrographs to determine the chemical composition, temperature, pressure, and density of the objects they are viewing.

The Faint Object Spectrograph will be used to analyze the properties of extremely faint objects in both visible and ultraviolet light. It will be able to isolate individual light sources from those surrounding them at very great distances. The Faint Object Spectrograph is equipped with devices that can block out light at the center of an image so the much fainter light around a bright object can be viewed. It will study the chemical properties of comets before they get close enough to the sun for their chemistry to be altered, as well as probing to see what the mysterious quasars are made of. This instrument will offer comparisons of galaxies that are relatively near Earth with those at great distances, helping researchers determine the history of galaxies and the rate at which the universe is expanding.

The Goddard High Resolution Spectrograph, though its work is similar to that of its faint object companion, has a specialized job to do. It is the only science instrument entirely devoted to studies of ultraviolet light. Its detectors are designed to be insensitive to visible light, since the ultraviolet emissions from stars are often hidden by the much brighter visible emissions. The "high resolution" in this instrument's name refers to high spectral resolution, or the ability to study the chemical fingerprints of objects in very great detail. The combination of this spectral resolution with the high spatial resolution of the cameras will allow scientists to determine the chemical nature, temperature, and density of the gas between stars. Its investigations will range from peering into the center of far-away quasars to analyzing the atmospheres of planets in our own solar system.

The High Speed Photometer, a relatively simple but precise light meter, will measure the brightness of objects being studied, as well as any variations in that brightness with time, in both the visible and ultraviolet ranges. The photometer will be able to study the smallest astronomical objects of any of the telescope's instruments. One of the photometer's tasks will be to look for clues that black holes exist in binary star systems. Variations in brightness would occur as one star revolves around the other. Irregularities in that variation might indicate that matter is being lost to a black hole--an object so dense that nothing, not even light, can escape from it.

The photometer will also provide astronomers with an accurate map of the magnitude of stars. A star's magnitude is its relative brightness in comparison with other stars. The brighter the star is, the lower its magnitude number will be.

The three fine guidance sensors serve a dual purpose. Two of the sensors lock on to reference stars to point the telescope to a precise position in the sky, then hold it there with a remarkable degree of accuracy. The third sensor, in addition to serving as a backup unit, will be used for astrometry.. the science of measuring the angles between astronomical objects. These measurements will be combined with information from other instruments to prepare a more accurate distance scale of the universe.

Deployment and Verification

The Hubble Space Telescope will be carried into space in the cargo bay of the Space Shuttle Discovery. When the assigned 370-mile orbit is reached, Shuttle astronauts will supply full electrical power from the orbiter to the Hubble telescope. Most of its systems will undergo a short checkout to make sure they are operating properly.

The day after launch, the telescope's own internal power will be switched on, and the power supplied from the orbiter will be turned off. Then, the crew will use the Shuttle's remote manipulator arm to lift the telescope from the bay and suspend it above the crew cabin. Slowly and carefully, ground controllers will command the energy-collecting solar arrays to unfurl and the high-gain antennas to be deployed. With this accomplished, the telescope will be ready to operate on its own, because the electrical power can be replenished with the solar arrays and ground controllers can send commands through and receive data from the antennas. The arm will release the telescope; then the Shuttle will move to a parallel orbit about 45 miles away. The Shuttle crew will remain on call for about two days to be sure everything is functioning correctly. If there is a problem--for instance, if the door which admits light to the telescope does not open in response to ground commands--the Shuttle will rendezvous with the telescope, and the astronauts will suit up for a space walk to correct the malfunction. After the two days, the astronauts will complete their mission and return home.

The Hubble Space Telescope will not be ready to begin taking pictures as soon as it is deployed. A methodical process called orbital verification, where the telescope's systems are activated one by one and the mirrors are carefully aligned, will come first. The telescope is an extremely complex instrument, and this process will take several months. After orbital verification is completed, another few months will be required to fine-tune the images received by the scientific instruments. During this period, scientists who were guaranteed observing time because of their contributions to the telescope's development will begin their investigations.

Day-to-Day Operations

After the telescope and its instruments are pronounced fit by their designers, the Hubble Space Telescope will begin doing what it was designed to do--opening new windows to the universe for astronomers around the world.

The Space Telescope Science Institute began accepting observing requests from astronomers in 1986. After carefully evaluating the scientific merits of the proposed observations, they chose the most promising and formulated an initial observing schedule for the telescope. Institute scientists will continue to accept, evaluate, and process requests for the 15 years or more that the Hubble telescope will remain in operation.

The Space Telescope Science Institute schedule will be translated into commands for the telescope at Goddard's Space Telescope Operations Control Center. Controllers there will relay the telescope's orders through a commercial communications satellite to a ground station at White Sands, New Mexico. The White Sands station will then send the signal on to one of NASA's Tracking and Data Relay Satellites. These tracking satellites are in geosynchronous orbit, an extremely high orbit that allows them to stay over the same relative position to Earth at all times. The relay satellite will then forward the commands to the telescope.

Pictures and other scientific data, along with information on the health and safety of the systems, will be transmitted from the space telescope to controllers along the same route in reverse. Computers at Goddard will convert the information received from the telescope, called "telemetry," into a format usable by scientists. It will be forwarded to the Science Institute where it will be analyzed and stored in permanent archives. After the scientists who planned the observations have studied the results for a period of time, the information will be made available to astronomers around the world.

Servicing in Orbit

The Hubble Space Telescope was designed specifically to allow extensive maintenance in orbit. This is the most practical way to keep the equipment functioning and current during its 15 years or more in space with a minimum of down time. Some of the components have a life expectancy shorter than 15 years and will need to be replaced from time to time. New technology will make it possible to design more sophisticated scientific instruments over the years. As a matter of fact, several improved instruments are already under development. In-orbit servicing allows worn parts to be replaced and new instruments to be substituted for the original equipment without the great expense, risk and delay of bringing the telescope back to Earth.

The modular design of many space telescope components means that units may be pulled out and a replacement plugged in without disturbing other systems. Doors on the exterior of the telescope allow astronauts access to these modular components, called Orbital Replacement Units. Handrails and portable foot restraints make it easier for them to move about in the weightless environment while working on the telescope. A special carrier has been designed to fit in the orbiter's cargo bay to hold replacement parts and tools.

Astronauts will visit the space telescope about every five years on servicing missions. In case of an emergency, special missions may be launched between the scheduled ones.

On servicing missions, the Space Shuttle will rendezvous with the orbiting telescope. Astronauts will use the Shuttle's remote manipulator arm to pull in the observatory and mount it on a maintenance platform in the orbiter's open bay. Astronauts will don space suits and go out into the bay to complete required maintenance. They may change out batteries or solar arrays, a computer, one of the scientific instruments, or any of the more than 25 units that can be replaced in orbit. The Shuttle also may be used to push the spacecraft back up to its original orbit if atmospheric drag has caused it to descend.

Once the maintenance is finished, the telescope will be released and carefully reactivated by ground controllers to resume its exploration.

The Hubble Space Telescope Team

The Hubble Space Telescope is the product of not just one group or agency, but a cooperative effort of many dedicated people from across the United States and around the world. Following is a brief summary of the institutions that are a part of the Hubble Space Telescope Program.

NASA Headquarters Office of Space Science and Applications, Washington, D.C.: Responsible for the overall direction of the Hubble Space Telescope Program.

Marshall Space Flight Center, Huntsville, Alabama: Manages the overall Hubble Space Telescope project, including supervision of design, development, assembly, pre-launch checkout, and orbital verification.

Goddard Space Flight Center, Greenbelt, Maryland: Responsible for development of the scientific instruments and for day-to-day operation of the telescope through its Space Telescope Operations Control Center and its oversight of the Space Telescope Science Institute on the campus of Johns Hopkins University in Baltimore, Maryland.

Johnson Space Center, Houston, Texas: Furnishes orbiter and crew services during deployment and maintenance missions.

Kennedy Space Center, Florida: Provides pre-launch processing and Space Shuttle launch support, assuring safe delivery of the telescope to orbit.

European Space Agency: Provided the solar arrays and Faint Object Camera, and will furnish operations support at the Science Institute; in return will be allocated 15 percent of the observing time.

Lockheed Missiles and Space Company, Sunnyvale, California: Designed and developed the support systems module; responsible for systems engineering and space telescope assembly and verification.

Perkin-Elmer Corporation, Danbury, Connecticut: Designed and developed the optical telescope assembly and fine guidance sensors.

Universities whose staff members have made major contributions to the program include the California Institute of Technology (Wide Field/Planetary Camera), the University of Wisconsin (High Speed Photometer), the University of California at San Diego (Faint Object Spectrograph), and the University of Texas at Austin (astrometry).

««« Previous... Time | Top of Page | Next... February Skys »»»