Observatories and Other Types of Land Based Space Exploration

Since before recorded history man observed his evironment and wondered what was out there. Above the surface of man's existence on earth, early man invented stories and fables to explain the workings of the heavens. Measurements of the rising and setting of the sun, moon, and planets with sticks and stones began the scientific observation of the heavens. Some of the oldest sites in civilivation, ancient ziggurats of Babalonya, stone circles of Callanish or Stonehenge, or merely observations of the time of day, were early observatories. Ancient man observed sunspots on the sun, when the sun was suitably obscured by clouds, but it took the development of science and technology to allow an observational basis for understanding the space around us. The Greeks, with their concepts of geometry, calculated the size and shape of the earth. The continuing development of scientific principles and knowledge during the Renassance brought about the scientific contemplation of what was in space. Christian Huygens with his early telescope and telescopes built by the Dutch in the 16th century, were used by natural philosophers of the day to observe the motions of the sun, stars, and planets. Galileo Galilei, a 16th century Italian scholar, used the telescopes and his imagination to look at the craters on the moon, the sun and stars, and the planets. Possibly his most famous observation was the moons of Jupiter, now known as the Galilean moons; Io, Europa, Ganymede, and Callisto. Even within a few hours these moons can be seen through a small telescope to move in their orbits around Jupiter. Galileo's observation and extrapolation of the motions of Jupiter's moons to those of the planets around the solar system was anathema to the Church, still clinging to a man and earth centered universe. Although the Greeks theorized a sun centered system, it took until N. Copernicus in the 1500's to describe a sun centered solar system and the mathematical proofs of J. Kepler to overcome opposition. Through direct observation of the heavens, humanity's intellectual knowledge expands through observing the physical reality of our environment.

In the space age of the latter half of the 20th century humans are no longer bound to the surface of the earth. Scientific theories of electromagnetism and quantum mechanics have opened up observations heretofore impossible. Both on the ground and in space, telescopes now focus on the astronomically big and distantly small throughout the electromagnetic spectrum. Telescope automation results in remotely controlled operations, whether on the ground or in space, to cost effectively explore not just the solar system, but space at distances back to the beginning of time.

Foundations of observational space exploration include not only the frequencies of electromagnetic radiation visible to our eyes, but frequencies throughout the spectrum from long radio waves to microwave, to infrared through visible, ultraviolet, x-rays, and gamma rays. The electromagnetic spectrum consists of photons of increasing energy associated with different events producing radiation. From earth, only radiation whose physical characteristics allow it to pass through our atmosphere can be observed. Thus, until the dawn of the space age only visible light and some radio wavelengths could be observed. Now space based observatories are able to see wavelengths in the infrared, ultraviolet, x and gamma ray spectrum unable to pass through our atmosphere. Space based observatories combined with new techniques and instruments for earth based observatories are now able to look at the entire electromagnetic spectrum, ushering an era of observational exploration with vastly more potential than Galileo's.

Observatories on earth and in space depend in great part on the portion of the electromagnetic spectrum they are designed to observe, and the following sections are based on a general separation of the spectrum.

Radio Wave and Microwave Observations

The radio wave portion of the electromagnetic spectrum wavelengths extend from about one million centimeters to 1 centimeter. Radio waves are long wavelength, and hence low frequency and energy. They are produced when electrons spiral or oscillate in magnetic fields. On earth electron oscillation in antennas is responsible for our radio and television transmissions, while from space radio waves may be associated with collisions between galaxies or electron jets spiraling through magnetic fields. Since the ability to detect radio waves only began in the later portion of the 19th century, notably by M. Marconi, radio wave observations are relatively new. Most portions of the radio spectrum travel well through the earth's atmosphere, but sensitive and directional detection of radio waves from space did not occur until the mid-20th century. In 1931 Karl Jansky at Bell Labs detected the first radio waves from space while investigating radio static. He and another early radio wave investigator, amateur G. Reber, found radio emissions from the sun, Jupiter, and perhaps most interesting from a deep space object they named Sagittarius A. Sagittarius A is now know to be located at the center of our Milky Way galaxy. Radio emissions are probably produced by energetic particles and electrons spiraling around the black hole at the very center of our galaxy.

The field of radio astronomy developed quickly, enhanced by sensitive directional big dish antennas. Radio astronomers detected hydrogen around our Milky Way galaxy arms, establishing our galaxy as a spiral type galaxy. A Nobel Prize went to A. Penzas and B. Wilson at Bell Labs for detecting in 1965 the emission, at a wavelength of 7.35 cm, of the echo of the beginning of the universe. This was a powerful validation of the "Big Bang" model of the origin of the universe-cosmology. While working to determine the source of interference of microwave receiver horns, they found a peak of radio noise coming from all apparent directions, not localized to a place like the sun or Sag A galactic center. Astronomers and astophysicics who studied how the universe began had two principal theories; 1) the universe has always existed - the so called "Steady State", and 2)  the "Big Bang", that the universe exploded from a singularity in space and time about 14 billion years ago. Like the heat from a primordial fireball slowly cooling, their detection of the residual radiation at 7.35 cm was a powerful validation of the convergence of theoretical physics and observation. Although the 7.35 cm microwave radiation can be observed from the ground, space based telescopes enjoy enhanced clarity and directional freedom. The NASA COBE (Cosmic Background Observatory) orbiting satellite has measured this cosmic residual radiation to a very high degree of accuracy, adding important information to theories of the very beginning of the universe.

Radio wavelength observations also have benefited by another powerful technique, that of interferometry. To simply describe this technique, consider that the detail that can be observed depends on the size of the wavelength used and size of the receiver. Long wavelengths like radio waves are inherently less detailed than shorter wavelengths, such as visible wavelengths, and must have very large receivers- big antennas. Radio wave receivers are generally large dishes, but physical size limits how big these dishes can be built. The largest single steerable dish at Jodrell Bank England is 250 feet across, and the Arecibo Puerto Rico fixed dish is 1,000 ft. Like a big telescope can see smaller and fainter, so a big dish "sees" smaller and fainter. However the application of the technique of interferometry allows observations from different observatories to be combined into a really big disk. This interferometric technique enhances resolution and sensitivity. Uniquely applicable to radioastronomy, combining observation from dishes separated around the face of the earth allows observations of very small objects very far away. The observations of these small radio objects, such as jets emanating from neutron stars and black holes at the center of active galaxies, represents an achievement of technology. The premier ground based radio interferometer, finished in 1981, is the Very Large Array in Soccoro New Mexico. It consists of 27 movable dishes each 85 feet in diameter that can be spread over 20 miles. Additional radio telescopes around the world can contribute to the interferometric technique and the first satellite for radioastronomy was launched in January 1997 by the Japanese. This satellite in earth orbit provides an even longer baseline "dish" looking farther and smaller. Two satellites in a solar earth type orbit would provide an even longer baseline.
 

Infrared Observations

Unlike radiowaves, infrared radiation, from about 1cm to10-3 cm in wavelength, is strongly absorbed by earth's atmosphere making ground based observation difficult. Infrared detector equipped telescopes have been flown on balloons, rockets, and high flying aircraft such as the NASA Kuiper Airborne Observatory. Satellite infrared observatories include the IRAS (Infrared Astronomical Satellite, the European Space Agencies ISO (Infrared Space Observatory), and NASA's SIRTF (Space Infrared Telescope Facility). The SIRTF is one of the four NASA Great Observatories and when launched in 1998 will provide unprecented details of the infrared sky.

Infrared wavelengths are useful to image dust, such as protoplanetary disks forming around stars and dusty clouds in star birthing regions. Since the earth's atmosphere absorbs most infrared radiation, space based earth orbiting observatories have a natural advantage.

Visible Observations

The visible wavelengths of light are a very narrow portion of the entire spectrum, registering in our eyes from the red to the blue. Evolution conspired to give earth bound creatures detectors and collectors, retina's and lenses, useful in our water and atmosphere. When we think of the "Great Observatories", probably the biggest optical telescopes come to mind; Palomar, Wilson, Kitt Peak, Pic du Midi, Anglo-Australian, Lick, Yerkes, Allegheny, Keck, and, of course, the orbiting Hubble Space Telescope. Since Galileo looked through his instrument we imagine refracting and reflecting telescopes of increasing size, cost and complexity. Pretty pictures are certainly part of the usefulness of visible wavelengths of light. We understand picture information readily, but observation of visible spectra also provides detailed scientific information on the composition of stars, planets, atmospheres, interplanetary and intergalactic space, and confirms the existence of complex molecules in space. Earth based telescopes have always been limited by the ocean of air we live in. This ocean of air, besides the obvious weather, is constantly moving and producing waves -twinkles- in the light as it passes through the atmosphere. While the Hubble Space Telescope sits above the ocean of air, it is very expensive to get up there. Consequently although space based visible telescopes provide the most stable images, getting a telescope the size of the twin Keck's to orbit, each 10 meters in diameter, is not likely to happen soon. New and improved techniques to reduce the effects of the atmosphere including adaptive optics to partially correct for "twinkling" caused by atmospheric motion, and visible wavelength interferometry. The combination of earth based telescopes with new techniques with space based observatories allows earth based observatories to compliment those in space.

Any review of space based telescopes must include interplanetary spacecraft that return the most detailed and impressive images from our own solar system. Since Sputnik was launched in 1957 as the first artificial earth satellite, a succession of solar system exploring spacecraft have revolutionized our understanding of our nearest neighbors. Our solar system is tiny in comparison to even our nearest star neighbors and therefor spacecraft traveling will never be able to return images from much beyond our solar system. However unmanned spacecraft including Explorer, Voyager, Surveyor, Pathfinder, Galilieo, Hipparcos, and many others have returned startling images unavailable from even the biggest earth bound telescopes. Manned exploration including the Apollo missions to the moon, returned snapshots that fire the imagination and lift the spirit of our civilization. Consider such famous images as earthrise over the surface of the moon or astronauts posing with a flag.

Finally, consider space based observations of earth. These observatories explore the mission to earth, providing enormous benefit in weather forecasting and climate modeling, practical application of space based observatories. Space and ground based observations are perhaps best made in visible light, the most familiar and rich wavelengths for humans to enjoy.

Ultraviolet

Electromagnetic wavelengths above those of visible blue light are not able to penetrate our atmosphere. This is very good since much is this high energy radiation of ultraviolet, x-rays and gamma rays, are harmful for carbon based life. If our atmosphere did not shield us from these high energy rays it is likely we would not have evolved. Early life on earth used carbon dioxide in the atmosphere and released oxygen, building over billions of years to the levels we now enjoy. Oxygen in the upper atmosphere is ionized by sunlight into ozone, blocking high energy ultraviolet light. Ozone and other atmospheric gasses shield us from these high energy rays, so space based telescopes step into this void. Space based telescopes that image in the ultraviolet include the International Ultraviolet Explorer (IUE ), observations made from the cargo bay of the Space Shuttle by the Astro UV telescope, the Extreme Ultraviolet Explorer (EUVE), and the Hubble Space telescope.

High energy radiation is produced by high energy events, ultraviolet, x-ray and gamma ray wavelengths are produced by highly ionized atomic transitions. Space based observations at these wavelengths will provide information of stellar atmospheres and high energy collisions.

X-RAY

X-rays and extreme ultraviolet rays do not pass through our atmosphere. Space based observatories include the Array of Low Energy X-ray Imaging Sensors(ALEXIS), the Einstein Observatory launched in 1978, the German ROSAT and the Japanese ASCA satellites, UK Advanced Satellite for Cosmology and Astrophysics (ASCA), the US-Japan Kavant astrophysics attached to the Mir space station, Rossi X-ray Timing Explorer (RXTE), and SAX and Italian x-ray satellite. Perhaps most important of the bunch is AXAF, one of the four NASA Great Observatories, the Advanced X-ray Astrophysics Facility (AXAF) scheduled to launch in August 1998. This 1.3 billion dollar satellite will do for x-ray astronomy what Hubble has done for visible wavelength observations. AXAF will be 100 times as sensitive as any other satellite with 10 times the resolution. It will se though to the core of our galaxy, since x-rays are so powerful they punch through interstellar dust and gas. Imaging among the most energetic events in the universe, AXAF will provide an eye on matter falling into black holes and neutron stars, supernova, and galactic collisions.

Gamma-ray

Gamma rays are the most energetic electromagnetic waves, associated with black holes, coalescing neutron stars, and solar flares. The Compton Gamma-Ray Observatory (CGRO) is one of the four NASA Great Observatories (SIRTF, Hubble, AXAF, and CGRO) launched by the Space Shuttle in 1991. Other gamma ray satellites include a constellation of military satellite sensors looking at earth to detect nuclear explosions. Astronomical gamma ray satellites include the Russian GRANAT, now exhausted of control motor fuel, and a US-Russian mission Spectrum X-Gamma. Among unusual observation by these satellites was the detection by the military satellites of gamma ray bursters, bright flares of gamma rays distributed evenly around the sky. Observations with Compton CGRO and Rossi x-ray timing satellites allowed the Keck earth based telescope to image what are the energetic merger of black holes or neutron stars coalescing at great distances in the early universe. Cosmological flashbulbs perhaps as the quasars and galaxies settled into the universe as we now know it. The synergy between space and ground based observatories continues.

Ground and Space Based Observatories in the future

The completion of big ground based telescopes at the end of the 20th century and completion of the NASA Great Observatories in space, are complimentary developments in exploring space and back to the instant of creation. Knowledge obtained cannot be foreseen and most likely an observer at the end of the 21st century will see the 20th Century view of the universe as simple as we now see Galileo's. Observational astronomy and theoretical physics will likely settle on a theory of everything, only to discover what is old is new again.


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