SPACE VEHICLES
High-altitude aircraft and balloons are the least expensive way of investigating invisible extraterrestrial radiation. Jet aircraft can ascend to about 15 kilometers, while balloons are useful up to about 30 kilometers, above which only 5 percent of the atmosphere remains. Rockets, though their flights are short compared to balloon flights, lasting for minutes instead of hours, can climb five times higher than balloons can. Artificial satellites cost much more than rocket flights, but satellites can continuously monitor events over different regions of the electromagnetic spectrum for long periods of time, an advantage that outweighs their additional cost. Since 1958 the United States has launched hundreds of instrumented satellites that have either orbited the earth or been sent to search the solar system.
Up until about 1983 the means of getting satellites off the su rface of the earth have been rockets. The advent of Space Shuttle has now provided another way to launch satellites. Space Shuttle is a true aerospace launch vehicle in that it takes off like a rocket, maneuvers in earth orbit as do other spacecraft, but lands like an airplane. This launch vehicle was designed to carry heavy loads into space and to be, reusable. Satellites can be carried into orbit in the Shuttle's cargo bay; when the Shuttle is in orbit, the satellites can be lifted out by a retractable arm and placed in their orbits. This also means that satellites can be retrieved from orbit to be brought back to the earth's surface or serviced and returned to orbit. Space Shuttle gives us the capability of carrying pieces of immense spacecraft, including manned space stations, into orbit to be assembled there. Its versatility signals a new generation of space exploration.
An important group of space vehicles has been the observatory satellites placed in orbits several hundred kilometers above the earth. Two of the most sophisticated and costly observatory satellites have been the Orbiting Astronomical Observatories; OAO-2 and OAO-Copernicus.
The Skylab program cost $6 billion. Three crews of three men each spent 171 days in Skylab between May and November, 1973. These astronauts carried out dozens of astronomical, biomedical, and technological experiments. The abandoned station was to remain in orbit for several years; it was hoped that with Space Shuttle astronauts would be able to reuse the station in the future. However, Skylab was dragged down by the atmosphere to a fiery demise over the Indian Ocean, scattering pieces over western Australia on july 11, 1979.
In early 1985 NASA expects to place a 2.4-meter unmanned reflecting telescope named Space Telescope in orbit at an altitude of 500 kilometers. Its optics and instrumentation will be enclosed in a cylindrical tube 13 meters long and 4.3 meters wide. Auxiliary apparatus includes two imaging cameras, faint-object and high-resolution spectrographs, a photometer, and other specialized devices. These analyzing instruments are designed to cover the wavelength range from about 1000 angstroms in the ultraviolet to 8000 angstroms in the near infrared. Data from the telescope will be radioed in digital (number) form through the Goddard Space Flight Center in Greenbelt, Maryland, to the Space Telescope Sciences Institute on the johns Hopkins University campus for processing.
Out in space no atmospheric absorption or turbulence will distort the images produced by the telescope. Thus the telescope should see astronomical sources up to 50 times fainter than those visible from the earth's surface; in terms of distance a faint object can be seven times farther away than could be seen from the surface of the earth. Space Telescope's spatial resolution will be 10 times better than the best earth-based reflectors. With proper maintenance from Space Shuttle, the telescope could operate for at least a decade. Unlike its ground-based counterparts, Space Telescope will scan the electromagnetic spectrum from the deep ultraviolet to the infrared.
Other space observatories are the planetary probes, which are literally the most exotic. Their role is to go to a planet to photograph and analyze from a close flyby, to orbit the planet, or in some cases to land. As examples, the Viking 1 and Viking 2 spacecraft landed on the surface of Mars (we shall discuss them later). Other examples are the Voyager 1 and Voyager 2 spacecraft launched to encounter jupiter, Saturn, and perhaps Uranus and Neptune. Much of our understanding of the nature of the universe is changing-rapidly and dramatically-because of these space observatories.
