This argument could only be persuasive, however, if one assumed that the sole purpose of a manned flyby mission was to accomplish its flyby or flybys. In fact, NASA planners expected that the flyby astronauts would perform a wide range of experiments and observations throughout their mission. A manned flyby spacecraft would amount to a multi-purpose space station in orbit about the Sun instead of the Earth. Manned Mars/Venus flybys would also serve as experience-building steps toward manned Mars landing missions, much as Project Gemini had been for the Apollo Program.
In August 1966, H. London, an engineer with Bellcomm, NASA's advanced planning contractor, described how the 1975 manned Mars flyby spacecraft could explore asteroids in the Main Belt between Mars and Jupiter (middle link below). Its crew would, he explained, have at their disposal a 40-inch reflecting telescope intended primarily for high-resolution Mars photography during the flyby. Following its Mars flyby, the spacecraft would reach aphelion (its farthest point from the Sun) within the Main Belt. This would bring it to within 20 million miles of at least two major Main Belt asteroids.
In a February 1968 follow-on to the 1966 memorandum, Bellcomm engineer W. Grobman estimated that a one-meter (39.6-inch) reflecting telescope on board a flyby spacecraft would be capable of photographing objects as faint as 27.4 magnitude. For comparison, the 200-inch reflector on southern California's Mt. Palomar could photograph no object fainter than about 23.5 magnitude.
Grobman cited two factors that accounted for the flyby spacecraft telescope's superlative performance. First, the instrument would operate beyond Earth's obscuring atmosphere, well away from moonlight, aurorae, humidity, clouds, and city lights. Second, it would be capable of very long exposures.
A telescope in low-Earth orbit would circle the planet in about 90 minutes, so even with carefully controlled slewing could in most cases observe an astronomical target for no more than 45 minutes before losing sight of it behind the Earth. The flyby spacecraft telescope, on the other hand, would not follow a fast planet-centered orbit, and would spend most of its time far from any object that could block its view of the universe.
According to Grobman, the only obvious limit on exposure duration would be the ability of astronomical film to record arriving photons. He calculated that scientifically useful exposures lasting as long as 40 hours might be possible with existing films. He noted in passing, however, that radiation in interplanetary space might darken film. One solution, he wrote, would be to replace film with an unspecified electronic imaging system.
The flyby spacecraft telescope would be capable of recording spectra of very faint objects. Recording the spectra of distant galaxies would, Grobman explained, help to improve understanding of the relationship between galactic distance and speed of recession first noted by Edwin Hubble in 1929. Because the flyby spacecraft telescope would operate outside of Earth's atmosphere, the spectra it recorded could include regions of the electromagnetic spectrum invisible to Earth-bound telescopes, he added.
Grobman cited as another class of potential targets objects that change brightness over periods of an hour or less, such as the mysterious distant quasars (now known to be giant black holes in galaxy centers). The flyby spacecraft telescope, he wrote, would be capable of detecting subtle and rapid variations in the brightness of even very faint objects.
"Targets of opportunity" - that is, objects that appeared unexpectedly during the manned flyby mission - would be fair game for observation, Grobman added. Such targets might include supernovae and novae within or beyond our Milky Way Galaxy and comets newly arrived in the inner Solar System.
Grobman completed his memorandum after August 1967, when the U.S. Congress specifically forbade further NASA work on manned planetary flybys. Though no manned Mars/Venus flyby astronauts ever pointed a telescope at a distant galaxy or nearby asteroid, the Hubble Space Telescope in low-Earth orbit, the Spitzer Space Telescope in Earth-trailing solar orbit, and other automated instruments have decisively demonstrated the capabilities of space-based telescopes.
In the past decade and a half, NASA has worked toward placing large automated observatories into halo orbits around the Sun-Earth libration (L) points. Astronauts might voyage to the Sun-Earth L points to assist in deployment of new observatories and to upgrade and perform repairs on existing ones. Manned telescope servicing missions to the Sun-Earth L points could also serve as experience-building steps toward manned Mars missions (bottom link below).
Optical Astronomy on a Manned Planetary Flyby Mission - Case 710, W. D. Grobman, Bellcomm, Inc., February 8, 1968.
http://beyondapollo.blogspot.com/2010/03/planetary-jag-manned-mars-flyby-1966.html
http://beyondapollo.blogspot.com/2009/03/astronomy-from-piloted-mars-flyby-1966.html
http://beyondapollo.blogspot.com/2011/07/100-day-mission-to-sun-earth-l2-1999.html
No comments:
Post a Comment