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SPACE FLIGHT PROJECTS
Spacecraft Classification
Spacecraft designed and constructed to achieve science data gathering are specialized systems intended to function in a specific hostile environment. Their complexity varies greatly. They may be broadly categorized according to the missions they are intended to fly. This chapter identifies a selection of different classifications.
Flyby SpacecraftFlyby spacecraft follow a continuous trajectory, never to be captured into a planetary orbit. They must have the capability of using their instruments to observe passing targets, and ideally, compensating for the target's apparent motion in optical instruments' field of view. They must downlink data at high rates to Earth, storing data onboard during the periods when their antennas are off Earthpoint. They must be able to survive for many years of long interplanetary cruise. Examples of flyby spacecraft include Pioneers 10 and 11, Voyagers 1 and 2 (each of which have achieved solar escape velocity), and the Pluto fast-flyby mission currently being considered. Flyby spacecraft were used in the initial reconnaissance phase of solar system exploration.
Orbiter SpacecraftA spacecraft designed to travel to a distant planet and enter into orbit must carry with it a substantial propulsive capability to decelerate it at the right moment to achieve orbit insertion. It has to be designed to live with the fact that solar occultations will occur, wherein the planet shadows the spacecraft, cutting off solar panels' production of electrical power, and subjecting the vehicle to extreme thermal variation. Earth occultations will also occur, cutting off uplink and downlink communications with Earth. Orbiter spacecraft are being used in the second phase of solar system exploration, following up the initial reconnaissance with in-depth study of the planets. These include Magellan, Galileo, Mars Global Surveyor, and Cassini.
Atmospheric Probe SpacecraftSome missions employ one or more smaller instrumented craft which separate from the main spacecraft prior to closest approach to a planet to study the gaseous atmosphere of the body as it drops through it. The atmospheric probe spacecraft is deployed by the release of springs or other devices that simply separate it from the mother ship without making significant modification to its trajectory. The mother ship typically would then execute a trajectory correction maneuver to prevent its own atmospheric entry so that it can continue on with other mission activities. An aeroshell protects the atmospheric probe spacecraft from the thousands of degrees of heat created by atmospheric friction during entry. The shell is ejected, and a parachute then slows the craft's descent while it undertakes its agenda of scientific observations. Data is typically telemetered from the atmospheric probe to the mother craft where it is recorded onboard for later transmission to Earth. Galileo, the Jupiter orbiter spacecraft, carries an atmospheric probe that will descend into Jupiter's atmosphere during the orbiter's first pass over the planet. The Pioneer 13 spacecraft carried four atmospheric probes which radioed their data directly to Earth during descent into the Venusian atmosphere. Cassini, being designed to orbit Saturn, will carry a probe to be released into the hazy nitrogen atmosphere of Titan, Saturn's largest satellite.
Atmospheric Balloon PackagesBalloon packages are designed for suspension from a buoyant gas bag to float and travel with the wind. Tracking of the balloon's progress across the face of a planet yields data on the circulation of the planet's atmosphere. They have a limited complement of spacecraft subsystems aboard: for example, they may have no need for propulsion subsystems or attitude and articulation control system (AACS) subsystems at all. They do require a power supply, which may simply be batteries, and telecommunications equipment to permit tracking. They may also be outfitted with instrumentation for direct-sensing science experiments to take measurements of an atmosphere's composition, temperature, pressure, density, cloud content and lightning.
Lander SpacecraftLander spacecraft are designed to reach the surface of a planet and survive long enough to telemeter data back to Earth. Examples have been the highly successful Soviet Venera landers which survived the harsh conditions on Venus while carrying out chemical composition analyses of the rocks and relaying color images, JPL's Viking landers at Mars, and the Surveyor series of landers at Earth's moon, which carried out similar experiments. The Mars Pathfinder project, which launches in 1996, is intended to be the first in a series of landers on the surface of Mars at widely distributed locations to study the planet's atmosphere, interior, and soil. A system of actively-cooled, long-lived Venus landers designed for seismology investigations, is being studied for a possible future mission.
Surface Penetrator SpacecraftSurface penetrators have been designed for entering the surface of a body, such as a comet, surviving an impact of hundreds of Gs, measuring, and telemetering the properties of the penetrated surface. Penetrator data would typically be telemetered to the mother craft for re-transmission to Earth. The Comet Rendezvous / Asteroid Flyby (CRAF) mission included a cometary penetrator, but the mission was cancelled in 1992 due to budget constraints. Plans for the Russian MARS '98 mission include a surface penetrator craft.
Surface Rover SpacecraftElectrically-powered rover spacecraft are being designed and tested by JPL as part of Mars exploration effort. The Mars Pathfinder project includes a small mobile instrument (rover) to be deployed on Mars. Mars rovers are also being developed by Russia with a measure of support from The Planetary Society. These rover craft will be semi-autonomous, and will be steerable from Earth, taking images and soil analyses for telemetering back to Earth. |
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