Page:Advanced Automation for Space Missions.djvu/59



NEP technology has been studied for a long time but has no current planned application beyond possible cargo transport operations from LEO to Geosynchronous Earth Orbit (GEO). Planetary missions such as the proposed Titan TABLE 3.5.-TITAN MISSION SPACECRAFT ACCOMPLISHMENTS

Spacecraft type Possible accomplishments Nuclear electric Spiral escape from low Earth orbit; propulsion interplanetary transfer to Saturn; rendezvous with Titan; and spiral capture into 600 km circular polar orbit. Main orbiting Automated mission operations dur spacecraft ing interplanetary and Titan phases: this includes interfacing with one supporting other spacecraft before deployments; deploying other spacecraft; communicating with other spacecraft and with Earth; studying Titan's atmosphere and surface using remote sensing techniques at both global characterization and intensive study levels; and selecting landing sites. Lander/Rover Lands at preselected site, avoids hazards; intensive study of Titan's surface; selects, collects and analyzes samples for composition, life, etc., explores several geologic regions. Subsatellite Lagrange point satellite monitors environment near Titan and is continuous communications relay; tethered satellite measure magnetosphere and upper atmosphere properties. Atmospheric Determines surface engineering probe properties and atmospheric structure at several locations/times. Powered air Intensive study of Titan's atmo vehicle sphere; aerial surveys of surface; transport of surface samples or surface systems. Emplaced science Deployed by long	range rover to package form meteorological and seismolog

ical network. (Alternatives are penetrators or extended lifetime probes.) Demonstration represent significant new possible applications. However, a NEP development program must be initiated in the 1980s to be operational in time for a Titan mission around the turn of the century.

The only major alternative propulsion technology is a chemical system using cryogenic liquids (the so-called Orbit Transfer Vehicle or OTV) for Earth escape, followed by gravity assists from Jupiter (in 1998) or from Earth and Venus, followed by aerocapture at Titan in the 20052010 time frame.

Main orbiting spacecraft. The principal vehicle for exploration in near-Titan space is an orbiter craft which remains with the NEP system. During the spiral capture process, the spatial structure of fields and particles around Titan can be measured. Following capture, the main spacecraft is parked in a circular polar orbit roughly 600 km above the surface of the body. Such an orbit has relatively little atmospheric drag and is highly desirable for close measurement and deployment of subsidiary system components into the atmosphere and to the surface of Titan.

During operations in near-Titan space, the main spacecraft must support a set of sophisticated remote-sensing instruments needed for global characterization and intensive study. In addition, it must continue to provide essential functions initiated during the interplanetary phases and support for deployed subcraft including navigation and communications with Earth. The estimated data collection volume is estimated at 1010-1011 b/day, significantly greater than the 109 b/day characteristic of previous planetary missions. Most of this is accumulated from instruments aboard the main orbiter, with perhaps 10% supplied by subsatellites and surface vehicles. Assuming that all raw data are returned to Earth, the required downlink communications capability is 10s-106 b/sec or 3-30 times the Voyager mission capacity from Saturn. However, significant amounts of data compression using advanced machine intelligence techniques should greatly reduce the transmission burden on the terrestrial downlink and also between elements of the Titan Mission.

The technologies developed in present and future planetary missions (especially Galileo, VOIR, and Earth-orbital) are generally applicable to this spacecraft. For instance, while in Titan orbit, the main orbiter is nadir-pointing much like VOIR and many Earth-sensing satellites. Major advancements are expected in the areas of machine intelligence and smart sensors, which suggests an increased capacity for data handling and communications as compared to previous planetary missions by the time of the Titan Demonstration.

Lander/rover. A lander/rover is needed to perform detailed surface and atmospheric measurements as well as the intensive level of study. Deployment of this spacecraft system is deferred until Titan's ground terrain has been fully mapped and an appropriate target site selected.