system component are shown in table 3.5. surface mobility, and physical sample selection, collection, and analysis. Other candidate system elements have more specialized functions, the management of which can be TABLE 3.4.- CANDIDATE SPACECRAFT FOR THE TITAN DEMONSTRATION MISSION
Spacecraft type
Typical number
Operational location
Mass, kg
Power, kW
Nuclear electric propulsion
1
Earth to Titan orbit
10,000?
400
Main orbiting spacecraft
1
Circular polar Titan orbit at 600 km altitude
1,200
..P
Lander/Rover
2
Surface
1,800
1
Subsatellites
~3
One at a Lagrange point others on 100 km tethers from NEP
300
0.3
Atmospheric probe
Through Titan atmosphere to surface 200 0.1 Powered air vehicle 1 Atmosphere 1,000 10 Emplaced science ~6 Surface 50 0.1
aDoes not include propellant. ^Uses NEP power.
The minimum duration of Titan operations is 1 year. While this would be barely sufficient to complete a nominal mission, it is a short time in comparison to seasonal changes in the Saturn system. (Saturn's solar orbital period is 29 years.) The most significant seasonal effects may be expected within about 5 years of the solar equinox of Saturn and Titan - which occurs in 1980, 1995, and 2010 AD. Hence, the preferred arrival dates are 2005 or 2010 AD, with a nominal mission duration of 5 years. Adding 5 more years for interplanetary flight, the preferred Earth-launch dates are 2000 or 2005 AD.
The success of the Titan Demonstration Mission depends on two essential elements - (1) the main orbiting spacecraft and (2) the lander/rover ? and on the machine intelligence which they possess. High-level AI capabilities are needed by the main orbiter to coordinate other system components and to conduct an ambitious program of scientific investigation, and are required by the lander/rover to complete its tasks including safe and accurate landing, assumed, at least in part, by advanced sensors and machine intelligence aboard the orbiter or landing craft.
Nuclear electric propulsion. The early phases of the mission, beginning with launch from Earth and continuing through Saturn arrival, require a high-performance propulsion system which can deliver the payload within a reasonable flight time (4 to 6 years). Low-thrust Nuclear Electric Propulsion (NEP) is the preferred technology for this purpose. The entire NEP system can be delivered to LEO, then be used for spiral escape from Earth, Earth-to-Saturn transfer, for Titan-rendezvous from a circular orbit around Saturn, and finally for spiral capture into Titan orbit and all subsequent spacecraft orbital adjustments. The main orbiter spacecraft and the NEP system share responsibilities for navigation, guidance, control and sequencing, system monitoring, and communications with Earth.
