Atmospheric data are taken during the lander descent phase, and this continues as long as the vehicle remains operational on the planetary surface. Small rocket thrusters are used to guide the craft to a safe place free of large boulders, deep crevasses, or steep slopes. After a soft landing, the surroundings are characterized in preparation for site selection for sample collection. Physical samples are then acquired using extensible manipulators (scoops, drills, slings, etc.) and are immediately analyzed to determine chemical composition, layering effects, evidence for indigenous lifeforms, etc.
After this has been accomplished, the lander requires samples taken from a wider area to complete its preliminary investigations. The general solution to this problem is the rover, a vehicle deployed by the lander and used to explore the local neighborhood and to bring back samples. The simplest rover design might operate no more than 100 m from the lander and would remain almost totally dependent upon it. Such a machine is useful for collecting samples more free of contamination and more representative of the surface than those taken nearer the landing site. However, the Space Exploration Team prefers a more ambitious design, an autonomous rover able to operate up to 10 km from the lander. This larger-area capability permits the lander/rover system to return data which better contributes to an overall understanding of the geological structures of complex sites. Such advanced rovers already have been considered for lunar and martian applications.
It is also necessary to provide the capability of performing intensive studies at more than one surface landing site. This flexibility is possible by deploying multiple lander/ rover teams which may be carried from site to site using powered air vehicles for very-long-distance transport. Physical samples could also be returned to stationary landers by similar means. Another possibility is a highly sophisticated long-range rover having a complete set of instruments and sample collection and analysis equipment, and designed for higher speeds, longer traverses (more than 100 km), and enhanced survivability over more difficult terrain with more challenging obstacles. Long-range rovers could visit any number of distinct geologic regions during their lifetimes and might be used to deploy a network of stationary science packages across the surface of the entire planet. The orbit of the main spacecraft is such as to permit regular contact with surface vehicles twice each Titan day (once each Earth week).
The lander/rover system needs extensive machine intelligence capability. Technology requirements are greatest for a long-range rover operating independently in the absence of continuous communications with the main orbiting spacecraft or with Earth. This capability is highly desirable, since without it the operational demands placed on other mission elements ? such as the subsatellites for ground-to- orbit Titan uplink or powered air vehicles necessary for sample and system component transport rapidly may become unmanageable.
A significant heritage may be expected from experience gained with the Viking landers and from any future martian or lunar missions, several of which might be approved and flown prior to the Titan Demonstration. One potential major difference is the unknown character of the surface including the possible existence of open liquids on Titan. If fluidic features are widespread it may be necessary to devise new methods of surface mobility and long-distance planetary exploration. New rover concepts for the reduction of machine intelligence requirements by decreased susceptibility to hazards should also be investigated.
Subsatellites. In addition to the main orbiter. subsatellites may be needed for certain specific purposes. One example is a free-flying spacecraft stationed at the LI Lagrangian point between Titan and Saturn. This could be used to monitor the particle/field environment beyond Titan's magnetosphere, to observe the target atmosphere, and to communicate with mission elements located on the Saturn side of Titan. Another example is a tethered sub- satellite system operating within 100 km of the main orbiter - such multiple devices can more easily distinguish spatial and temporal variations in particles and fields and probe the upper atmosphere (which would cause unacceptable drag on the main spacecraft if it attempted these measurements directly).
The subsatellite concept is new to planetary mission planning. However, these devices currently are projected for use on the Space Shuttle and also are under consideration in connection with manned and unmanned orbital platforms. This technology should become available by the time of the Titan Demonstration (e.g., the spin-stabilization of Mission relay subsatellites). There may also exist some commonality with previous planetary missions such as Pioneer 10/11 and Pioneer Venus.
Atmospheric probes. Several mission components must be sent into Titan's atmosphere at selected locations to make in situ measurements of the air and to carry small instrument packages to the surface. These probes are deployed by the main orbiter from its 600-km circular polar orbit, thus permitting considerable flexibility in choice of geographical entry points and timing. Atmospheric entry probes measure vertical profiles of the atmosphere at the time of deployment, and provide sufficient information to meet mission objectives at the "exploration" level. The Pioneer Venus, Galileo, and proposed Saturn Orbiter Dual Probe (SOP2) missions all include atmospheric entry probes among their equipment.
One large entry probe and at least three small probes are necessary to fulfill the major objectives of Titan exploration. As in the Pioneer Venus mission, all probes measure atmospheric structure, pressure, temperature, etc., whereas
