target planets without stopping and eventually returns to the Earth. The spacecraft may use a close approach to the target planets to modify its trajectory and control mission duration. Trip duration for Mars fly-by missions is of the order of 500 to 700 days, while trip duration for Venus fly-by missions is of the order of 400 days.

4. An orbiting mission is one in which the spacecraft goes into orbit about the target planet; there may or may not be a landing. After a specified time, the spacecraft departs from the orbit and returns to Earth.

5. A Mars landing mission (fig. 90) might use a Mars orbit rendezvous technique, an adaptation of the lunar orbit rendezvous (LOR) technique being developed for the APOLLO program. The minimum duration landing mission will vary from 330 to 500 days and provide a short stay at Mars. The minimum energy mission would require 800 to 900 days, the stay times ranging from 0 to 500 days. These mission characteristics require long duration life support systems and the development of advanced launch vehicles, propulsion, on-board power systems, and other.

Two types of stopover trips to Mars are of importance:

6. An opposition-class trip to Mars is one in which the spacecraft arrives at and departs from Mars within a span of a few days near the time of opposition. These trips are characterized by high energy requirements, high entry speeds, and masses on Earth orbit, for a landing mission, of the order of 1 million pounds.

7. Conjunction-class trips to Mars are characterized by arrival at Mars after one opposition date and departure from Mars before the

next opposition date. These are long duration missions typically on the order of 750 to 950 days and provide a stay time at Mars of from 0 to 550 days. This class of missions shows a mass in Earth orbit which is appreciably less than the opposition class of "fast" trips and unlike those trips is invariant across the synodic cycle. Figure 91 illustrates the variation in performance requirements with time for both opposition-class and conjunction-class missions.

Past studies

The first NASA contract in the manned planetary area was an effort in fiscal year 1962 to develop round-trip trajectories for high acceleration vehicles. As a result, all such trajectories to Mars and Venus from now until 1999 have been calculated and presented in a three-volume planetary trajectory manual. This data is completely independent of whatever spacecraft systems will ultimately be used for the missions.

The first set of system studies was begun in 1962 to examine the requirements for performing manned missions to Mars and Venus. These studies provided information on the basic requirements of manned planetary travel, and served as a basis for definition and direction of further studies, as well as for related research and development programs.

Conceptual designs of spacecraft and spacecraft systems were evolved during these studies. One spacecraft configuration shown for a fly-by vehicle is shown in figure 92. The command module and service module are at opposite ends of a rigid interconnecting structure and rotate about a central hub section. The centrifugal force pro

duced by this rotation acts outward, radially, and the crew experiences this force as gravity.

The hub at the center of rotation contains a heavy solar flare shelter and has a midcourse propulsion unit at one end. The primary power supply is at the end of the hub opposite the midcourse propulsion system. An airlock incorporated into the hub permits the crew to leave the spacecraft for external repair or inspection.

Utilizing the information from the previous studies, our recent efforts have been devoted primarily to continuing exploratory and feasibility studies. These studies investigated the feasibility of performing Mars and Venus missions by using existing or planned hardware. Missions utilizing nuclear engines and advanced Earth launch vehicles were also examined and methods for accomplishing Mars missions during the difficult mission period, 1975-84, were investigated.

In addition, a method of performing opposition-class Mars stopover missions by flying near Venus was uncovered. This mission combines the short duration of opposition-class trips with the low energy requirements of conjunction-class trips. Spacecraft systems studies were performed to determine conceptual configurations and important systems parameters, such as size, weight, packaging effects, power requirements, vehicle shapes, and crew size. Efforts to define the objectives of a manned planetary exploration program were also intensified.

In order to examine the critical requirements of subsystems for these long-duration mission systems, the spacecraft required to fly a

Mars orbit rendezvous profile were investigated. The spacecraft consisted of a mission module, an excursion module, and an Earth reentry module.

The mission module is a spacecraft which travels between Earth and Mars orbits carrying the crew and the other spacecraft modules. A cutaway of one configuration (fig. 93) shows the concept of packaging the other spacecraft modules within the mission module. Distribution of living space and systems is shown, the solar collectors for providing on-board power being extended. The Earth reentry module shown here (fig. 94) is reentering the Earth's atmosphere and shows the position of the crew during reentry.

Present studies

Present study activities consist of an evaluation of several alternate mission profiles and methods of mission accomplishment resulting from previous studies. In addition, system requirements to determine the feasibility of performing early fly-by missions using hardware currently being developed for other NASA space flight programs will be examined.

Conjunction-class Mars landing trips will be investigated to determine those problems critical to this class of missions. The methods of alleviating the severity of these problems and the most feasible mission mode as effected by surface or orbital stay times of 0 to 550 days will be determined. Necessary modifications to the spacecraft design to permit manned orbiting missions to Venus will also be determined.

In-house work on a master plan for manned interplanetary exploration is continuing. In addition, an intraagency effort to integrate the manned and unmanned planetary programs is underway. This effort will assure that vital engineering-oriented environmental data on the near planets is obtained on a timely basis. Also being examined are the requirements which may be placed on manned planetary missions for obtaining scientific data to supplement the unmanned programs with probes carried and launched by early manned missions.

VEHICLE STUDIES

To accomplish the earth launch phase of these future manned missions, a variety of vehicles are being studied. Figure 95 shows the three major areas of launch vehicle study effort. We must first look to increasing the capabilities of the launch vehicles already under development SATURN IB and SATURN V. Some of the manned space flight missions which could follow the APOLLO manned lunar landing program are within the capabilities of the present SATURN vehicles but the rest to varying degrees, are not. However, many could be accomplished by improved or uprated SATURNS. Therefore, we are studying the growth potential of these vehicles.

The study of reusable orbital vehicles is the second area which looks very promising. Anticipated levels of manned activity in Earth orbit from the middle 1970's and into the 1980's indicate a strong need for a more economical launch vehicle. Further, the desire to provide for the travel of nonastronaut passengers places limitations of 21⁄2 to

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