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TABLE IV.-Potential Missions for Electrically Propelled Spacecraft

Power source Departure Arrival Destination Elects;gover Solar electric 1974 1975 Asteroid flyby (Eros) 8 1976 1977 Asteroid landing (Eros) 8

1976 1976 Venus orbiter 16

1976 1978 Comet (Kopf) 24

1977 1978 Execliptic 8

1977 1978 Solar probe (0.1 AU) 16

1977 1979 Jupiter flyby (or highly 24

elliptic orbiter)

1977 1980 Saturn flyby 24

1978 1978 Mercury orbiter 16

1978 1978 Mars orbiter 24

1978 1979 Asteroid landing 24

1978 1980 Comet (Encke) 24

1979 1980 Mars landing 24

1979 1982 Uranus flyby 24

Nuclear electric 1980 1982 Jupiter orbiter 150 1980 1982 Comet (d'Arrest) 150

1981 1984 Saturn orbiter 150

1981 1985 Neptune orbiter 150

1982 1986 Comet (Halley) 150

1982 1986 Uranus orbiter 150

1982 1987 Pluto orbiter 150

NOTE. —All launch vehicles would be of the Titan class except Eros flyby, which would use the Atlas/Centaur; see p. xxv.

The total masses of the spacecraft listed in table IV, at the beginning of their planetary trajectories, vary between about 500 and 6000 kg. Most of them could be launched also with a space shuttle instead of a Titan. In this case, a chemical kick stage must be provided to accelerate the spacecraft from the shuttle orbit into the planetary transfer trajectory.

The list of potential projects on table IV starts with an asteroid flyby. Asteroid Eros has a fairly eccentric trajectory; every 2 yr it approaches the Sun with a perigee of only 1.3 or 1.2 AU. In 1977, the Earth-Eros distance will be less than one-third the shortest Earth-Mars distance.

The proposed asteroid flyby mission in 1975 would represent a relatively easy mission with modest velocity requirements and with a guidance system that would have to guide the spacecraft only to a distance of about 100 km from the asteroid for photography, temperature measurements, and some other unsophisticated observations. Most of the instruments for this flyby mission would be available from previous projects, such as Surveyor, Mariner, and Lunar Orbiter. The spacecraft could be launched with an Atlas/Centaur. Although this flight would not yet provide all the desired information on Eros, it would obtain important data on size, shape, mass, rotation, surface features, and other properties of the asteroid that must be known for a successful follow-on project, the landing and data-return mission to Eros. Equally important would be another objective met by this simple flyby mission, a full-scale test flight of the complete electric propulsion system. From the standpoint of the spacecraft engineer, such a test flight would be very desirable before a spacecraft as complex and expensive as an asteroid landing and sample-return vehicle is committed for flight. Evidently, a project of this kind would not only be a highly valuable preparatory step for an asteroid lander mission but also a most important achievement with respect to the evolution of electric spacecraft for planetary exploration. Although data from the flyby mission would become available not more than a year before the launching of the lander mission, these data would be valuable for instrument settings and other details of the lander mission and as a confirmation of design data chosen for the lander. If the flyby should reveal severe deviations of the asteroid features from anticipated properties, the launching of the lander would have to be postponed.

None of the projects listed in table IV has attained the status of an approved project; all of them have only been the subjects of very preliminary studies. However, the list of potential projects may indicate the great promise that electric propulsion holds for a broad program of planetary exploration that could begin in the midseventies and would provide a rich harvest of knowledge of our entire solar system.

REFERENCE

Irving, I. H., and Blum, E. K. 1959, Comparative Performance of Ballistic and Low Thrust Vehicles for Flight to Mars. Vistas Astronaut. 2, 191.

DISCUSSION

WETHERILL: I think it would be a mistake to overemphasize differences in the merits of cometary and asteroidal missions. There are considerable savings in a program of missions using similar spacecraft; the cost per launch is much less for multiple missions than for single missions. These savings could be realized by development of a multipurpose solar electric spacecraft suitable for both asteroid and comet missions. From the point of view of scientific priorities, I think as much emphasis should therefore be placed on the sum of the value of these two types of missions as on the difference in their value.

I understand that the NASA Office of Advanced Research and Technology (OART) is requesting funds for a solar electric interplanetary mission in the budget currently before Congress, and I wonder what plans exist for obtaining scientific advice in planning these missions.

SOBERMAN: In the past, mission definition has taken place with little input from the scientific community. At the time of the “Announcement of Flight Opportunity,” the proposing scientists are faced with a vehicle for which at least a preliminary design exists. This “preliminary design” in practice is difficult, if not impossible, to modify. Even in the case of the Grand Tour missions, the so-called scientific definition phase must contend with the constraints of the TOPS vehicle.

I propose that NASA set up a definition team of scientists to work with the mission planners at the earliest stages so that the result would be system-optimized to perform the science.

DWORNIK: At NASA Headquarters, the Office of Space Science and Applications (OSSA) in fact does conduct an early mission definition phase that involves the scientific community. Scientists were invited to participate in the early planning phase of the Viking and the Mariner-Venus '73 and Mariner-Mercury '73 missions in order that spacecraft and mission constraints would not freeze out certain types of experiments. Specifically, the Mariner 1973 program actually altered early spacecraft and mission constraints to accommodate experiments.

BARBER: In the final recommendations, OSSA requires sufficient information for mission selection. Also at NASA Headquarters, OART is trying to provide some of this information for evaluation by OSSA. The initial thrust of our studies is to decide the technology feasibility of such small-body missions. We at OART are now at the point where we have a fairly systematic approach to the technology problem. The remainder of the work is to evaluate the scientific impact of the solar electric trajectory and spacecraft. The question as to what should be the science is not being considered at this time because the investigation is primarily technology oriented. When the actual mission is chosen, one would expect to turn to the scientist. At that time, the primary responsibility will transfer from OART to OSSA.

ASTEROID RENDEZVOUS MISSIONS

D. F. BENDER AWD R. D. BOUR KE
Jet Propulsion Laboratory

The earliest flights to concentrate solely on the nature of particular asteroids will probably be of the simple flyby or rendezvous type. That is, data will be obtained by close investigation of these bodies without actually coming in contact with their surface. The technology of flyby missions to bodies at planetary distances is well established; there have been five successful Mariners to date and more flights are scheduled. Unfortunately, ballistic flyby missions suffer from the fact that the spacecraft is near the body of interest for a short period of time (relative velocities at asteroid encounters are 5 to 12 km/s). This problem is acute because spatial resolution of the onboard instruments must be very high, and typical flyby velocities will preclude detailed observations.

These problems lead to consideration of rendezvous missions wherein the spacecraft is placed into the same heliocentric orbit as the asteroid and therefore remains close to it over a long period of time. This will allow long-term observations of the body over a range of aspects, distances, and phase angles. Probably most of the surface would be available for observation by this technique.

There are two principal means of achieving rendezvous and these are classified as “ballistic” (or “impulsive” or “high thrust”) and “low thrust.” In the first method, the spacecraft is given substantial velocity changes at various points in its flightpath and travels ballistically in between them. These velocity changes occur upon leaving Earth, upon arriving at the asteroid, and possibly at one point in between. They are imparted by a conventional rocket engine and occur over a very short time compared with the total flight, hence the term “impulsive.” The other method is to continuously thrust the vehicle over most of its flight with a high-specific-impulse, low-acceleration engine. The current concept for doing this uses solar electric propulsion wherein solar power is converted to electricity that drives electron-bombardment mercury-ion thrustors." This requires very large lightweight solar arrays. These engines operate for nearly the whole flight and are directed in such a way as to continuously and optimally change the orbit of the spacecraft until it coincides with the orbit of the asteroid. One concept for a solar electrically powered spacecraft suitable for an asteroid rendezvous is shown in figure 1. The technology of

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