Engineering topics
Publications and source records attributed to Stuart, J. R..
An overview of the Planetary Observer Program formulated by the Solar System Exploration Committee, including the key assumptions intended to constrain the cost of each program mission, is presented. Several candidate missions for the program and their scientific objectives and launch opportunities are described. A missions operations concept for controlling the operations costs of these projects is presented.
The fundamental objectives of the Mars Geoscience/Climatology Orbiter (MGCO) 1990 mission are related to the determination of the surface composition and topography of the planet Mars, its gravitational and intrinsic magnetic fields, and the seasonal behavior of volatiles, dust, and the atmosphere of Mars. These objectives would be achieved through a global mapping of the planet over a Martian year. For the baseline mission, a single spacecraft would be launched in August 1990, arrive at Mars in August 1991, and map the planet from a Sun-synchronous, near-circular, polar orbit for one Martian year. Attention is given to a science rationale and objectives, a mission description, the flight system, and mission operations.
The evolution of NASA's planetary navigation techniques is traced, and radiometric and optical data types are described. Doppler navigation; the Deep Space Network; differenced two-way range techniques; differential very long base interferometry; and optical navigation are treated. The Doppler system enables a spacecraft in cruise at high absolute declination to be located within a total angular uncertainty of 1/4 microrad. The two-station range measurement provides a 1 microrad backup at low declinations. Optical data locate the spacecraft relative to the target to an angular accuracy of 5 microrad. Earth-based radio navigation and its less accurate but target-relative counterpart, optical navigation, thus form complementary measurement sources, which provide a powerful sensory system to produce high-precision orbit estimates.
Previously cited in issue 08, p. 1191, Accession no. A82-22092
The Mars Orbiter Water Mission (MOWM) is discussed as an example of a low-cost, low-risk focused-science Mars mission which might be implemented in the near term within recently imposed fiscal constraints. MOWM is designed to act as a follow-on to the Viking results in the areas of comparative climatology and water, carbon dioxide and dust inventories. The STS-compatible space vehicle would be based on proven earth-orbiting satellite hardware, fulfilling requirements for both spinning and despun components, operation with nonoptimum solar cell orientation, PAM-A upper stage compatibility, and data storage for delayed transmission. Study results have shown the technical and economic feasibility of missions based on any of four submitted designs which meet these requirements, and of low-cost, well-focused science objective Mars missions in general.
An assessment is presented of the results of recent feasibility studies on a low cost dual mission concept, employing a single spacecraft for the sequential, intensive survey of two planets. After establishing the features of a basic lunar-Martian polar orbiting mission in the context of existing spacecraft and propulsion technology, an examination of trajectory options shows that an earth return maneuver allows the application of the same concept to lunar-asteroid missions and offers substantial savings for the lunar-Martian mission. The implications of Centaur availability for extension of this concept to main-belt asteroids and to tripple missions are considered, along with questions of cost and reliability.
Statistical studies of telemetry errors were made on data from the Solar Mesosphere Explorer (SME). Examination of frame sync words, as received at the ground station, indicated a wide spread of Bit Error Rates (BER) among stations. A study of the distribution of errors per station pass, however, showed that there was a tendency for the station software to add an even number of spurious errors to the count. A count of wild points in science data, rejecting drop-outs and other system errors, yielded an average random BER of 3.1 x 10 to the -6 with 99% confidence limits of 2.6 and 3.8 x 10 to the -6. The system errors are typically 5 to 100 times more frequent than the truly random errors.
Mars oblateness has been found to provide sun-synchronous orbits, including orbits with stationary apsides, similar to those used at earth. A low mass and low data rate complement of scientific instruments placed in such orbits can provide exciting planetary investigations such as the Mars Orbiter Water Mission described herein. Use of a modest Shuttle kickstage (PAM-A) and existing spacecraft hardware makes this mission low-cost. A preliminary mission and spacecraft design is described. The major emphasis of the paper is on the mechanics of heliocentric transfer for the 1986 and 1988 launch opportunities, Martian sun-synchronous orbit geometries, injectable mass capabilities, and methods of achieving these scientifically useful orbits.
The Solar Mesosphere Explorer (SME) satellite experiments will provide a comprehensive study of atmospheric ozone and the processes which form and destroy it. Five instruments to be carried on the spacecraft will measure the ozone density and altitude distribution, monitor the incoming solar radiation, and measure other atmospheric constituents which affect ozone. The first objective of the SME satellite experiments is to determine what changes occur in the ozone distribution as a result of changes in the incoming solar radiation. A second objective is to measure all changes in the ozone density distribution in the altitude range of 30 to 80 km and to determine the causes of these changes.
Mission overview of the Solar Mesosphere Explorer is presented along with design analysis and summaries of results. The Solar Mesosphere Explorer is a spin stabilized satellite carrying a complement of four Ebert-Fastie spectrometers and a four-channel Mersenne radiometer. Description of the spectrometer is given including a telescope and its aberrations. The radiometer is also described with consideration given to isothermal and thermal design, a Winston paraboloid, and optical tolerances. These five instruments are for measuring the earth's ozone density and distribution and providing quantitative data about those processes which govern the formation and destruction of ozone.
The next logical step in the exploration of Mercury is an orbiter mission. A conflict exists between those in the field of planetary sciences who desire a mission with a low circular orbit, and scientists in the fields and particles disciplines, who generally prefer a highly elliptical spacecraft orbit. The thermal environment imposed by the sun and planet render the low orbit intolerable for spacecraft using previous thermal control methods. A thermal control concept and a spacecraft mission concept have been developed which resolve these problems and promise a scientifically significant mission for the mid-1980s.