R. H. Tolson

Viking First Encounter of Phobos: Preliminary Results

During the last 2 weeks of February 1977, an intensive scientific investigation of the martian satellite Phobos was conducted by the Viking Orbiter-1 (VO-1) spacecraft. More than 125 television pictures were obtained during this period and infrared observations were made. About 80 percent of the illuminated hemisphere was imaged at a resolution of about 30 meters. Higher resolution images of limited areas were also obtained. Flyby distances within 80 kilometers of the surface were achieved. An estimate of the mass of Phobos (GM) was obtained by observing the effect of Phoboss gravity on the orbit of VO-1 as sensed by Earth-based radiometric tracking. Preliminary results indicate a value of GM of 0.00066 � 0.00012 cubic kilometer per second squared (standard deviation of 3) and a mean density of about 1.9 � 0.6 gram per cubic centimeter (standard deviation of 3). This low density, together with the low albedo and the recently determined spectral reflectance, suggest that Phobos is compositionally similar to type I carbonaceous chondrites. Thus, either this object formed in the outer part of the asteroid belt or Lewiss theory that such material cannot condense at 1.5 astronomical units is incorrect. The data on Phobos obtained during this first encounter period are comparable in quantity to all of the data on Mars returned by Mariner flights 4, 6, and 7.

Viking Lander Location and Spin Axis of Mars: Determination from Radio Tracking Data

Radio tracking data from the Viking lander have been used to determine the lander position and the orientation of the spin axis of Mars. The areocentric coordinates of the lander are 22.27�N, 48.00�W, and 3389.5 kilometers from the center of mass; the spin axis orientation, referred to Earths mean equator and equinox of 1950.0, is 317.35� right ascension and 52.71� declination.

Reducing flexible base vibrations through local redundancy resolution

Future space systems will use teleoperated robotic systems mounted on flexible bases such as the Shuttle Remote Manipulator System. Due to dynamic coupling, a major control issue associated with these systems is the effect of flexible base vibrations on the performance of the robot. If uncompensated, flexible vibrations can lead to inertial tracking errors and an overall degradation in system performance. One way to overcome this problem is to use kinematically redundant robots. Thus, this article presents research results obtained from locally resolving kinematic redundancies to reduce or damp flexible vibrations. Using a planar, three-link rigid robot example, numerical simulations were performed to evaluate the feasibility of three vibration damping redundancy control algorithms. Results showed that compared to a zero redundancy baseline, the three controllers were able to reduce base vibration by as much as 90% in addition to decreasing the required amount of joint torque. However, similar to locally optimizing joint torques, excessive joint velocities often occurred. To improve stability, fixed weight, multi-criteria optimizations were performed.

Mars Dynamics, Atmospheric and Surface Properties: Determination from Viking Tracking Data

Approximately 3 months of radio tracking data from the Viking landers have been analyzed to determine the lander locations, the orientation of the spin axis of Mars, and a first estimate from Viking data of the planets spin rate. Preliminary results have also been obtained for atmospheric parameters and radii at occultation points and for properties of the surface in the vicinity of lander 1.

Lunar Orbiter: Tracking Data Indicate Properties of Moon's Gravitational Field

After only a few days in orbit, the first U.S. satellite of the moon has already produced data which have provided new information about the moons gravitational field. Results indicate that the spacecraft will probably not impact on the moon before it completes its photographic mission, but that it will probably do so in about 8 months. Preliminary indications are that the moon has a relatively large pear-shaped component and that the gravitational properties will be of considerable scientific interest.