April 1983:JPL space report

PLANETARY MISSION CONCEPTS

Planetary missions can be classified in many ways. One method describes how close the mission is to coming into existence. In the category of highest cer¬ tainty lies a "flight project," for which a project approval document, or PAD as NASA calls it, has been issued. An example is the 1986 Galileo mission to Jupiter. A "development flight project" is next, and it represents a concerted effort to define and develop a project to the stage where it will become approved with a PAD. Current examples are the Venus Radar Mapper (VRM) or the TOPEX mission described below. The two remaining categories are somewhat arbitrary but could be labelled "studies" and "advanced-concept studies," with the latter at the bottom of the ladder.

Kerry Nock supervises the Advanced Projects group in the Mission Design section at JPL, and recently he talked to Spaceflight about some of his group's advanced-concept studies. One of these missions is outlined briefly here, with others to follow in future installments of "Space at JPL.”

The Voyager and Pioneer missions began the detailed study of the rings of Saturn, and a Saturn orbiter may be approved in the future, similar to Galileo with Jupiter. However, the most definitive study of this fascinating swarm of particles needs a spacecraft positioned very close to the rings, or even in them, to make in situ measurements of their structure and composition. One such advanced-concept mission is the "Saturn Ring Rendezvous." This uses a nuclear-powered electric propulsion system (from a low Earth orbit, reached by the Shuttle) which continually expels mercury ions from the space¬ craft, resulting in an ever-widening spiral trajectory until Saturn is encountered after more than eight years of flight. This spiral path contrasts with the usual elliptical orbit which is used to transfer from planet to planet after brief pushes provided by chemical engines such as those onboard the Shuttle/Centaur launch system. At the time the spacecraft begins its outward spiral from Earth it weighs about 17,000 kg with about 10,000 kg of mercury propellant. After one minute of thrusting the vehicle has moved only 0.3 m and has increased its speed by only 0.01 mps, yet the steady push will drive this spacecraft across hundreds of millions of kilometres to Saturn. On reaching its target, the spacecraft is made to spiral into the planet.

The value of the electricpropulsion system shows itself as the probe approaches Saturn's outermost ring. Here, the,spacecraft rotates so that its thrust is partially directed toward the plane of the rings, which boosts it slightly above the ring system while it continues to spiral into the planet. From this advantageous position, only 1 to 20 km above the ring plane, the entire radial span of the rings can be observed by instruments as they move from the outermost to the innermost ring. Coming across particle-free regions, the probe could be lowered gently into the rings themselves by adjusting the direction of thrust, achieving true rendezvous. Scientific examination of ring material taken onboard would be possible by means of chemical analyses, mass spectroscopy and even electron microscopy.

SATELLITE ALTIMETRY

A possible follow-on mission to Seasat is being studied at JPL: the Ocean Topography Experiment, or TOPEX, satellite. If funded, TOPEX would be launched late in this decade into a 1,300 km orbit at an inclination of 65°. The main purpose would be to study the general circulation of the oceans by precision altimeter measurements. Ocean currents cause a rise in the ocean surface above the marine geoid; for example, the Gulf Stream lifts the water level by about 2 m over a large section of the ocean.

The orbital height of 1,300 km would reduce the effects of atmospheric drag to a minimum and slightly reduce (compared to Seasat) the influence of errors in the model of the Earth's gravitational field. The ground track of the satellite would be designed to provide coverage of the same spot on the ocean every 10 days, thereby allowing studies of ocean variability even where the gravity field is poorly mapped. The mission would last 3 to 5 years.

The TOPEX altimeter is planned to have a precision of 1 to 2 cm and a second instrument, a microwave radiometer, would be carried to make measurements of the radiation emitted from water vapour in the atmosphere between the satellite and the ocean. From these measurements, corrections to the altimetric data could be made which would compensate for the presence of the vapour.

Charles A. Yamarone, Jr. of JPL manages the TOPEX development flight project.