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.