The first comprehensive study of an early 21st century extra-solar probe concept was published by L. D. Jaffe and other members of a NASA/Jet Propulsion Laboratory (JPL) study team in 1980. The mission goal for TAU was to propel a scientific payload in the year 2000 on an extra-solar trajectory such that the spacecraft traversed 1,000 AU during a 50-year flight time. The average velocity of the TAU probe relative to the Sun would be about 100 km s-1. About 10,000 years would be required for the TAU probe to reach the nearest extra-solar star system, if it were moving in the appropriate direction.
Many propulsive options were considered for TAU. These included direct launch from Earth; Jupiter gravity assist and powered flyby; solar and laser sailing; solar-electric and laser-electric propulsion; fusion; and antimatter. It was concluded that in the 2000 AD time-frame, only two propulsive options existed for the TAU mission: nuclear-electric propulsion (NEP) and the ultralight solar sail unfurled as close to the Sun as possible.
Primary TAU mission objectives included determination of heliopause and interstellar medium characteristics, accurate measurements of stellar and galactic distances using long-baseline astrometry (possibly with two or more TAU probes moving along different trajectories), examination of cosmic rays excluded by the heliosphere, and determination of large-scale Solar System characteristics from afar. Secondary mission objectives included close observation of Pluto (if a TAU probe were directed to pass near that planet), extra-galactic observations and evaluation of the possibilities of observing other solar systems from spacecraft.
Candidate scientific instruments for TAU included magnetometers and electric-
field meters, spectrometers and radiometers, radio-astronomy detectors and optical cameras. Excluding power and propulsion, the estimated mass of the TAU probe was 1,200 kg. Estimated data transmission rates were in the region of 10 kilobits per second. Astrometric photographs of distant galactic objects could be transferred to Earth at the rate of several images per day.
Exercise 3.1. Because of the speed-of-light limitation, an extra-solar or interstellar probe requires a high degree of intelligence and autonomy. Estimate the time required for a signal radioed from the Earth to reach a probe at 100, 500 and 1,000 AU from the Sun.
Sadly, the TAU study did not lead directly to an extra-solar mission. It does, however, serve as a valuable baseline study for contemporary extra-solar mission plans.
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