TAU (spacecraft)

TAU (Thousand Astronomical Units) was a proposed uncrewed interstellar probe that would go to a distance of one thousand astronomical units (1000 AU) from the Earth and Sun by the NASA Jet Propulsion Laboratory in 1987 using tested technology.

[1] The primary goal of the mission was to improve parallax measurements of the distances to stars inside and outside the Milky Way, with secondary goals being the study of the heliopause, measurements of conditions in the interstellar medium, and (via communications with Earth) tests of general relativity.

[4] After launch it would accelerate to about 106 km/s (about 22.4 AU/year, or ~0.04% the speed of light) over 10 years, using xenon as propellant and a nuclear fission reactor for power.

Once deployed, a central boom would have telescoped the three main units listed above to a total 40 meter length to separate the payload from the nuclear reactor.

The TAU payload module would have separated from the rest of the spacecraft after ten years of constant thruster firing at a distance of 12 billion km (80 AU)[7] as the xenon propellant tanks would have been depleted.

TAU concept art
Stellar parallax is the basis for the parsec , which is the distance from the Sun to an astronomical object that has a parallax angle of one arcsecond . (1 AU and 1 pc are not to scale, 1 pc = ~206265 AU) What TAU would do is use its distance from the Earth to make the parallax measurement, so rather than just 1 AU as with an Earth-based annual parallax it would be hundreds of AU. This would increase the distance measurement horizon from about 500 light years for the Hipparcos satellite to 250 000 light years.