TAU (Thousand Astronomical Units) was a proposed

uncrewed An uncrewed vehicle or unmanned vehicle is a vehicle without a person on board. Uncrewed vehicles can either be under telerobotic control—remote controlled or remote guided vehicles—or they can be autonomously controlled—autonomous vehicl ...

interstellar probe that would go to a distance of one thousand astronomical units (1000 AU) from the Earth and Sun by the

NASA The National Aeronautics and Space Administration (NASA ) is an independent agency of the US federal government responsible for the civil space program, aeronautics research, and space research. NASA was established in 1958, succeedin ...

Jet Propulsion Laboratory The Jet Propulsion Laboratory (JPL) is a Federally funded research and development centers, federally funded research and development center and NASA field center in the City of La Cañada Flintridge, California, La Cañada Flintridge, California ...

in 1987 using tested technology. One scientific purpose would be to measure the distance to other stars via

stellar parallax Stellar parallax is the apparent shift of position of any nearby star (or other object) against the background of distant objects, and a basis for determining (through trigonometry) the distance of the object. Created by the different orbital p ...

. Studies continued into 1990, working with a launch in the 2005–2010 timeframe.

Overview

TAU was a proposed nuclear electric rocket spacecraft that used a 1 MW fission reactor and an ion drive (with a burn time of about 10 years) to reach a distance of 1000 AU in 50 years. The primary goal of the mission was to improve parallax measurements of the distances to stars inside and outside the

Milky Way The Milky Way is the galaxy that includes our Solar System, with the name describing the galaxy's appearance from Earth: a hazy band of light seen in the night sky formed from stars that cannot be individually distinguished by the naked eye. ...

, 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 General relativity, also known as the general theory of relativity and Einstein's theory of gravity, is the geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of gravitation in modern physics ...

. One of the tasks envisioned for TAU would be a flyby of

Pluto Pluto (minor-planet designation: 134340 Pluto) is a dwarf planet in the Kuiper belt, a ring of trans-Neptunian object, bodies beyond the orbit of Neptune. It is the ninth-largest and tenth-most-massive known object to directly orbit the S ...

. A Pluto flyby was achieved in 2015 by the

New Frontiers program The New Frontiers program is a series of space exploration missions being conducted by NASA with the purpose of furthering the understanding of the Solar System. The program selects medium-class missions which can provide high science returns. ...

mission '' New Horizons''. Some of the instruments proposed for the design included a 1.5-meter telescope for observations and a 1-meter telescope for laser communication with Earth. 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 Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...

as propellant and a nuclear fission reactor for power.

Description and mission profile

*Payload module (5,000 kg mass including a 10-watt laser transponder+1-meter-

aperture In optics, an aperture is a hole or an opening through which light travels. More specifically, the aperture and focal length of an optical system determine the cone angle of a bundle of rays that come to a focus in the image plane. An ...

laser communications telescope capable of transmitting data at 20 kilobits/second at 1000 AU, and a 1.5 meter astrometric telescope plus other experiments) *Propulsion module (4,000 kg dry mass including ten 4.45 newton thrust ion propulsion clustered in groups of five and fired paired for two years each. The total specific mass of ion thrusters, power processor units, etc. would be 4 kg/kWe (kilograms per kilowatt electrical power)) *1-MWe nuclear reactor+shield+radiator (6,000 kg mass or 12.5 kg/kWe specific mass) The 25,000 kg (gross launch mass including 10,000 kg of

propellant) ''TAU'' spacecraft would have been launched into a low Earth orbit by the

Space Shuttle The Space Shuttle is a retired, partially reusable low Earth orbital spacecraft system operated from 1981 to 2011 by the U.S. National Aeronautics and Space Administration (NASA) as part of the Space Shuttle program. Its official program n ...

in 2005–2010. 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 ion propulsion/xenon propellant module would have been positioned close to the

center of gravity In physics, the center of mass of a distribution of mass in space (sometimes referred to as the balance point) is the unique point where the weighted relative position of the distributed mass sums to zero. This is the point to which a force ma ...

, its 250 km/s exhaust velocity ion engines providing an acceleration of 0.35 mm/s². ''TAU'' would have attained Earth escape velocity in 250 days in a spiralling orbit, followed by Solar System escape speed 700 days later. 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)Solar System Log by Andrew Wilson (page 125), Jane's Publishing Company Limited, 1987, as the xenon propellant tanks would have been depleted. ''TAU'' would have reached 200 AU in 15 years after launch, 400 AU in 23 years, 600 AU in 32 years, 800 AU in 41 years and the full 1000 AU in half a century. Even so, it would have traversed less than 0.4% of the 4.3 light years to

Alpha Centauri Alpha Centauri ( Latinized from α Centauri and often abbreviated Alpha Cen or α Cen) is a triple star system in the constellation of Centaurus. It consists of 3 stars: Alpha Centauri A (officially Rigil Kentaurus), Alpha Centa ...

, the nearest star.

References

External links

Tau (Thousand Astronomical Unit) Mission
in ''The Encyclopedia of Astrobiology, Astronomy, and Spaceflight''.
Abstract of "TAU -- A MISSION TO A THOUSAND ASTRONOMICAL UNITS"
by K. T. Nock of the

.
Abstract of "Preliminary scientific rationale for a voyage to a thousand astronomical units"
by M. I. Etchegaray of the

and the California Institute of Technology. {{Pluto Cancelled space probes Interstellar travel Space astrometry missions

Overview

Description and mission profile

See also

References

External links