The Thousand Astronomical Unit (TAU) Probe is a conceptual mid-century NASA space exploration mission involving an advanced-technology robot spacecraft that would travel on a 50-year journey into very deep space—out to a distance of about 1,000 astronomical units
(some 93 billion miles [150 billion km]) away from Earth. The astronomical unit (AU) is a unit of distance in astronomy and space technology defined as the distance from the center of Earth to the center of the Sun. One AU is equally to approximately 93 million miles (149.6 million km), or 499 light-seconds.
The Thousand Astronomical Unit (TAU) Probe would feature an advanced multi-megawatt nuclear reactor, ion propulsion, and a laser (optical) communications system. Initially, the TAU Probe would be directed for an encounter with Pluto and its large moon Charon, followed by passage through the Kuiper belt and the heliopause, possibly reaching the innermost portion of Oort cloud. The heliopause is the theoretical end of the solar system—the region space where the Sun's influence ends and the solar wind smashes into the thin gas between the stars.
The advanced robot spacecraft would investigate low-energy cosmic rays, low-frequency radio waves, interstellar gases, and deep-space phenomena. It would also perform high-precision astrometry, the precise measurement of distances between stars. One key technology for this mission is an advanced space nuclear-reactor power system capable of autonomously and automatically providing a nominal 100 kilowatts of electric power for a period of at least 50 years. The power plant must be reliable and perform its task unattended by human controllers for five decades or more. With thrust provided by an advanced electric propulsion system, the TAU Probe would travel at a cruising speed of approximately 20 AU per year. This means that the journey would take roughly 50 years to achieve a total distance from Earth of 1,000 AU.
As a pre-interstellar probe mission, the TAU Probe would demonstrate sustained autonomous operations for more than 50 years. Spacecraft systems, subsystems, and components would require lifetimes and levels of reliability from one to two orders of magnitude greater than that currently available. The machine intelligence of this robot probe must be capable of autonomous assessment of the external environment and internal conditions onboard the probe. The smart robot probe must be able to make appropriate decisions and implement physical changes within itself, as circumstances warrant. In particular the TAU Probe must perform spacecraft health management, which involves the capability to predict, detect, and correct system performance. The robot spacecraft must be designed to practice fault management through repair, redundancy, and performance of workarounds—all without human guidance or assistance. Finally, the TAU Probe must be smart enough to perform unsupervised resource management, involving electric power usage and distribution, thermal control, use of consumables, the commitment of spare parts and emergency supplies, and data flow and data management.
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