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The Road to Space. The First Thousand Years.
Space: From Firecrackers to Interstellar Flight
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Spacecraft at Solar System Galactic Frontier

Voyager 1 and Voyager 2 reach the termination shock

Termination shock. NASA's Voyager 1 spacecraft crossed the termination shock in December 2004 at 94 AU from the Sun. Voyager 2 reached the shock in early September 2007at 84 AU from the Sun. In contrast to Voyager 1, the plasma instrument on Voyager 2 is operational. At the termination shock the expanding supersonic solar wind slows down and becomes subsonic. This is an exciting news for space scientists, who waited for these events for many, many, many years.

Have we reached the galactic matter surrounding the solar system? No. The boundary separating the galactic and the solar matter, the heliopause, is predicted to be 40–60 AU further away from the sun. The Voyager 1 spacecraft leaves the solar system with the escape velocity about 17.1 km/sec, or 3.6 AU/year. (Voyager 2 is slower with the velocity 3.1 AU/yr.) By the time Voyager 1 reaches the heliopause it will likely be turned off because of the gradual reduction in power producing capacity of its power source – radioisotope thermoelectric generators (RTGs).

Are the Voyager spacecraft properly instrumented to explore the galactic frontier of the solar system? No. Voyager 1 and 2 spacecraft were designed primarily for study of giant planets during planets flybys. Instruments will measure some very important physical properties of the region but the detailed study of the processes at and beyond the frontier will be performed in the future by a precursor interstellar mission – Interstellar Probe. The Interstellar Probe mission has been considered for the last thirty years. (See Instrumentation for interstellar exploration and references therein.) Depending on programmatic priorities, NASA will launch a spacecraft into interstellar space in 10–20 years. The mission goal is to reach the pristine, unperturbed (by sun's presence) interstellar medium (300–400 AU from the sun) surrounding the solar system and explore it in-situ.

New science questions. While Voyagers have answered some important questions about the physics and processes at the heliospheric boundary, they also raised new ones. In particular, the existing concept of acceleration of ions to high energies (giving rise to anomalous cosmic rays) at the termination shock is apparently not valid. Voyager 2 also clearly showed that the kinetic energy of the solar wind flow is not transformed into heating of the bulk of the shocked solar wind plasma but goes into unknown elsewhwre.

IBEX. Now, the attention is on the Interstellar Boundary Explorer (IBEX) mission launched in October 2008. IBEX will remotely (from 1 AU) probe – for the first time – plasma properties between the termination shock and the heliopause. The new experimental technique – imaging in fluxes of energetic neutral atoms (ENAs) – promises new discoveries about the galactic frontier.

ENA Imaging Tutorial

What is next? The complementary remote techniques (ENA imaging and EUV imaging) will establish the properties of the asymmetric heliosphere. Then the Interstellar Probe will penetrate the surrounding galactic medium.

Recommended books on history of astronautics, rocketry, spaceflight, and space technology

Recommended textbooks and monographs on astronautics, rocketry, and space technology

Three-dimensional asymmetric heliosphere. The region controlled by the sun is called the heliosphere. The heliosphere is expected to be essentially asymmetric (see figure on the right). There are three reasons for the asymmetric heliosphere.  First, the sun moves with respect to the surrounding galactic medium with the velocity 26 km/sec, or about 5 AU/year. Second, the solar wind flow is not isotropic. Third, the interstellar magnetic field will add asymmetry. Distance to the termination shock is 90 AU in the upwind (with respect to the interstellar wind) direction; distance to the heliopause is 140-180 AU.


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Remote techniques needed. Voyager 1 and future Interstellar Probe will establish the distance to the solar system frontier and its properties only in two points. The size of the heliosphere calls for remote techniques to probe the essentially asymmetric three-dimensional boundary. There are two experimental techniques that will probe the galactic frontier remotely: imaging of the termination shock and the heliosheath plasma in fluxes of energetic neutral atoms (ENA) and mapping the heliopause in extreme ultraviolet radiation (EUV).

Energetic Neutral Atom imaging (ENA imaging). ENA imaging is a mature experimental technique and ready for implementation. In January 2005, NASA selected a new space mission, Interstellar Boundary Explorer (IBEX), to image the heliosphere in ENA fluxes. More on history of ENA imaging and instrumentation.

Extreme Ultraviolet Mapping (EUV) of the Heliopause. EUV mapping of the heliopause is a relatively new experimental concept and important advancement in instrumentation is required to enable such space experiments. The development work is under way. The concept of heliosphere imaging in EUV and instrumentation were recently presented at a SPIE meeting. More details are at: 

An overview of remote study of the solar system frontier:

See also on the future Interstellar Probe

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